Assessment of the Volume, Spatial Diversity, Functioning, and Structure of Sediments in Water Bodies Within the Słubia River Catchment (Myślibórz Lakeland, Poland)

Abstract

Water reservoirs play a crucial role in the environment in many aspects: hydrology, geochemistry, sediment lithology, geo- and biodiversity, landscape, etc. First of all, it is necessary to have accurate information about the spatial distribution of these objects in a given area to assess their size and functioning. Maps and contemporary spatial databases are often incomplete or outdated, especially in regard to small objects, of variable surface area and condition. This article uses the following approach: high-resolution terrain models derived from airborne laser scanning (ALS) were used for visual interpretation of extensive, flat depressions representing water body basins, thus determining the total number of objects, and classifying them as kettle holes, lakes, ponds, and other types of reservoirs (e.g., overbank basins, oxbow lakes). Using an aerial orthophotomap, the objects were subsequently verified as to how many basins are currently occupied by water bodies. The next step was to determine a number of topographic and morphometric parameters for each object in order to assess their functioning conditions. For selected objects, the assessment was expanded to include a geochemical and lithological analysis of the sediments. The study was conducted in the catchment of the Słubia River (136 km²), located in Central Europe, in northwestern Poland. In the Słubia catchment, a total of 931 water body basins were mapped. The dominant forms are kettle holes (<1 ha), representing nearly 80% of all objects. At present, kettle holes are largely devoid of water bodies and subject to a strong human impact. In addition to those, 118 lake basins were identified (>1 ha, the largest being Lake Morzycko, 360 ha), half of which are occupied by water reservoirs. Ponds and other reservoirs were represented by 37 and 47 objects, respectively. From the perspective of contemporary sediment-forming processes in the documented sedimentary basins, the most favorable conditions for biogenous sediment accumulation exist in the catchments of the upper and medium courses of the Słubia River valley. Although the lithological diversity and thickness of individual sediment types in the Słubia catchment represent local features, they corroborate the results of previous telmatologic research conducted in Myślibórz Lakeland.

1. Introduction

Surface inland water bodies are hydrographic objects that represent a crucial link in the water cycle, on both local and global scales. They gather, filter, and purify water, constitute important habitats, and serve as geodiversity and biodiversity refuges. They include lakes and smaller water reservoirs, both natural and anthropogenic. Surface water reservoirs are distinguished by a variety of possible origins, shapes, surfaces, depths, trophic regimes, etc., which result in the existence of various classifications. Currently, increasing attention is being paid to small water bodies such as kettle holes. They are important for biodiversity, providing habitats for amphibians, birds, and wild animals, and they also positively impact the landscape. Such water reservoirs are also highly sensitive to environmental changes. In recent years, a large proportion of them have disappeared, become overgrown, or eutrophicated as part of a natural cycle of the basin functioning, but also due to climate change and economic activity in their vicinity [1,2,3,4,5].

The functioning of water bodies depends on the type and intensity of water supply from the catchment and precipitation. Lake balance type, including open (flow-through lakes and lakes with outflow) and closed (drainageless) basins [6], and percent contribution of lakes to the surface area, reflecting the lake position within a catchment [7], are among the parameters characterizing the degree to which the catchment exerts influence on water bodies. Both features, for instance, impact the rate of lake water renewal [8], nutrient cycling [9,10,11], and the rate of sediment infilling [12], which resulted in the identification of multiple hydrologic types [13,14]. The Ohle index, i.e., the relationship of the catchment surface area to the lake surface area [15], is another measure of the catchment resistance to the impact of the surroundings. This relationship influences, e.g., the water exchange intensity, or the character and volume of the supplied matter [16].

Palaeogeographical data for the young glacial landscape belt indicate that changes to land use structure led to significant changes in water relations not only in river valleys [17,18,19], but also in small catchments located on plateaux [20,21]. The scale of these changes, however, varied according to the relief diversity and geology, as drainage potential represents an important cause for groundwater level changes and fluctuations in lake basin extent. With respect to hydroclimatic changes of the past <20,000 years, such events have been recorded in—among other proxies—biogenic sediment lithology, soil cover changes, and in selected geochemical parameters [20,22,23,24,25,26,27,28,29,30,31,32,33,34]. In the cases cited above, drainageless basins occupying small areas, often referred to in the literature as kettle holes or pot holes, are especially prone to the supply of pollution.

Assessing the ecological and geochemical status of water bodies requires comprehensive information about their spatial distribution and topographic parameters. However, existing data sources, such as maps and topographic databases, often contain incomplete, outdated information, or other errors (misassigned parameters, defunct water bodies, etc.).

Remote sensing methods and geoinformation analyses are playing an increasingly important part in determining the present-day topographic parameters of a given water body. They enable precise measurement of the area and shape of an object [35]. The decreasing costs of remotely piloted aircraft/unmanned aerial vehicles, and using them to acquire high-resolution materials, also enable the basin functioning to be monitored with respect to surface area variations throughout the year, quantitative and qualitative changes in water resources, including pollution levels [36,37], and taking into consideration their episodic or cyclic character [38]. Indirect remote sensing methods, such as visual interpretation of vegetation on an orthophotomap, or visual interpretation of terrain relief in high-resolution models, also enable preliminary inventorying of presently non-functioning objects, or objects whose extent has significantly decreased [39].

Existing studies based on aerial/satellite imagery analysis were discussed, among others, by Choiński and Kijowski [35]. Studies from Polish lakelands focused mostly on single, large-area lake basins [19,40]. Within Myślibórz Lakeland, the kettle holes mentioned above are notable for a number of reasons. Calculations by Pieńkowski [41] indicate that as late as the end of the 19th century, their number in the study area equaled as many as 3715, of which slightly more than 83% comprised ecosystems characterized by the lowest surface area (<0.189 ha). Due to the widespread degradation of kettle holes in northern Poland [26,41,42,43,44], various measures are being undertaken in order to grant them efficient protection. This is because such objects have a high significance for water and carbon cycling at various spatial scales [45,46]. To that end, assessments have been made of, among others, natural values of basins and their catchments, including the water balance components [47] and vegetation changes [48,49]. Other studies offered methodological considerations of geosystem functioning theory [50,51].

The present paper is an attempt at assessing the number and functional state of water bodies located in the southwestern part of Myślibórz Lakeland in western Poland, comprised within the Słubia River catchment. The main goal was to be achieved by carrying out the following detailed aims:

• Determining the number, spatial distribution, and density of water bodies identified in the landscape, with a distinction between kettle holes, lakes, ponds, and other types of reservoirs;
• Determining the number and percentage of basins presently occupied by water bodies;
• An attempt at identifying the reasons for the decline of water bodies, including an interpretation of the surface relief, and lithogeochemical analysis, along with a classification of bottom deposits sampled in selected kettle holes.

2. Materials and Methods

2.1. Study Area

The Słubia River catchment is located in northwestern Poland, close to the German border, approximately 60 km south of Szczecin (Figure 1). The study area is confined between 52°46′01″ N and 52°54′54″ N latitude, and 14°14′01″ E and 14°35′17″ E longitude. The catchment surface area equals 136 km². The largest community located within the Słubia catchment is the town of Moryń, located to the west of Lake Morzycko. The Słubia River is a 32 km-long right-side tributary to the Odra River (German Oder; one of the main rivers of Poland, discharging into the Baltic Sea; at this section of its course, the Odra River defines the state boundary). The Słubia River originates from Lake Białęgi. It flows initially to the northwest, toward the lakes Narost and Mierno, and subsequently turns to the southwest, flowing through Lake Witnickie Małe, Lake Witnickie Wielkie, Lake Witnickie (the three hereafter referred to collectively as Witnica Lakes), Lake Morzycko, and Lake Słubie. In the terminal part of its course, over a distance of about 4.5 km between Stare Łysogórki and Siekierki, the Słubia River again flows toward the northwest, parallel to the Odra River, within the eastern part of the latter river’s valley floor. The confluence of the Odra River with the Słubia River is located in Siekierki, at an elevation of just 4.7 m a.s.l. Along the course of the river (following the Map of the Hydrographic Division of Poland), eight subcatchments have been distinguished within the Słubia River drainage area—as labeled in Figure 1: four catchments of the individual river segments (#1, #3, #5, #8), one tributary catchment Channel “A” (#6), and three direct lake catchment areas for Lake Narost (#2), Lake Morzycko (#4), and Lake Słubie (#7).

Both the Słubia River valley and the catchment area belong to the young river network system located within a landscape inherited from the last deglaciation, comprising a series of kettles, presently occupied by lakes, and alternating erosional and aggradational segments [52]. The Słubia catchment is located in the southwestern part of Myślibórz Lakeland [53]. It features one of the highest lake densities in Poland. The lakes arose from the erosional activity of the ice sheet and glacial waters, melting of dead ice lumps, or uneven accumulation of glacial deposits [42,54]. A 2021 physiographic analysis of the Słubia catchment indicated that its average terrain slope equals 3.08°, and maximum slope angle equals 36.65°, thus providing favorable conditions for rapid surface runoff of rain and meltwaters, and contributing to diminished infiltration and evaporation [36].

The Słubia River catchment is located at the junction of several geomorphological macroforms associated with the ice sheet and meltwater activity from the period of the last 16,000 years [55]. The dominant terrain forms of the Myślibórz Lakeland are flat and undulating morainic plateaux; the Odra River valley is a minor form with respect to the occupied surface area [41]. Compared to other mesoregions of Western Pomerania, Myślibórz Lakeland displays a relatively high percentage of area occupied by outwash plains, postglacial furrows, end moraines, and plateaux with a high number of kettle holes (Figure 2A). Some of them host mires, others are occupied by flow-through lakes [56,57]. Valley floors adjacent to the lakes, and those located along the upper course of the Słubia River in the northeastern part of the catchment (from Lake Białęgi to Lake Morzycko), are covered with forests, as is the southwestern, lower part of the catchment (Figure 2B). The morainic plateaux are deforested and farmed. Areas labeled as ‘built-up areas’ on the map are associated with towns and settlements highlighted in Figure 1. Meadows and pastures are associated with the Odra River valley (Figure 2B).

In Myślibórz Lakeland, hydrogenous areas are common and differ with respect to the occupied surface area, geology, biotope diversity, and land use, which is typical for a young glacial landscape [44,58]. The spatial variability of plant cover of aquatic areas and wetlands indicates that the discussed part of Myślibórz Lakeland is dominated by wet meadows of the order Molinietalia and cane associations of the order Phragmitetalia [59]. The largest and deepest basin located within the discussed part of the Lakeland is Lake Morzycko, with a maximum depth of about 60 m. The bottom of the lake is a crypto-depression, infilled with limnic deposits of variable lithology and chemical composition [36]. Despite the high ecological significance, the aquatic habitats of the study area, predominantly kettle holes, are undergoing strong transformations associated with human activity [41,44]. Further, the entire studied catchment is located within the zone of decreasing lake area, with the rate of decline over the past nearly 100 years equal to several percent [60].

2.2. GIS Data Sources and Database Construction

In the article, two spatial datasets were used, both publicly available online via the Geoportal platform [61], administered by the Polish Office of Geodesy and Cartography (pol. Główny Urząd Geodezji i Kartografii—GUGiK, Warsaw, Poland):

• Digital Elevation Model resulting from airborne laser scanning. The model originated from the ISOK project (pol. Informatyczny System Osłony Kraju przed nadzwyczajnymi zagrożeniami—IT System of State Protection against natural hazards). It is based on elevation data from the ISOK project Standard I, where the measurement point density equaled 4–6 pts per m², and measurement accuracy equaled 0.2 m. For our study area, the data were generated in 2014. The spatial resolution of this dataset equals 1 × 1 m.
• Aerial orthophotomap made in 2023 using the Color Infrared (CIR) technique, with a spatial resolution of 0.25 × 0.25 m.

Additionally, the original dataset concerning morphological units and terrain cover was used (Figure 2), set up for the Adamek et al. [36] study. The database was prepared using QGIS 3.22 ‘Białowieża’ [62], along with using the modules of SAGA GIS and PCRaster, implemented in the software.

From the source DEM, three digital terrain models (DTMs) emphasizing the relief were generated: slope model, shaded relief model, and multi hill-shading model. The extent of a water body basin is visible as a flat area encompassing a depression. Its margin is visible in the model as a narrow terrain belt characterized by a high slope angle increase resulting from erosional and abrasional processes [36]. Individual basins visible in the model were marked on the polygonal vector layer and classified as representing one of four types (

Table 1. Object classification used in the article.

Object Type	Features	Appearance (Shaded Relief Model)
Kettle holes /Pot holes *	Small (<1 ha) and shallow depression, usually nearly circular or oval in outline; no visible hydrographic network (neither supply nor surface discharge); Basin occupied by shallow water bodies (permanent/periodic/episodic) or wetlands); Occurring in agriculture areas (mid-field) as well as forests (mid-forest), often in groups comprising multiple such objects next to one another.
Lakes	Natural aquatic basins with area exceeding 1 ha; The largest objects distinguished in this study; Variable object shape; the lake basin surrounded by an abrasional shore visible in the model is often considerably larger than the existing water body (if the latter still exists).
Ponds	Artificial, shallow water bodies; Variable surface area, polygonal outline (basin shores are near-linear); Often form complexes/clusters of densely-packed basins separated by levees.
Other reservoirs	Natural surface water bodies fitting in neither category, e.g., overbank basins and oxbow lakes; Variable surface area and outline (from circular to spindle-shaped, often arcuate).

* A synonymous name, often used in papers focusing on North American sites [63]. In this article, the term “kettle holes” is used consistently.

).

Subsequently, for each of the identified basins, the orthophotomap was visually inspected for the presence or absence of a water body [35]. Water bodies were marked on a separate vector layer. The extent of the water body may considerably differ from the extent of the basin visible in the terrain model (Figure 3). There are also cases when one basin is occupied by more than one water body. In case a water body is absent, the basin is often occupied by wetlands, wet meadows, or swamps, visually different from the adjacent areas on the orthophotomap (Figure 3). In the ALS ISOK model, water bodies are also indirectly visible as surfaces characterized by very low density of empirical points: the beam of the infrared laser used for the ISOK project is strongly absorbed by the water column, which gives the water bodies low, randomly distributed values of slope / shading parameters, or a triangulation visible in the texture [39].

The analysis of the number and spatial distribution of water bodies was performed by dividing the catchment into subcatchments (Figure 1). For each subcatchment, we determined the number of objects representing individual object types (Table 1) and their surface area percentage relative to the subcatchment area.

Each identified object was assigned an object type (Table 1), and information on the presence or absence of a water body was expressed verbally as yes or no. The object surface area was subsequently computed using an attribute field calculator, following which layers from the vector database were superimposed on the objects, assigning each object with (a) location in the catchment, (b) in the morphological unit, and (c) land cover in the adjacent area (Figure 3). Based on the DEM input sampling, the mean topographic elevation value was also determined for individual basins.

Morphometric impacts on water body functioning were investigated using one of the topographic wetness indices (TWIs). These are quantitative spatial measures enabling inferences on the occurrence of field conditions conducive to moisture concentrating [63,64,65]. In this study, the SAGA WI (SWI) was applied, which is a modification of TWI that limits the influence of elevation differences in relatively flat areas [66]. SWI was computed based on the source DEM that underwent downsampling to 5 × 5 m prior to the analysis [65], using the SAGA GIS algorithm, available from the QGIS package interface. SWI is a natural logarithm from a quotient of a specific local catchment area (SCAm), a tangent function of the local slope angle (tanβ) [66]:

$$\mathrm{SWI} = \ln \left({\displaystyle \frac{SCAm}{tan\beta }}\right)$$

(1)

As a subsequent step, for individual objects occupied by water bodies, the Ohle index was calculated as a coefficient of catchment impact exerted on the water body. It is defined as the relationship between the water body surface area and the catchment surface area [15]:

$$\mathrm{Ohle} \mathrm{index} = {\displaystyle \frac{water reservoir catchment surface area}{water body area}}$$

(2)

To calculate the Ohle index, the source DEM was first preprocessed by downsampling to 5 × 5 m, and subsequently subject to sinking and filling depressions [67]. Generating the catchment extent was the final operation. The distribution of the Ohle index values was analyzed assuming the following ranges: <10, 10–40, 40–150, and >150, as proposed by Bajkiewicz-Grabowska [7].

2.3. Field Work and Laboratory Analyses

In order to determine the spatial variability in sediment lithostratigraphy, 40 geological sections were examined (5 for each subcatchment), with a total thickness slightly exceeding 221 m. This study exclusively targeted drainageless sedimentary basins representing kettle holes, as they guaranteed the recovery of a continuous sequence of Holocene biogenous and mineral sediments. A sediment core was taken using an Instorf corer from the central part of each kettle hole [68]. Core collection was performed with a 50 cm long, 5 cm wide corer in a manner enabling the recovery of undisturbed and uncontaminated sediment. Visual sediment descriptions prepared in the field also included the degree of autochthonous organic matter decomposition. According to the key by Tobolski [68], this parameter was determined following a percent scale for a small lump of fresh peat, immediately following its collection from the deposit.

The aim of this work did not require a detailed geological study leading to the description of a complete sequence of sediment lithology. The present article does, however, present the results of the organic matter, calcium carbonate, and mineral matter examination for the surface sediment layer, i.e., 10 samples from each section, involving a division into particular sediment types. Despite the rather low sampling resolution (1 sample per 2 cm of core), the results presented herein are the first attempt at determining the spatial relationships in present-day sedimentary processes within basins that are especially important for the denudation balance of the young glacial landscape of Myślibórz Lakeland.

The laboratory analyses involved the determination of percent organic and mineral matter content in sediment samples dried at 105 °C. Both sediment components were determined by means of loss on ignition performed at 550 °C [69]. The resultant ash content is indicative of the mineral matter content and is the reverse of loss on ignition. CaCO₃ content was determined in the remaining part of the sediment using a Scheibler apparatus and 10% HCl [70]. Notably, the protocol described above was also used in other paleolimnological studies carried out in Western Pomerania, including Podlasińska [71], Korzeniowski et al. [29], and Okupny et al. [31], thus enabling intercomparable results.

3. Results

A total of 931 basins have been identified within the Słubia River catchment. Of these, 301 objects are currently occupied by water bodies, the remaining 630 representing depressions that are either dry or occupied by wetlands. Kettle holes represent 78% of all objects (728), with lakes ≥ 1 ha comprising 13% (118 objects). Ponds and other water reservoirs constitute the remaining 9% of objects, 37 and 47, respectively.

The highest number of objects is found in the northeastern part of the Słubia catchment (Figure 4). The upper course of the Słubia River down to Lake Morzycko (subcatchments #1, #2, and #3) comprises 2/3 of all basins—597 objects. Within this area, kettle holes constitute 90% of objects, with the remaining 10% represented by lakes. The segment of the Słubia River between Lake Narost and Lake Morzycko (subcatchments #2 and #3) is characterized by a >12 per cent ratio of the identified basins relative to the subcatchment surface area.

Water bodies have the highest share in the specific catchment area of Lake Morzycko (Figure 4, subcatchment #4), whose basin occupies nearly 33% of the discussed subcatchment. In addition, the discussed area comprises a further five smaller lakes and four ponds. Kettle holes, whose number equals 43, represent > 75% of the objects.

In the Słubia River catchment between Lake Morzycko and Lake Słubie (Figure 4, subcatchment #5), there is a high percentage of ponds. Fifteen ponds and the same number of kettle holes were identified in this area.

The “A” channel catchment, located in the northwestern part of the Słubia River catchment (Figure 4, subcatchment #6), harbors 140 basins, which represent 15% of all objects in the study area. In addition to a high percentage of kettle holes (75%), there is also a significant share of lakes within this subcatchment, equal to 20% of objects. This is largely due to the occurrence of the cluster of lakes between the communes Stare Objezierze and Nowe Objezierze (Figure 1).

With respect to surface area, the catchment of Lake Słubie is the smallest subdivision within the Słubia River catchment (Figure 4, subcatchment #7). Only nine objects, representing all the basin types considered here, were identified within this subcatchment. The catchment of the lowermost course of the Słubia River has the lowest percentage of water body basin surface area (Figure 4, subcatchment #8). Among its 97 objects, other reservoirs reach the highest contribution to the surface area. These include mostly oxbow lakes present within the Odra River valley floor. Kettle holes represent <30% of the objects identified in this subcatchment.

3.1. The Ohle Index Distribution

Of the 301 objects occupied by water bodies within the Słubia River catchment, 2/3 (202) were kettle holes. Forty-two of these, located close to the northern and eastern borders of the catchment (Figure 5), displayed Ohle index values < 10. The remaining water body-occupied kettle holes were located in an area constrained by Lake Narost, Lake Mierno, Witnica Lakes, and Lake Morzycko, adjacent to the Słubia River and its valley. Their Ohle index values were considerably higher. The kettle hole cluster located in subcatchment #3 (Figure 5) is distinguished by especially high Ohle index values.

Among the 52 lakes, six objects displayed an Ohle index value above 150. These were lakes that the Słubia River flows through: Słubie, Morzycko, Witnickie, Witnickie Wielkie, Witnickie Małe, and Mierno. The lakes located within the upper reaches of the Słubia River catchment (Narost, Białęgi), located outside of the immediate vicinity of the Słubia River, display lower values of the Ohle index, which decreases with increasing distance from the river, and increasing proximity to watershed areas (Figure 5).

Ponds (27) have very low values of the index. The extents of their catchments were often generated along the axes of levees separating individual ponds in groups of these objects (Figure 5). Other reservoirs (18) are located between the Odra and Słubia riverbeds, and therefore their catchments usually display low Ohle index values (<10).

3.2. Kettle Holes

Kettle holes are the most common object type identified in the present study. Their spatial distribution within the Słubia River catchment is heterogeneous: there is a high concentration of kettle holes in the northern and northeastern, upper parts of the catchment, while only single such objects occur in the lower, southwestern part of the catchment (Figure 6A). The map also shows an additional spatial feature: a relatively high number of kettle holes are located close to the boundaries of the catchment or of its subdivisions (adjacent to watersheds). Nearly 505 objects were located on farmlands, 201 in forested areas, and 12 and 10, respectively, in green open spaces and in built-up areas. Two hundred two kettle holes (27.7%) are presently occupied by water bodies; 62% of these function on agricultural lands, 47% in forests, and the remaining 1% in green spaces and built-up areas.

Kettle holes are dominated by small objects: 78% of these occupy an area < 0.3 ha (Figure 6B). The average basin surface area equals 0.19 ha (median 0.11 ha), and the surface area distribution is clearly skewed toward low values. The basin size is not related to the presence or absence of a water body: in each interval, the percentage of water body-occupied objects ranges from 25.3 to 28.1%.

With respect to topography, kettle holes are located at the highest elevations compared to all the studied water bodies: 706 objects are located above 50 m a.s.l., with only a single object located below 20 m a.s.l. (Figure 6C). The percentage of water body-occupied objects increases with elevation: they are located mostly in the northeastern, upper part of the catchment (Figure 6A).

Five hundred seventy-five objects are located within morphological units associated with morainic plateaux (Figure 2), among which Karczewski [55] distinguished a class based on the presence of numerous kettle holes (Figure 6D). The slightly lower number of kettle holes in the class “morainic plateaux with numerous kettle holes” compared to “flat and undulating morainic plateaux” is due to a considerably smaller area occupied by the former morphological unit in the catchment—despite the higher percentage of water body-occupied objects. Apart from morainic plateaux, 75 kettle holes are located on the Chojna subphase outwash plain, and 78 objects are located within the remaining morphological units of the catchment.

The SWI shows a normal distribution, with the highest share of objects displaying medium values for the drainage basin, between 7.01 and 8.00 SWI (Figure 6E). The number of identified kettle holes increases slightly at SWI values > 9.01, including also water body-occupied objects.

3.3. Lakes

Footprints of 118 lake basins were identified in the Słubia River catchment, 56 of these presently occupied by water reservoirs (Figure 7). Except for the distalmost subcatchments (#8 in the southwest and #1 in the east), the lakes are rather non-uniformly distributed in the remaining subdivisions of the Słubia catchment, both along its borders and in the center. Most basins were located in agricultural areas and forests. Single objects were assigned to the remaining land cover classes. The basins occupying the largest areas (lakes Morzycko, Narost, and Białęgi) were treated as complex objects, as their basins are large enough to come in contact with various kinds of land cover, and thus they cannot be assigned to a single land cover class.

Lake basins in the Słubia River catchment are mostly small objects—more than half occupy an area of <2.5 ha (median 2.41 ha). Three lake basins occupied areas exceeding 100 ha: the basins of lakes Morzycko, Narost, and the basin harboring Witnica Lakes. The percentage of water body-occupied objects increases with increasing object area. Among objects larger than 20 ha, each basin is occupied by at least one water body (Figure 7B).

More than 100 objects were located at topographic elevations > 40 m a.s.l. (Figure 7C). The percentage of water body-occupied objects increases with increasing topographic elevation as well. The SWI distribution for lake basins is skewed toward higher values (9.01–10.00), which is also associated with the increase of water body-occupied object percentage (Figure 7E).

Seventy-two lake basins were located on flat and undulating morainic plateaux along with kettle holes (Figure 7D). Sixteen objects were located within the Chojna subphase outwash plain. The same number of lake basins were identified within the valley and the lake channel bottoms. These particular objects were also the largest with respect to surface area. The fewest lake basins were located close to the end morains—such objects occupied small areas and, at present, are devoid of water bodies.

3.4. Ponds

As an exception among all the object types distinguished in this study, the majority of the identified pond basins (27 out of 37) were occupied by water bodies. This is due, however, not to natural hydrologic or topographic tendencies, but to fish culturing in artificial objects. They usually form groups of objects located in close proximity to one another. One such large group is located west of Lake Morzycko and the city of Moryń; several smaller ones are located adjacent to the communes Objezierze and Stare Łysogórki (Figure 8). Ponds are also the only objects in this study that were mostly located proximally to urbanized areas (20 out of 37 objects).

Half of the pond basins occupy an area < 0.2 ha, and ten ponds occupy an area > 1 ha (Figure 8B). Most ponds are located at an elevation between 40 and 80 m a.s.l., in subcatchments #5 and #6 (Figure 8C). Two-thirds of pond basins are located on end morains and other marginal forms, the rest occurring on morainic plateaux and glacial valley floors (Figure 8D). The SWI for pond basins is skewed slightly toward higher values: the highest number of objects fall within the range from 8.01 to 9.00. With increasing SWI, there is a considerable increase in the percentage of water body-occupied objects. All objects displaying SWI values > 9.01 are occupied by water bodies (Figure 8E).

3.5. Other Reservoirs

This object type includes overbank basins and oxbow lakes. A total of 47 of these objects were identified and mapped, 18 of which are occupied by water bodies. Thirty-nine such objects are located next to the southwestern border of the study area, in the eastern part of the Odra River valley floor, and most of them probably represent former oxbow lakes and segments of the Odra River rather than the Słubia River (Figure 9). A group of such basins is located at the lowest elevation within the studied catchment (<20 m a.s.l.). The remaining eight basins are located 30–40 m higher, adjacent to Lake Słubie (Figure 9A,C).

Most objects of this type occupy small areas: the average area equals 0.33 ha, with 30 basins occupying < 0.1 ha (Figure 9B). All these objects are located within valley floors and subglacial furrows (Figure 9D).

3.6. Lithology and Key Geochemical Features of Bottom Sediments

For the interpretation of the contemporary processes responsible for the deposition of sediments in the studied lakes and kettle holes, we use the classification of Markowski [59]. Based on the relationship between the contributions of organic matter, calcium carbonate, and non-carbonate mineral matter in sediments infilling the basins of the study area, detritus gyttja, clayey gyttja, detritus–calcareous gyttja, and clayey–calcareous gyttja were distinguished (Figure 10). In some kettle holes, such sediments are covered by deluvial deposits composed of sandy silt with poor streaking and organic matter content as low as 2–14%. A distinctive feature of detritus gyttja is organic matter content in the order of 32–65% associated with low (usually < 10%) CaCO₃ content, or its total absence. In clayey–calcareous and detritus–calcareous gyttja, CaCO₃ content exceeds 20%, but, notably, maximum values close to 40–50% were found only in several samples from subcatchments #3, #4, and #8. In the case of clayey gyttja, this component was usually missing, but maximum values do not exceed 7% (Figure 11). Importantly, the classification scheme by Markowski [59], reproduced below, pertains to sediments laid down in lakes and mires. In the lithogeochemical classification reproduced in Figure 10, deluvial deposits would be classified as clayey gyttja.

The thickest deluvial deposits in kettle holes were documented in objects located within subcatchments #2, #3, and #6 (Figure 12). This part of the Słubia River catchment, in addition to numerous elevations, features a fine-scale relief, where Ohle index [15] values typically vary from 10 to 40. Regardless of the location, a typical sedimentary succession in the central part of a kettle hole in the central and northern part of the study area conforms to the following scheme: clay/organic mud/gyttja → peat, of variable decomposition degree → deluvial deposits/peaty aggraded mud. Analyzing the obtained data, it was concluded that organic matter percentages usually vary from 0.5 to 1.2% for clays, through 8–12% for deluvial deposits, 5–12% for organic muds, 40–60% for gyttjas, and up to 78–92% for peats (Figure 9). In drainageless sedimentary basins of the southwestern part of the Słubia River catchment (subcatchment #8), in turn, mostly a series of peats were documented (Figure 12). Apart from the variable degree of decomposition, these deposits are characterized by an average organic matter content as high as 94% (Figure 11).

4. Discussion

4.1. Assessment of the Object Inventory Method

The use of GIS-based methods in studies on the spatial distribution of hydrological network elements enables precise measurement of topographic and morphometric parameters of its constituent objects [40,67]. GIS also became a standard in modeling catchments and their associated processes: discharges, surges, and floods [67,72]. The method of DTM visual interpretation, relying on high-resolution input (e.g., DEM made using ALS technology) and placing emphasis on terrain relief, has already been used in hydrological studies aiming at river network detection. These involved the detection of the precise course of the network and riverbed shape, and thus enabled stream type determination, including a distinction between natural and anthropogenic streams such as channels and drainage ditches [73,74]. ALS ISOK data used in the article were also used previously to interpret the extent of raised bogs as positive terrain forms transformed by peat extraction [73,75]. In each of these cases, the course or the extent of objects displaying variable topographic parameters could be determined with high precision, including narrow and shallow streams or low-elevation surface forms. Importantly, the method is also independent of vegetation cover, which enables interpreting surface relief also in forested areas.

The remote sensing methods applied in this paper, however, do not enable the identification of the hydrological regime of a water body or wetland, or reconstructing the timing of their functioning, or—in the case of objects that are not presently occupied by water bodies—of their decline. To this end, in addition to traditional methods involving the installation of hydrometric devices, geochemical environmental proxies are becoming increasingly more popular in recent years [76].

The new inventory accomplished herein revealed a considerably higher number of water body basins, especially lakes, than previous studies based exclusively on topographic maps or aerial photographs [77]. Previously, 27 lakes were identified within the Słubia River catchment, displaying a high variability in surface area and a dominance of small objects. Twenty-one of these had surface areas < 10 ha and a total surface area of 76.2 ha, whereas for the remaining six lakes, the total surface area equaled 579 ha, i.e., 88.4% of the total area of all the mapped basins. An examination involving terrain relief interpretation indicated that there are more than twice as many (56) lakes, defined as natural basins having a surface area > 1 ha and being occupied by a water body. Such a high discrepancy could be due to incomplete spatial information reproduced on previous topographic maps, but also due to differences in classification. Individual water bodies examined in the present study often have significantly smaller surface areas relative to the extent of the basin, and thus, they could have been neglected or treated as kettle holes or objects of other types rather than lakes.

An estimated half of lakes and an even higher proportion (up to 2/3) of the remaining natural water bodies in Poland ceased to exist over the past several thousand years [24,77]. These estimates are consistent with our data from the Słubia River catchment, in which natural water body-occupied objects represent, respectively, 27.7% for kettle holes, 47.5% for lakes, and 38.3% for other water reservoirs (Figure 6, Figure 7, Figure 8 and Figure 9). In most cases, the water body decline resulted from the natural process of sediment infilling, overgrowing, and water regime changes occurring in the vicinity.

During visual interpretation of both spatial datasets, we made additional observations regarding human impact on the functioning and decline of the studied objects, especially in agricultural lands or in their close proximity, or in the case of built-up areas. Examples are shown in Figure 13.

Figure 13A shows the extent of the basin occupied by the so-called Witnica Lakes. The model interpretation shows that all three lakes actually occupy a single, large basin, divided in the central part by the railway embankment visible in the shaded relief model. A clear difference (photo-tone, texture) in vegetation present at the bottoms of the basins is also visible in the orthophotomap (determining the basin extent is also possible using the Color Infrared orthophotomap).

Close to Witnica (Figure 1), there is an additional, shallow lake basin, presently occupied by wetland, with a small water body in its southern part (Figure 13B). The northern part of the basin includes a trace of a probable islet on a former lake, but also traces of a network of drainage ditches and clay or peat pits. Similar footprints also occur in other mapped basins. For instance, traces of drainage works are present within the large basin in Figure 13D.

The Słubia River flows through a total of seven lakes (Figure 1). During this study, however, the footprint of one more flow-through lake was found, located between Lake Witnickie and Lake Morzycko (Figure 13C). The surface area of that basin equals 8.5 ha. The straight-line course of the Słubia riverbed crossing the axis of the former basin from the northeast to the southwest suggests that the lake may have been deliberately drained. In the commune of Przyjezierze, located to the west of the discussed point, there is a linear feature in the DTM suggesting this may have been the site of a stream driving a water wheel-powered facility (mill, sawmill) utilizing the water resources of that lake. The area of the basin of the former water body also differs with respect to photo-tone and vegetation texture from the surrounding area.

The datasets used in the present study are separated by nine years (the ALS model was made in 2014, the orthophotomap in 2023). Over this time, some of the kettle holes located in agricultural areas may not only have dried out, but their basins may have become incorporated into the adjacent arable fields (Figure 13D,E). Mid-field objects are often surrounded at some distance by unpaved roads used for tending the fields, visible both in the model and the orthophotomap (Figure 13D(I)). Two small kettle holes in the western part of Figure 13D and three in Figure 13E have been obliterated in the period separating the generation of our datasets and incorporated into the arable land. The former dirt roads, following a circular pattern, have also been obliterated and replaced by new roads following a straight-line pattern, parallel to the existing ones. A larger object visible in the eastern part of Figure 13E has also been incorporated into the adjacent fields in approximately 50%.

Based on the above observations, it can be summarized that previously water bodies may have been still more numerous, especially in agricultural areas. This would have been consistent with a relationship observed in historical and archeological studies, especially those employing visual interpretations of high-resolution models: traces of former objects are preserved more frequently in the relief of forested areas, while on farmlands, such forms become obliterated [39]. Although the precise extent of such objects would be difficult to reconstruct, in future studies, we attempt to locate former kettle holes using indirect topographic markers such as TWI [64,65], photo-tone, and vegetation structure visible in the orthophotomap, followed by the corroboration of the locality by applying geochemical and lithological methods [35,76].

4.2. Reasons for Spatial Variability in Morphometric Parameters and the Condition of Water Bodies

The study by Choiński [77] showed that number-wise, the Słubia River catchment is dominated by lakes occupying surface areas < 10 ha. That study inventoried 21 lakes with a total surface area of just 76.2 ha, while the remaining 6 lakes occupy a total surface area of 579 ha, i.e., 88.4% of the total area of all the mapped basins. This implied that the size structure of the analyzed lakes did not significantly differ from the structure of all lakes located in northwestern Poland, which is also dominated by lakes with surface areas smaller than 10 ha. About 60% of lakes in northwestern Poland occupy an area smaller than 10 ha, with only 6% of lakes being larger than 100 ha [77]. This means that within the borders of the studied catchment, the lakes are characterized by a similar level of resistance to the negative impact of the surroundings. Notably, in the case of the studied part of Myślibórz Lakeland, the higher resistance of water bodies to negative impacts of external factors, including especially human impact, can be attributed also to the prevalence of deep lakes over shallow ones. This concerns mostly postglacial lakes: nearly 1/3 of such objects reach a maximum depth ranging from 10 to 20 m. Water supply mode also displays a strong regional variability. Catchments of kettle holes, but also of some lakes, are small, as the Ohle index [15] values most frequently range from 10 to 40 (Figure 5). On morainic plateaux, surface water supply to water bodies is therefore low and restricted to the near-shore zone. Such a situation is recorded in sediment lithology and concentrations of selected geochemical components of bottom deposits in the remaining part of northwestern Poland [21,50,71,78].

Thus, spatial variability in the contribution of individual size groups of water bodies is due to the relief of the individual subcatchments, as well as due to their origin and the degree of development associated with natural overgrowing and human activity. The highest proportion of lakes was identified on flat and undulated morainic plateaux, in the 41–60 m a.s.l. elevation range (Figure 5). These are mostly flow-through lakes. In the upper parts of the catchment, however, vertical water exchange is dominant, while in the middle and lower parts, horizontal water exchange becomes gradually more important [78,79]. In turn, on flat and undulating morainic plateaux and on varied morainic plateaux, the percentage of water bodies existing in kettle holes was similar (Figure 6D); 280 and 270 objects, respectively, are located within each of these terrain macroforms. They occur most numerously in the elevation range of 61–80 m a.s.l. As such, kettle hole concentration areas occur predominantly in the morphologically highest part of Myślibórz Lakeland. Similar regularities in the distribution of drainageless sedimentary basins were documented, e.g., by Żurek [80] and Okupny et al. [21] for other physico-geographic regions of northern Poland. Further, the prevalence of drainageless water bodies in watershed zones of the Słubia River catchment points to spatial variability in deglaciation modes, and the key part played by the surface geology in shaping the permanent or periodic occurrence of water in terrain depressions. In the case of the Polish Lowland, it is the uneven glacial accumulation, melting of dead ice lumps, and glacio-tectonic processes that are jointly responsible for the fact that the plateaux composed mostly of clay are interspersed with drainageless terrain depressions [81,82,83]. In turn, the permanent or periodic occurrence of water in the discussed depressions in the Słubia River catchment is dependent on the preservation of the form itself and on the type of sediments covering its floor.

Land use is another important factor differentiating the preservation and supply of matter into water bodies in the Słubia River catchment. The distribution of the identified basins with respect to land use is not homogeneous, with prevalent arable land (584 cases, 62.8%); 255 objects (27.4%) are located adjacent to forests, and the remaining 91 objects (9.8%) are located next to mostly pastures and meadows (55 objects). These results, however, point to a pronounced variability in individual subcatchments. In subcatchment #8, comprising the lower course of the Słubia River, over 80% of water bodies are located adjacent to forests, while in subcatchments #5 and #6, both with an overall agricultural character, their percentage is negligible (3%). Forest areas occur in the surroundings of water bodies located in the middle and upper courses of the Słubia River valley as well. Catchments with agriculture as the dominant land use form prevail in the case of water bodies located to the east of Lake Narost, northwest and south of the so-called Witnica Lakes, west of Lake Morzycko, and in the vicinity of Nowe Objezierze (respectively, subcatchments #2, #3, #5, and #6). These regions feature fertile cambisols developed on clays and clayey sands [55]. Following centuries of exploitation, these soils are locally leached [84]. Notably, the processes of soil erosion and mineral matter supply to basins located in the Słubia catchment also depend on the plateau slope and river valley orientation. As the Słubia River valley is predominantly longitudinally aligned (Figure 1), water relations in the discussed catchment differ in relation to the north-facing slopes (north-facing slopes are more common in subcatchments #4 and #6, and south-facing slopes prevail in all the other subcatchments). This diversity, in conjunction with surface sediment lithology, is reflected in the spatial distribution of individual water erosion classes by Józefaciuk and Józefaciuk [85].

4.3. Reasons for Spatial Lithogeochemical Variability in Sediments Infilling Lakes and Kettle Holes

Among the conditions responsible for lithostratigraphic diversity in sediments laid down in water bodies of the temperate zone, Borówka [79] enumerates terrigenous, biogenous, and chemical sedimentation. The character and intensity of a given type of depositional process depend, among other factors, on surface relief, which in turn controls the potential for shaping the surface runoff. The low slope angle of plateau surfaces, responsible for the low degree of concentrated surface runoff, is especially important in this respect. This principle is obviously locally modified by the polygenetic character of river valleys, or the presence of complex river/lake systems, as corroborated by previous geomorphological studies conducted in the Słubia River catchment [36]. Consequently, the morphogenetic considerations expressed above are reflected in sediments laid down in water bodies located in the studied part of Myślibórz Lakeland, where allo- and autochthonous sediment accumulation processes have been taking place for more than 10,000 years.

Detritus gyttja is the most frequent type of sediment occurring in the biogenous accumulation basins of Myślibórz Lakeland [56,59]. In most lakes of Myślibórz Lakeland, the thickness of detritus gyttja ranges from 1.15 to 3 m [56,59,86,87], whereas in the studied kettle holes of the Słubia River catchment, detritus gyttja thickness ranges from 2 to 3.6 m. This type of sediment makes up as much as 49.4% of the total thickness of all aquatic sediments (Figure 11). Moreover, Figure 11 is an introduction to the occurrence of the particular biogenic sediments, which have remained after the kettle holes basin as a result of support with geomorphology at the Słubia River catchment. The diversity of geology and geomorphology of the individual zones determined the depth and type of biogenic sedimentation. Mineral matter percentage in such sediments, however, rarely exceeds 40% [29,36], and the fact that their occurrence is restricted mostly to shallower bays points to high biological productivity of these basins and to their high retention capabilities. A considerable proportion of lakes accumulate calcareous gyttja or lacustrine chalk, with CaCO₃ content > 80% (Figure 8). According to Markowski [59], such sediments display a strong regionalization, as in this part of Western Pomerania, they are found predominantly on morainic plateaux. The study by Borówka [78], however, indicates that deposition of such sediments depends not only on surface and intrasoil leaching in the catchment, but also on atmospheric precipitation. In a thick, intermorainic level, water supply occurs from the northeast, which is why the watershed zone of three rivers: Słubia, Rurzyca, and Tywa may be considered as the source zone of CaCO₃ deposited in the studied limnic sediments. Deposits making up the discussed zone contain, on average, 8–24% of CaCO₃ [88,89]. Locally, however, marked decalcification occurs up to a depth of 2 m. The maximum thickness of calcareous gyttja in some of the basins slightly exceeds 5 m [56,59,86,87].

Clayey gyttja displays the lowest frequency in the study area. On average, it underlies the bottoms of slightly more than 20% of lakes [23,59]. The thickness of this type of sediment in biogenic accumulation basins inventoried by Jasnowski [56], Markowski [59], and Maślankowska [86] usually ranges from 0.15 to 0.7 m. Clayey gyttja makes up 23% of the total thickness of all limnic sediments (Figure 11). In the studied kettle holes, these values are very similar, as the thickness of clayey gyttja does not drop below 0.2 and does not exceed 0.6 m. The origin of this type of sediment is linked to the sedimentation of the most fine-grained mineral matter supplied to sedimentary basins from slope processes, deflation, and lake shore abrasion. For instance, the percentage of allochthonous mineral matter in clayey gyttja from Lake Morzycko exceeds 70%, and the first place in the migration series is occupied by Fe, Mn, and Mg [36]. This may be caused by reducing conditions prevailing in the deepest parts of the lake, making organic matter decomposition more difficult and thus responsible for sulfide enrichment. Further, the sediments of the lakes located within the Słubia River catchment usually display low concentrations of trace metals, and are close to the values of the local geochemical background [36,90,91,92].

A comparison against the lithostratigraphic record of changes in water relations by Żurek [93] indicates that the basins selected here for detailed investigation should be considered drainagaless depressions supplied topogenically. Such geoecosystems develop where the floor of a given concave form intersects the groundwater level, which receives an additional surface water supply characterized by short transport duration [80,94,95]. Initially, the depressions fed proximally were infilled with clay, organic mud, and gyttja. With respect to geochemistry, however, samples taken from organic muds classify the host sediment as clayey gyttja (Figure 8). With time, limnic sediments were covered with low and transitional peat, which are also distinguished by organic matter content (Figure 8). Notably, in the studied kettle hole basins from the Słubia River catchment, a transition to an ombrogenic water regime and high peat sedimentation was not documented to date. This is due to an intensification of farming-related erosion and the deposition of deluvial sediments, which, in many cases, have accelerated basin terrestrialization. According to Jania and Zwoliński [96], the most intensive processes of water erosion of soil in the young glacial landscape belt occur on slopes longer than 100 m and having an angle ranging from 4 to 6°. In the case of the Słubia River catchment, such conditions for denudational processes occur mostly in subcatchments #2, #3, and #6. Sienkiewicz [97] also emphasizes that the highest rate of slope degradation, and the highest thicknesses of aggraded muds in northern Poland occur within those catchments, in which positive terrain form with relative elevations from 1 to 3 m occur next to 1–2 m deep drainageless depressions. Further, the common occurrence of weakly permeable and impermeable deposits in the study area is conducive to stagnation of percolated water or intrasoil runoff, which ultimately leads to a change in slope morphology, from convexo-concave to convex. A consequence of the denudational processes mentioned above in the catchments of the smallest drainageless depressions, also in other parts of the European Lowland, is that they frequently become totally infilled with mineral deposits, and thus the water bodies occupying their floors become aggraded [21,84,98,99,100].

What follows is that a characteristic feature of sediments infilling kettle holes in the agriculturally utilized part of the Słubia River catchment is medium to strong mud content. The thickness of such deposits usually oscillates below 60 cm, but it may reach up to 120 cm. Notably, these are up to four times thicker than documented by Jasnowski [56] for other fossil biogenous accumulation basins within Myślibórz Lakeland. The content of allochthonous mineral matter, on the other hand, varies in a rather broad range, from 40 to 77%. In nearly 2/3 of the samples, however, its content does not fall below 55%. Compared to other regions of Western Pomerania, these values are considerably higher. For instance, the contents of mineral matter in the upper part of the bottom deposits in Bytów Lakeland, the Parsęta River catchment, or in Stobno (close to Szczecin), do not exceed 50%, with a maximum thickness of the deluvial sequence in the order of 30–40 cm [21,50,101].

In turn, a distinctive feature of sediment profiles recovered from drainageless but shallower sedimentary basins functioning within forest landscapes of the Słubia River catchment is the higher thickness of organic muds and peats. Maximum thicknesses of such sediments reach nearly 4 m, and organic matter content in their uppermost parts rarely falls below 85%. The type and features of sediments deposited in these basins are probably due to the supply of high-amplitude variability in intrasoil waters. As a result of a long-term lowering of the water level, there was an increase in the degree of organic material decomposition, and distinctive silt-rich peat layers were deposited. Accumulation of such sediments was previously documented for other parts of the Polish Lowland [93,102]. In turn, one of the highest CaCO₃ content values (always above 20%) in limnic deposits of the southwestern and western part of the Słubia River catchment is due to the supply of water-borne acidic calcium carbonate, which was intensely leached from morainic plateaux located to the northeast. Notably, maximum values of this component are usually twice as low as in limnic deposits from basins located in the nearby Wielkopolska Lowland or Lubuskie Lakeland [103,104].

5. Conclusions

The inventory of basins located within the Słubia River catchment revealed the presence of 301 water body-occupied basins, and a further 630 basins that are presently devoid of water bodies. Such a proportion is considerably higher than reported in existing studies on young glacial areas of Central Europe, which estimate that about 50% of the original number of basins have disappeared. The following relationships have been observed:

• Number-wise, kettle holes occupying small surface areas are the dominant basin type in the studied catchment. These also yielded the highest rate of water body decline (72.3% of objects devoid of water bodies). Kettle holes were frequently located close to watershed zones and displayed low values of indices for moisture concentration (SWI) and basin resistance (Ohle index).
• A large number of lakes were identified (188), with their preservation degree estimated at 44%. We also observed a considerable difference between the size of the present-day water body and the extent of the basin visible in the terrain model.
• Basin decline and desiccation are due not only to changing climatic and hydrologic conditions, but also to human impact. This is especially striking in the case of kettle holes located on arable land, despite the significant part they play in ecology and landscape, and despite the benefits their presence brings to the adjacent farmlands.
• Land cover structure in the catchment is a significant factor shaping the character of bottom sediments and the supply of matter to the studied water bodies. The most favorable conditions for highly organic sediment accumulation occur in basins located in the forests, while the least favorable conditions are observed where arable land prevails. Consequently, the organic matter content in sediments of the selected water bodies ranges from low to medium (20–75%), correlative to a medium to highly silt-rich sediment group. From the perspective of contemporary sediment-forming processes in the documented sedimentary basins, the most favorable conditions exist in the catchments of the upper and medium course of the Słubia River valley. In both cases, the future functioning of the water bodies will depend on hydroclimatic conditions, including fluctuations of the groundwater level.
• Despite the strong human impact on water bodies of the Słubia River catchment, limnic sediments display a low enrichment in most elements relative to the local geochemical background, and a >10% higher organic matter content compared to kettle holes.