In many Western European countries, naturally flowing rivers have been channelized in the 20th century to suit agricultural practices [1
]. However, this has caused several unforeseen problems, such as hydrological flashy responses resulting in increased flooding downstream and desiccation of naturally wet ecosystems [3
]. Therefore, since the 1990s, nature and water management policies have been in place, attempting to re-create natural rivers systems [7
]. There are many examples of rehabilitation programs that include re-meandering projects, often in small streams (e.g., [6
]). Re-meandering of these previously channelized streams is generally based on the assumption that meandering is a natural channel-forming process and therefore, the channel planform characteristics of the historical situation before channelization is mimicked [4
]. However, despite the sinuous planform, the extent to which meandering processes play a natural role is often minimal in systems characterized by a low valley slope and relatively low energy levels [10
]. Although such low-energy streams should be regarded as “non-dynamic”, they often show lateral migration to some degree [12
]. However, in general, meandering processes in low-energy rivers have largely been overlooked thus far. Therefore, there is a need for describing factors that contribute to lateral migration in low-energy rivers.
Processes and forms of individual river channels are influenced by a complex set of factors, such as flow energy (valley gradient, bankfull discharge, frequency of high discharge events), sediment load, sediment size, floodplain heterogeneity, and bank cohesion (composition and vegetation) (e.g., [9
]). Human interventions in hydrological systems also influence laterally moving behavior [22
]. Although it is commonly perceived that in temperate lowland rivers, sinuosity tends to increase if the valley slope is steeper, and there are some examples of rivers where sinuosity and valley slope are positively correlated (e.g., [23
]), specific relations between valley slope and sinuosity cannot be generalized for rivers with mean annual discharge of more than 10 m3
]. In addition, it is also often perceived that in cases with a higher sinuosity, a higher lateral migration can be expected [24
]. Sinuosity of a stream is often not a stable value, but is temporally and spatially variable. For example, vegetation influences, extreme weather conditions, clustering of meander cut-offs, or human influences can have an impact on the sinuosity [10
Rivers can be classified based on river channel processes or on their resulting planform [19
]. Nanson and Croke [12
] created a genetic classification, in which they distinguished river systems into high-energy non-cohesive floodplains, medium-energy non-cohesive floodplains, and low-energy cohesive floodplains. According to this classification, banks in meandering systems are laterally active in a floodplain environment composed of gravels, sands, and silts, whereas in irregular sinuous systems, the channels are stable in sedimentary systems of abundant silts and clays with organic sediments. The latter show no lateral migration, as the discharge power is below a certain threshold to erode bank material. This thus implies that when no erosional processes are taking place, a channel cannot be defined as a meandering stream since no migration will take place. Nanson and Knighton [25
] proposed a single-channel river classification based on planform, where laterally active regularly sinuous channels show meandering and braided planforms and laterally inactive channels consist of what they termed ’straight’ and ‘irregular sinuous’ planforms. Gurnell et al. [24
] make a distinction in rivers based on sinuosity P
, in which they define ‘straight’ (P
< 1.05), ‘sinuous’ (1.05 < P
< 1.5), and ‘meandering’ (P
> 1.5) planforms.
Since meandering is defined by the level lateral activity or the planform, or both, in this paper, the focus is on the lateral migration of the river channel rather than on the definition of whether a river is ‘meandering’ or not.
To classify the extent of lateral activity in single-thread rivers, Van den Berg [17
] developed an empirical stability diagram based on data from a large number of rivers. The diagram was subsequently refined by Bledsoe and Watson [26
], Makaske et al. [27
], and Kleinhans and Van den Berg [10
]. Classes include highly braided rivers, moderately braided and meandering rivers with scrolls and chutes, meandering rivers with scrolls, and laterally immobile rivers without bars. The discrimination borders between classes should be interpreted as lower limits rather than hard thresholds [10
]. Following the empirical diagram, the lateral stability can mainly be described by three variables: The median bed grain size, the valley slope, and the bankfull or mean annual discharge [10
Although the separation between actively meandering and laterally inactive irregular sinuous rivers has been studied extensively, it remains unclear what factors influence lateral migration in low-energy rivers, as the empirical stability diagram has not been validated for these cases. Low-energy rivers in this case are defined by a mean annual discharge of less than 10 m3
] or by a specific stream power of less than 10 W/m2
]. In general, there has been great interest in relatively highly lateral instable systems, where rivers with relatively stable planforms due to strong resistance to erosion and/or low specific stream power have received considerably less attention [29
Eaton et al. [30
] demonstrate that predicting channel patterns is a three-variable problem, based not only on slope and (dimensionless) discharge, but also on bank strength. Erosive banks and fresh bars are observed even in small streams. The erosivity or bank cohesion may depend on different factors, such as soil type, sediment deposition structure, and riparian vegetation [31
]. In cases where banks are stable, there are still some very slow rates of lateral migration [12
]. Generally speaking, banks consisting of loose material, like (aeolian) sand, are more erodible than banks consisting of more cohesive materials like silt, clay, or peat [9
]. Bank stability also depends on the amount of organic matter and the amount of plant roots which consolidate bank material [32
]. However, the composition of valley fills and its resistance also act as a factor in determining lateral migration in low-energy rivers, but is relatively under-exposed thus far [9
]. In addition, research has focused on lateral activity in either alluvial or, to a lesser extent, in peatland environments (e.g., [34
]), but not in valley systems with heterogeneous valley fills. Although it is acknowledged that the erosional resistance of spatially heterogeneous floodplains affects planform evolution of meandering rivers, the relative importance of this effect is limited [19
]. Special attention needs to be paid to floodplain heterogeneity, as it may cause local variation in lateral channel migration [29
Therefore, there is a need for case studies that describe which factors contribute to lateral migration in low-energy rivers in heterogeneous valley fills. This is important for river management, since it may determine the way river rehabilitation projects should be designed. Therefore, this paper aims to (1) quantify historical and contemporary lateral migration in a low-energy irregular sinuous river and (2) assess the factors that influence lateral channel migration in a heterogeneous valley fill. The river is positioned in the context of the stability diagram of Kleinhans and Van den Berg [10
] and in the river planform classification system as proposed by Gurnell et al. [24
]. The paper then focuses on the spatial variability of lateral migration rates and relates this to local conditions of valley and channel slope, sinuosity, and valley fill composition. The research involves a detailed case study of the Drentsche Aa mixed alluvial and peatland lowland river situated in the Northern Netherlands. The outcomes are compared to the presumed naturalness of the river, and findings are discussed in the light of river rehabilitation projects.
2. Regional Setting
The Drentsche Aa (Figure 1
) is a semi-natural, lowland river system in the north of the province of Drenthe in the Netherlands. Due to its high natural, cultural, and geo-heritage values, the area was designated as one of the 20 Dutch National Landscapes [5
] and is part of the UNESCO Global Geopark De Hondsrug. The total catchment size is around 300 km2
with a length of around 16 km. The area can be characterized as a relatively flat area, ranging from 22 m AMSL near the source area in the south to 0.6 m AMSL at the catchment outlet in the north [5
The shallow subsurface consists of subglacially deposited till of Saalian origin, in which periglacial snow meltwater runoff has eroded shallow valleys with a NNW–SSE orientation [41
]. The area was covered with undulating aeolian cover sands and periglacial fluvial deposits of Weichselian origin about 0.5–2 m in thickness, which form the Pleistocene basis of the stream valleys [42
]. During the late Weichselian and Holocene, peat formation and local alluvial deposits filled the valleys [42
]. Since the 1600s, the peat has been subsiding due to surface drainage by people. The current-day peat thickness is usually limited to about 0.5–2 m in thickness, but can locally reach a thickness of around 7 m [45
]. In some areas, aeolian dunes of late Pleistocene age are embedded or partly covered by Holocene deposits. This means that in some locations, the bottom of the streambed is situated in the Pleistocene aeolian or fluvial deposits, and in others, the stream is flowing through a peaty environment. The composition of the backswamps in the floodplain is variable and mainly consists of peat with local alluvial material due to the nature of obliquely aggrading streams during the Holocene [11
]. Natural levees are present along the active river channel, although they may have been slightly raised in places due to local small-scale manual deepening of the channel as drainage improvement by farmers in the past. Abandoned channels or oxbow lakes are almost entirely absent [11
]. The higher areas mainly consist of infiltration areas, whereas in the valleys, groundwater seepage reaches the surface [40
]. Figure 1
shows the geomorphological map [41
] and the current-day river network of the study area.
The moderate climate has an annual average rainfall of about 750 mm, evenly distributed over the year [5
], which has been slowly increasing due to climate change [48
]. Typical channel widths are 2–3 m in upstream sections to around 10–15 m near the catchment outlet. The current-day mean annual river runoff near Schipborg is around 1.8 m3
/s, which has a typical stream power of around 0.5 W/m. The bankfull discharge is 6.8 m3
/s or 1.6 W/m2
], which classifies the stream as a low-energy river. Typical bankfull discharge levels are lower in the tributaries, with around 1.5 m3
/s in the Gastersche Diep and 2.9 m3
/s in the Loonerdiep (Figure 1
). The stream is free-flowing and is not influenced by sea level rise or tidal fluctuations. The stream consists of a number of tributaries or reaches (locally called ‘diep’; Figure 1
). The stream and its valley can be described as ‘unconfined single thread’ according to the Gurnell et al. [24
] classification scheme.
Agriculture is the main land use in the area (50%), of which around half is pasture. Another 35% consists of natural areas, both as grasslands on the valley floors, as well as pine and deciduous forests (20%) in the higher, drier areas. During the last century, European and national agricultural policies focusing on food security transformed farming practices and countryside management in large parts of the Netherlands. However, the catchment of the Drentsche Aa was relatively untouched by the large-scale land consolidations [5
]. Some short upstream tributaries were channelized in the 1960s and 1970s, which has some influence on the hydrological conditions in these channels, but they are still generally considered as ‘semi-natural’ in a Western European context [5
]. The majority of the stream network still has its sinuous planform [42
]. The studied part of the network system is approximately 16 km in length and elevation difference from upstream to downstream is about 7 m.
In 1944, Kuenen [46
] investigated lateral migration in the Drentsche Aa by conducting a field survey and analyzing map data spanning a 20-year period. He concluded that the Drentsche Aa river is morphologically largely inactive and only shows activity on small stretches of the channel during short periods of time. Candel et al. [11
] have shown that lateral migration and the resulting planform appear to be inherited by the Pleistocene subsurface morphology and composition during the build-up of the valley fill during the Holocene. Due to oblique aggradation, lateral activity in sandy environments appears to be stronger than in peaty conditions, suggesting that local conditions are dominant in lateral migration. Their research was based on six lithological cross sections of the valley fill in the study area, showing the long-term processes in a limited number of places. This paper has a more contemporary focus and uses data from a multitude of locations along the river channel.
5. Conclusions, Discussion, and Recommendations
The planform of the Drentsche Aa river is spatially variable and, based on its sinuosity, it can be classified on both sides of the (irregular) sinuous and meandering boundary using the Gurnell et al. [24
] river planform classification system. With relatively low bankfull discharge levels (Qbf = 6.8 m3
/s; ωpv =1.6 W/m2
), the stream is regarded as a low-energy river. By plotting four river sites in the Kleinhans and Van den Berg [10
] empirical stability diagram, the river is positioned in the lowest field. Although the empirical stability diagram was not designed for rivers with such low bankfull discharges, the diagram was applied here as a frame of reference rather than a hard classification method, as placing the river in the diagram may also help to validate the diagram for low-energy rivers. According to the position in the diagram and using the Gurnell et al. [24
] classification, the Drentsche Aa river should be nearly immobile.
However, field observations reveal that the Drentsche Aa shows active signs of erosion and sedimentation processes, indicated by bare vertical cut-banks and fresh point bars in the channel. Moreover, the historical map analysis also shows proof of lateral migration, with average migration rates of 0.08 m/y over the last 85 years. Locations with observed active erosion in the field correspond to statistically significant higher lateral migration rates than locations where no or little active erosion has been observed, according to the historical map analysis. Our interpretation, therefore, is that historical lateral migration rates for the period 1924–2009 are representative for current-day conditions. This means that, although the river is classified as a low-energy stream, this does not mean it is a non-dynamic river. This confirms that the stability diagram is not applicable for this low-energy stream, since the diagram classifies the stream as ‘lateral immobile’. Energy levels (e.g., bankfull discharge and stream slope) and sediment bedload grain size alone, therefore, do not explain the lateral migration in the Drentsche Aa, or at least not in the way as they are related to each other in the current stability diagram.
We observe that lateral migration rates are spatially heterogeneous. Average local migration rates vary between 0 and 0.33 m/y. Some river sections show higher lateral migration rates than others based on CUSUM and LOESS analyses. The degree of lateral migration cannot be explained by valley slope, as the correlation between slope and lateral migration is very weak and statistically insignificant. Moreover, a higher sinuosity does not mean that the river is more actively migrating, since the correlation between lateral migration and sinuosity is also insignificant. This corresponds with the findings by, e.g., Kleinhans and Van den Berg [10
] and Gurnell et al. [24
]. The spatially heterogeneous nature of lateral migration therefore indicates that lateral activity is controlled by other local scale conditions.
We show that lateral migration of the low-energy river is best explained by valley fill deposits at landscape scale, as indicated by both geomorphological and soil maps. Lateral migration of river bends in peaty valley fill deposits were significantly lower than bends in aeolian sand deposits. Similarly, bends in peaty soil map units showed statistically significant lower lateral migration rates than sandy soil units. This indicates that sandy valley fills and valley sides show less resistance to erosion than organic-rich sediments, and are hence the most important factor in controlling lateral migration in the case study area. This corresponds with findings by several authors that bank composition and texture (e.g., [9
]) and organic matter content (e.g., [32
]) influence lateral migration rates. These observations add contemporary evidence to the oblique aggradation model based on several cross-sections in the same study area by Candel et al. [11
]. The stream tends to be more laterally active in sandy deposits, resulting in the stream adhering to the sandy valley sides or aeolian dunes embedded in the peat valley fill. We show that this process has also been active over the last 85 years.
As soil and geomorphological maps on a 1:50,000 scale were used, the influence of valley-scale heterogeneity on lateral migration was captured. Bank composition variability at the local scale is not covered in this analysis. Banks composed of deposits formed in local conditions, such as former stream beds or overbank deposits, are not represented in the maps. Detailed field data on composition and organic matter contents could provide more information. However, the process of lateral migration is self-destructive, meaning that former banks were eroded during the migration process and its composition cannot be established. Field campaigns are not useful for studying the lateral migration rates over the last 85 years, but if historical archived soil borings are available, they may prove useful to capture the plot-scale variability. Detailed determination of bank composition in combination with field measurements of lateral migration would improve the knowledge at the local scale.
In addition, in the case of typical Dutch low-energy river valleys, a more thorough understanding of direct and indirect human influences is necessary. We need to assess the extent to which human action in the past has influenced hydrological discharge levels at the catchment scale and channel cross sections at the local scale in order to quantify possible effects on lateral migration rates. In that way, it is possible to determine whether current-day migration rates can be regarded as natural reference levels, or whether they are to be seen (as part of) a human artefact. This would require an interdisciplinary approach, combining knowledge from geomorphological, historical hydrological, and cultural historical disciplines.
We can conclude that the heterogenic composition of the valley fill deposits is more dominant in determining lateral migration in the low-energy river than factors such as valley slope, specific energy, and bedload grain size. This explains the heterogenic character of migration rates along the river, as was already observed but not explained in this catchment by Kuenen [46
Extending these findings to low-energy lowland rivers in general, river rehabilitation plans should not only be based on the planform of the last situation before channelization, but also study earlier planforms and the composition of the valley fill to assess historical morphological processes that may reflect natural conditions for the specific water course. Dominant local conditions may have implications for how rehabilitation projects should be designed, as well as for the way the streams should be managed after the reconstruction of the channel. This will eventually lead to rehabilitation designs that truly reflect a more natural state of the river.