Long-Term Variability of Ice Phenomena in Selected Rivers of the Central Vistula River Catchment

Abstract

The phenomenon of surface freezing in lakes, rivers, and reservoirs, has been an essential part of Poland’s winter landscape for centuries. It plays critical ecological roles, such as regulating heat balance and influencing the state of biocenoses. Due to progressive climate warming, we have observed significant changes in ice cover duration, thickness, and timing in recent decades. Ice phenomena on rivers are temporary. They strongly depend on air temperature, which has recently been increasing worldwide. This paper analyzes the variability of ice phenomena formation in selected river profiles of the central Vistula River catchment, central Poland. The research period covers the years 1968–2016. The data come from the Institute of Meteorology and Water Management-State Research Institute (IMGW-PIB). The duration (including the dates of occurrence and disappearance of the phenomenon) and the frequency of occurrence of ice phenomena over the long-term were determined with particular attention to ice cover. The long-term occurrence of ice phenomena shows a decreasing trend (shorter duration, later onset dates) with a simultaneous increase in the average air temperature during the winter half of the hydrological year.

1. Introduction

Ice phenomena are typical of rivers in temperate climate zones. Ice on rivers in Poland is one of the constant factors influencing the variability of water levels and runoff magnitude. During the winter, a river’s hydrological conditions depend on various factors, including air temperature and ice cover [1]. The thermal and ice regimes of river streams are also shaped by groundwater inflow [2] and anthropogenic changes, such as land use and development in the river valley [3,4]. Ice phenomena on rivers occur periodically, and their formation is a complex process influenced by numerous hydrological and meteorological factors, as well as the morphological characteristics of the riverbed. Key factors determining the presence of ice include temperature and water dynamics, as heat transfer between water and air is much greater than between the water and the riverbed [5]. Ice phenomena vary in form and duration. Ice cover is often the shortest-lived among the different forms, with its formation encouraged by sustained sub-zero air temperatures [6]. One significant consequence of ice phenomena is the formation of ice jams, which can cause flooding not only on river terraces but also in residential areas, resulting in material damage [7]. Knowledge of ice characteristics is essential for the design of bridges, culverts, dams, sills, and other hydraulic structures in regions where ice phenomena occur. Accurate understanding of ice characteristics contributes to better assessment of the risk of ice jamming, planning of winter operations (e.g., icebreaking), and infrastructure protection [8]. Lowland areas are particularly vulnerable to the catastrophic effects of ice jams [9]. To mitigate flood risks, rivers are regulated, and interventions are made to reshape their beds, thereby reducing the duration of ice phenomena [4]. The occurrence of ice phenomena also has a substantial impact on the riverine ecosystem, affecting both plants and animals. Prolonged ice presence often forces animals to migrate, disrupting their life cycles [10]. Along the length of a single river, the course of ice phenomena can vary significantly between the upper, middle, and lower sections [11]. River freezing begins when air temperatures drop below 0 °C. Ice formation in the riverbed is initiated by suspended particles that act as nuclei for crystallizing ice molecules [12]. The mechanism of ice cover formation generally involves three phases: partial freezing of the river, complete ice cover formation, and eventual disappearance [5,13]. Morphometric characteristics also influence the formation and decay of ice, contributing to variability among different water bodies [14]. The cycle of freezing and thawing significantly affects numerous factors shaping the stability of riverbanks, usually contributing to a decrease in their cohesion and resistance to erosion. These processes can increase the inflow of sedimentary material into the riverbed by, among other things, uncovering fresh layers of sediment that are more susceptible to erosion by water, wind, and moving ice blocks [15]. At the same time, ice cover is one of the key indicators of winter temperature changes [6,16]. The presence of ice cover on rivers restricts inland navigation, weakens flood embankments and bridges, and adversely affects the region’s economic conditions [9]. A notable example is the 1982 flood in Włocławek, which caused extensive residential and industrial infrastructure destruction.

The intensity and variability of ice phenomena on Polish rivers have changed significantly over the years. In recent decades, increasing global warming has led to significant changes in the duration, thickness, and timing of ice cover. Studies conducted both in the north and south of the country indicate a shortening of the ice cover period and a shift in the timing of ice formation and melting [14,17,18,19]. Ice phenomena now tend to last for shorter periods, appear later in the season, and result in thinner ice cover—or, in extreme cases, no ice cover at all. Long-term analyses, such as those conducted on the Biebrza River located in northeastern Poland (1959–2023), show that the ice is melting earlier and earlier, by an average of about 0.38 days per year (~3.8 days per decade), confirming a statistically significant relationship with the increasing air temperature [20,21]. Analyzing the occurrence of ice phenomena on the Bug River (eastern Poland) from 1903 to 2012, a reduction in the duration of their occurrence was also observed. At the beginning of the 20th century, ice phenomena on the River Bug lasted on average for about 100 days, while in the first years of the 21st century, their occurrence shortened to about 60 days. Also, the number of days with permanent ice cover has decreased from about 70 to an average of 60 days. A clear downward trend can be observed in the ice phenomena duration and periods with continuous ice cover [17,18] when studying ice phenomena on the Łeba and Parsęta rivers (both rivers are located in northern Poland, on the Baltic Coast). Similar trends were observed in the lakes of the southeastern Baltic Sea basin (1961–2020): a reduction in the duration of ice events, a decrease in maximum ice thickness, and an increase in the frequency of ice breakup events [16,22,23]. In lowland rivers, including the Vistula at Toruń, the ice cover period shortened from about 60–120 days to 30–80 days over recent decades [24,25,26]. The gradual disappearance of ice phenomena in rivers undoubtedly disrupts the typical hydrological cycle in the temperate climate zone, limiting the temporary retention of water in catchments and affecting the conditions for life and development of aquatic organisms, both plant and animal [27,28].

These changes may be linked to climate change, such as rising winter air temperatures, anthropogenic factors like pollution, alterations in riverbed morphology, and the construction of reservoirs and hydropower plants [6], highlighting that shifts in river freezing patterns are typical in the current climate. Precipitation in Poland shows considerable spatial variability and is influenced by elevation. Central lowland regions of Poland are characterized by the lowest annual precipitation totals, below 600 mm [29,30]. According to [30], annual precipitation totals in Poland are slowly changing, although not all studies show statistically significant increases. The average air temperature has risen by approximately 1.5 °C per century from the preindustrial era [31], and the decreasing tendency in the number of frost days was empirically validated in the area of the Central Vistula basin [32,33,34]. Changes in the freezing patterns of rivers, including the later onset and earlier disappearance of ice cover, are also observed all over the northern part of the globe [35]. This study examines the characteristics of ice phenomena in selected river profiles within the Middle Vistula River basin. It also seeks to identify trends in their occurrence based on hydrological data (ice phenomena observations) and meteorological conditions (temperature and precipitation).

2. Materials and Methods

2.1. Study Catchment

The study includes five hydrometric stations on the rivers—Pilica, Iłżanka, Kamienna, and Pokrzywianka—in the Mzowian Lowland, in central Poland. All of them are left-bank tributaries of the Vistula and are located within the borders of two voivodeships: Mazowieckie and Świętokrzyskie. According to the Atlas of the Hydrographic Division of Poland, the Vistula catchment has an area of 168,775 km², 55.8% of Poland’s total 311,889 km². In terms of administration, the Vistula River basin is divided into seven water regions [36]. All the hydrometric stations are located within the Middle Vistula River basin. This basin covers the area from estuary of the San River in the south to the Narew River in the north. The region is mainly used for agriculture. The left-bank part of the Middle Vistula constitutes only 16.6% of the total catchment area, while the right-bank part covers 83.4% of the area [37]. The characteristics of the selected hydrometric stations are presented in

Table 1. The location and the area of hydrometric stations (according to https://hydro.imgw.pl, accessed on 15 August 2025).

Number	Name of the Gauge Station	River	Zero Elevation of the Water Gauge [m a.s.l]	Location of the Station According to River Kilometer [km]	The Catchment Area of a Given Station [km2]
1.	Białobrzegi	Pilica	112.09	48.28	8659.53
2.	Kazanów	Iłżanka	145.24	27.91	861.15
3.	Czekarzewice	Kamienna	135.48	14.6	1881.00
4.	Włochy	Pokrzywianka	211.27	1.56	183.18
5.	Michałów	Kamienna	186.13	92.47	588.00

. The locations of the hydrometric stations are shown in Figure 1.

All hydrometric stations are located on the Mazovian Lowland, which is one of the largest regions in Poland by area. The soils of the Mazovian Lowland are generally classified as low fertility. Most of the area is covered by podzols developed on sands, clays, and stagnant sediments. An exception is found in river valleys, where locally fertile fluvial muds occur. The region is primarily rural, with rye, triticale, and potatoes commonly grown. Orchards dominate the central part. The length of the growing season is 200–220 days.

Poland’s climate is transitional due to the overlap of cool, dry continental air masses from the east and warm, moist masses from the Atlantic Ocean [38]. As a result of the lack of barriers in the meridional relief, the two masses intermingle, affecting the country’s weather variability. The average precipitation over Poland varies between 550 and 700 mm. It is variable in time and space. It most often occurs in summer due to convective movements of moist air masses carried by westerly winds. In the Mazovian Lowlands, precipitation is among the lowest in Poland, with values up to 550 mm per year. In winter, the Mazovian Lowlands are characterized by quite significant thermal differences. The average annual temperature is about 8 °C. The average winter season temperature is between 0 °C and −2 °C. Winters in the area are characterized by cold weather, especially in the north and east of the region. The average number of days with snow cover in winter is between 30 and 60 days. January is the coldest month.

2.2. Data

Poland’s climate is very diverse, and changeable weather is typical. Winter is usually considered to include the months from December to February or March [39]. In this study temperature and precipitation characteristics, as well as ice phenomena, are presented in terms of the hydrological year rather than the calendar year. The results obtained were not considered in terms of meteorological winter.

According to the World Meteorological Organization definition [40], a hydrological year is a continuous period of twelve months, selected in such a way that overall changes in storage are minimal, so that the transfer of reserves to the next year is kept to a minimum. The beginning of the hydrological year depends on the climatic and hydrological conditions of the region. There is no fixed definition of the beginning of the hydrological year. Individual countries choose it in such a way that the water balance is included in a single, consistent measurement period. In Poland the hydrological year begins on 1 November and lasts until the end of October of the next year. This is related to the water cycle in the catchment area. The winter half-year of the hydrological year therefore lasts from November to April. Both of these months are not considered winter in the meteorological sense, because they can be characterized by relatively high temperatures. However, meteorological conditions typical for Polish winter, such as low temperature, frost, cold wind, and snow cover, may occur in both November and April. They are typical of March. This is not unusual for Polish climate. Considering the main topic of this study and the variability of meteorological conditions, the division into winter half-year in relation to the hydrological year seems to be more appropriate than the typical definition of winter, as it takes into account the months in which these phenomena may still occur.

All the data used in the study come from the Institute of Meteorology and Water Management—National Research Institute (IMGW-PIB) and are available on the official website (https://danepubliczne.imgw.pl/, accessed on 15 August 2025). The air temperature characteristics for the studied period were determined based on maximum, minimum, and average air temperature data recorded at the Kozienice (1977–2016) and Puławy station (1951–2016).

Precipitation characteristics were based on daily precipitation totals recorded at the Białobrzegi and Annopol stations (1961–2016). From these records, monthly and half-yearly totals for selected hydrological years were calculated, and trends in their changes were analyzed.

The available ice phenomenon data were collected from the following monitoring profiles: Białobrzegi, Pilica River (1969–2016); Czekarzewice, Kamienna River (1981–2010); Kazanów, Iłżanka River (1991–2016); Michałów, Kamienna River (2003–2014); Włochy, Pokrzywianka River (2008–2016). The dataset classified ice phenomena, including slush, ice floe, shore ice, ice cover, ice jams, and combinations such as shore ice with slush, shore ice with ice floe, slush with ice floe, and slush jams. Additional classifications included water on ice, flowing ice, and decayed ice. Based on this classification, the occurrence of each phenomenon was determined, including the dates of its beginning and end, along with an analysis of trends in their occurrence and changes over time.

3. Results

3.1. Temperature and Precipitation

The distribution of the average air temperature during the winter half-year of the hydrological years 1951–2016 in Puławy and Kozienice (1977–2016) is shown in Figure 2. The long-term average air temperature during the winter season of the hydrological year at the Puławy station was 1.5 °C and 1.7 °C at the station in Kozienice. There is an apparent variability in the average air temperature from year to year. In Puławy, the most significant amplitude in the multi-year average temperature was 4.77 °C, while in Kozienice, it was just under 5 °C. Over the multi-year period of 1951–2016, in the southern part of the Middle Mazovian Lowland, an increasing trend in the average air temperature during the winter half-year is noticeable. The warmest year was 2007, when the average air temperature in Kozienice and Puławy exceeded 4.5 °C. The year 1963 proved to be the coldest year, with the average temperature of −2.42 °C in Puławy. Over more than six decades, only 13 years had a negative average air temperature, and in 7 of those years, the average temperature did not fall below −1 °C. It is worth noting that after 1990, for ten years, the average air temperature exceeded 3 °C. Before 1980, only two such occurrences were recorded (in 1961 and 1975).

Based on the concordance of the recorded average air temperature course in Kozienice and Puławy during the winter season for which data from both stations were available, it can be assumed that even before 1977, the temperature values recorded at both stations were similar.

Precipitation characteristics were based on data recorded at the Białobrzegi and Annopol stations. A long data series (1951–2016) was available for both stations, allowing for a complete characterization of the studied period. Precipitation totals for the winter half-year of the hydrological year ranged from 102 mm to 330 mm at the Białobrzegi station and from 99 mm to 421 mm at the Annopol station (Figure 3). The average total precipitation for the winter half-year is approximately 186 mm at the Białobrzegi station and 189 mm at Annopol, which is similar. Over the multi-year period, the wettest years, with precipitation totals exceeding 250 mm during the winter half-year, were the following hydrological years: in Annopol—1967 (421 mm), 1968 (278 mm), 2014 (273 mm), 1983 (265 mm), 2011 (254.6 m) 1977, and 2016 (253 mm); in Białobrzegi—1967 (330 mm), 2001 (314 mm), 1962 (268 mm), 2016 (261 mm), 2011 (253 mm), and 1970 (252 mm). Over 65 years, precipitation totals exceeded the long-term average 29 times in Annopol and 30 times in Białobrzegi. The driest winter half-year of the hydrological year was recorded in Annopol in 1974 (99 mm) and Białobrzegi in 1976 (102 mm). The lowest recorded winter half-year precipitation total in Annopol was four times lower than the maximum precipitation total recorded at the same station during a winter half-year. An increasing trend in winter half-year precipitation totals was observed (Figure 3).

Both cases were similar, considering the monthly distribution of precipitation totals in the winter half-year. Throughout the hydrological year, the distribution of atmospheric precipitation totals varies. The highest precipitation totals are recorded during the summer months. During the winter half-year, daily precipitation totals are significantly lower than those during the summer half-year. Figure 4 presents the long-term average monthly precipitation totals for the winter half-year. At both stations, average precipitation totals in November, December, and April exceed 30 mm, while in January, February, and March they are below 30 mm. The lowest precipitation was recorded in February. During January and March, precipitation is also relatively low. The monthly totals were similar at both stations: at Bialobrzegi, they were 25.8 mm in February, 27 mm in January, and 27.9 mm in March; at Annopol, 25 mm in February, 27.8 mm in January, and 29.2 mm in March. The highest precipitation was recorded in April and November, months that are not directly associated with the calendar winter, but are part of winter season during the hydrological year. The total precipitation in November amounted to 34.3 mm in Annopol and 38.8 mm in Białobrzegi, while in April it was 39.4 mm in Annopol and 35.7 mm in Białobrzegi. In December, the total precipitation reached 27.8 mm in Annopol and 27 mm in Białobrzegi. The pattern of average monthly precipitation during the winter half-year aligns with the data from other stations in the Mazovian Lowland [41].

To identify correlations between air temperature and precipitation totals during the winter half-year, years in which both parameters exceeded their long-term averages were compared. The results are summarized in

Table 2. Above multi-year average winter half-year air temperature and precipitation totals recorded at particular meteorological stations.

Station	Year
Puławy–Annopol	1951, 1967, 1968, 1971, 1975, 1977, 1983, 1988, 1994, 1998, 2000, 2001, 2004, 2007, 2008, 2009, 2011, 2014, 2016
Puławy–Białobrzegi	1952, 1967, 1968, 1971, 1983, 1989, 1992, 1993, 1994, 1995, 1998, 2000, 2001, 2004, 2007, 2008, 2011, 2014, 2015, 2016
Kozienice–Annopol	1977, 1983, 1988, 1994, 1998, 2000, 2001, 2004, 2007, 2008, 2009, 2014, 2016
Kozienice–Białobrzegi	1983, 1989, 1992, 1993, 1994, 1995, 1998, 2000, 2001, 2004, 2007, 2008, 2012, 2014, 2015, 2016

.

When comparing air temperature data from Puławy with precipitation records from Annopol, both parameters simultaneously exceeded their long-term averages 19 times during the study period, while in Białobrzegi this occurred 20 times. A similar pattern emerges when comparing temperature from Kozienice with precipitation from both stations—13 cases in Annopol and 16 in Białobrzegi. The years 1967, 1968, 1971, 1983, 1994, 1998, 2000, 2001, 2004, 2008, 2011, 2014, and 2016 (14 cases) were common to both raised temperatures in Puławy and increased precipitation in Annopol and Białobrzegi. Similarly, in 1983, 1994, 1998, 2000, 2001, 2004, 2007, 2008, 2014, and 2016 (10 cases), air temperature and precipitation simultaneously exceeded their long-term averages at the Kozienice, Annopol, and Białobrzegi stations. As shown in Table 2, both temperature and precipitation exceeded their long-term averages at all stations in 1983, 1994, 1998, 2000, 2001, 2004, 2007, 2008, 2014, and 2016—a total of ten occurrences, of which seven were recorded in the 2000s.

3.2. The Occurrence of Ice Phenomena

The following ice phenomena were observed on the Kamienna River at the Michałów and Czekarzewice profiles, and on its tributary, the Pokrzywianka River at the Włochy profile: frazil ice, border ice, ice cover, ice floes, water on ice, border ice with frazil ice, and border ice with ice floes. Throughout the study period, ice phenomena were observed on the Pokrzywianka River for 336 days, on the Kamienna River at Michałów for 283 days, and at Czekarzewice for 264 days. Among the identified ice forms, border ice and continuous ice cover were the most frequently occurring ones. The Pokrzywianka River showed the most tremendous variability in ice phenomena in 2012 and 2013 (Figure 5), with 2013 recording the highest number of days with ice forms (67 days). Between 2008 and 2016, ice cover accounted for 43% of all ice phenomena on Pokrzywianka, while border ice made up 45%. The least common forms were ice floes, ice water, and combinations of border ice with frazil ice, constituting only 12% of all ice forms.

During the winter half-year, the number of days with ice phenomena at stations in the Kamienna River region did not exceed 60 days. Figure 6 and Figure 7 illustrate the days with specific ice forms on the Kamienna River. The maximum number of days with ice phenomena in Michałów was 57 days (2010), while in Czekarzewice, this number did not exceed 50 days. Since 1995, border ice has been the dominant form on the Kamienna River. At the Michałów station, border ice was observed on 272 days, and at Czekarzewice on 141 days during the study period. After 1990, ice cover on the Kamienna River became rare, appearing only once every few years and lasting only a few days. Occasionally, two ice forms appeared simultaneously—border ice with frazil ice (in 1987, 1992, 1993, 1996, and 2008), persisting for 1–7 days, and border ice with ice floes, which appeared once in 2003 in Michałów and lasted for one day.

On the Iłżanka River near Kazanów, only three studied ice forms were observed: border ice, ice cover, and ice water. Between 1991 and 2016, ice phenomena were recorded for 613 days. The variability of ice phenomena on this river is relatively uniform. Border ice and ice cover were the most common, constituting 99.6% of all ice phenomena. Ice water accounted for the remaining 0.4%, lasting just one day. The longest durations of ice phenomena on the Iłżanka were recorded in 1996 (70 days) and 2010 (50 days, Figure 8). The longest-lasting ice cover in Kazanów occurred in 1996. Since 1999, the ice cover has ceased to appear annually, with the number of days it lasts decreasing over time.

The Pilica River exhibits the greatest diversity in ice phenomena. Based on data from 1968 to 2016, significant variability is evident in the appearance of specific ice forms, including frazil ice, ice floes, border ice, ice cover, and their combinations, such as border ice with frazil ice and border ice with ice floes (Figure 9). Over 48 years, the total number of days with ice phenomena was 2642. The longest duration of ice phenomena in Białobrzegi was in 1996 (120 days), and the shortest was in 2015 (10 days). Among all ice phenomena, border ice was the most frequent in Białobrzegi (747 days), while ice cover was recorded for the longest cumulative duration (881 days). The rarest were border ice with ice floes (14 days) and ice floes alone (49 days). Ice cover appeared annually from 1968 to 1987, except in 1975 and 1983, when the average winter half-year temperature did not drop below zero. Since 1987, ice cover has become less frequent and shorter, except for the cold year of 1996, which recorded 69 days of ice cover. In the last two decades, ice cover has appeared on the Pilica River only seven times (Figure 9), reflecting the impact of a warming climate. Since 2010, increasingly longer gaps of 2 to 5 years have been observed between the occurrences of ice covers. As the number of days with ice cover has declined, the number of days with border and frazil ice combinations has risen, accounting for 23.9% of all ice phenomena.

The lowest number of days with ice phenomena (fewer than 10 days) across all hydrological stations occurred in the warmest years: 1975, 1983, 1999, 2007, and 2015. Over the years, all profiles have shown a significant decline in ice phenomena. Rising average air temperatures strongly influenced this change. The most significant difference in ice phenomena was observed in Białobrzegi, where the number of days with ice phenomena dropped from around 90 days in the late 1970s to 33 days by the mid-2010s. The slightest difference was observed in Czekarzewice on the Kamienna River, with a reduction in only 10 days over 26 years.

Analysis of hydrological data for selected rivers in the Middle Vistula basin allowed for identifying ice phenomena and the timing of their appearance and disappearance. The recorded dates of appearance and disappearance of ice phenomena, with particular emphasis on ice covers, are given in

Table 3. The recorded dates of appearance and disappearance of ice phenomena, with particular emphasis on ice cover in the Middle Vistula catchment.

Station	Type of Phenomenon	Appearance Date		Disappearance Date
		Earliest	Latest	Earliest	Latest
Włochy	Ice phenomena	1 December	4 January	10 December	12 February
	Ice cover	10 December	21 February	10 December	24 February
Michałów	Ice phenomena	3 December	26 January	8 February	5 March
	Ice cover	28 January	29 January	30 January	2 February
Czekarzewice	Ice phenomena	19 November	19 February	5 January	12 March
	Ice cover	8 January	11 February	23 January	22 February
Kazanów	Ice phenomena	13 November	30 January	9 January	4 March
	Ice cover	20 November	13 February	10 December	24 February
Białobrzegi	Ice phenomena	4 November	8 February	5 January	1 April
	Ice cover	30 November	6 March	14 December	31 March

. The occurrence of ice phenomena and ice cover on individual stations is given in

Table 4. Occurrence of ice phenomena at Włochy station in the hydrological years 2008–2016.

Włochy station	Hydro. Year	Months divided by decades
		XI			XII			I			II			III			IV
	2008
	2009
	2010
	2011
	2012
	2013
	2014
	2015
	2016

,

Table 5. Occurrence of ice phenomena at Michałów station in the hydrological years 2004–2015.

Michałów station	Hyd. Year	Months divided by decades
		XI			XII			I			II			III			IV
	2004
	2005
	2006
	2007
	2008
	2009
	2010
	2011
	2012
	2013
	2014
	2015

,

Table 6. Occurrence of ice phenomena at Czekarzewice station in the hydrological years 1981–2010.

Czekarzewice station	Hyd. Year	Months divided by decades
		XI			XII			I			II			III			IV
	1981
	1982
	1983
	1984
	1985
	1986
	1987
	1988
	1989
	1990
	1991
	1992
	1993
	1994
	1995
	1996
	1997
	1998
	1999
	2000
	2001
	2002
	2003
	2004
	2005
	2006
	2007
	2008
	2009
	2010

,

Table 7. Occurrence of ice phenomena at Kazanów station in the hydrological years 1991–2016.

Kazanów station	Hyd. Year	Months divided by decades
		XI			XII			I			II			III			IV
	1991
	1992
	1993
	1994
	1995
	1996
	1997
	1998
	1999
	2000
	2001
	2002
	2003
	2004
	2005
	2006
	2007
	2008
	2009
	2010
	2011
	2012
	2013
	2014
	2015
	2016

and

Table 8. Occurrence of ice phenomena at Białobrzegi station in the hydrological years 1969–2016.

Białobrzegi station	Hyd. Year	Months divided by decades
		XI			XII			I			II			III			IV
	1969
	1970
	1971
	1972
	1973
	1974
	1975
	1976
	1977
	1978
	1979
	1980
	1981
	1982
	1983
	1984
	1985
	1986
	1987
	1988
	1989
	1990
	1991
	1992
	1993
	1994
	1995
	1996
	1997
	1998
	1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016

. In all the tables, yellow stands for ice phenomena, and blue for ice cover.

The timing of the formation of ice phenomena in rivers has shown considerable variation over the years. Before 1990, ice phenomena typically appeared in late November and December, as observed on the Pilica River. Since 1990, the onset of ice formation has shifted to December and January, particularly on the Kamienna, Pokrzywianka, and Iłżanka Rivers. Ice phenomena were observed on all the rivers several times during a single winter season, lasting from a few days to a few weeks. This variability depends on prolonged periods of low air temperatures. The cessation of ice phenomena on the studied rivers in the Middle Vistula region typically occurs in February and March. The highest ice formation and persistence intensity in the analyzed hydrological profiles is observed in January and February, the coldest months of the year.

The average number of days with ice phenomena during the winter half-year of the hydrological year in the analyzed profiles ranged from 9 days in Czekarzewice (1981–2010) to 73 days in Białobrzegi (1968–2016). On the Pokrzywianka River, the average number of days with ice phenomena was 37, while in Kazanów and Michałów, it was 23 days. The onset of ice phenomena in the Włochy profile (Table 4) during 2007–2016 occurred in December and January. The earliest onset was on 1 December 2011, lasting until 11 March 2012. During this period, ice cover appeared on 1 February 2012 and lasted for 22 days.

The longest duration of ice phenomena on the Pokrzywianka River occurred in the winter of 2013/2014, between 22 December and 30 March. During this time, ice cover appeared once and persisted for over two weeks. Ice cover did not occur during the winter of 2014/2015 in the past decade. In Michałów (Table 5), the longest duration of ice phenomena was recorded from 9 December 2004 to 2 March 2005. In the past decade, ice cover appeared at this location twice, lasting a maximum of 5 days. The first ice phenomena on the Kamienna River typically occur in the first week of December in Czekarzewice and persist until late February or early March (Table 6). In Czekarzewice, over three decades, ice cover appeared twice during the winter of 1985, twice in the winter of 1987, and once in 1991. In these cases, ice cover lasted between 4 days and 3 weeks. After 1991, ice cover did not recur (except for one day in 2010), and the duration of ice phenomena significantly decreased. Ice phenomena in Czekarzewice primarily form in December. Considering the last thirty years, only during eight of were there no recorded ice phenomena at the Czekarzewice station. The onset of ice phenomena on the Iłżanka River (Table 7) during the years 1990–2016 occurred in December and January, typically ending in February or the first days of March, depending on when the first ice forms appeared. The average duration of ice phenomena in the Kazanów region is 1.5 months. The earliest occurrence of ice phenomena was on 13 November 1994. The latest ice forms were recorded on 4 March 2002. Over the 26 years, ice phenomena did not occur twice, in 2007 and 2015. During the winters of 1993, 1994, 1995, 1996, 1997, and 2003, ice cover appeared twice, and in the winter of 2004, it occurred thrice. Since 2000, there have been six years without ice cover. Ice cover appeared most often in the second half of December and the first days of January. The longest-lasting ice cover persisted for one month during the winter of 1996. During the period of 1968–2016, the first ice forms on the Pilica River typically appeared in the second half of October and in December. The earliest ice phenomena in Białobrzegi were recorded on 6 November 1988. In the last decade, the onset of ice phenomena has shifted to the second half of December and the first days of January. Ice forms on the Pilica usually disappear in the second half of February and in March. The longest duration of ice phenomena occurred in the winter of 1969, ending on 1 April. Over the past 65 years, ice cover on the Pilica River has occurred 44 times, and in some years, it has appeared twice or even three times during winter season. There was no ice cover in 1975, 1980, 1983, 1988, 1989, 1995, 2000, 2001, 2003, 2004, 2007, 2008, 2011, 2012, 2013, 2014, and 2015. As shown in Table 8, a complete lack of ice cover was noted mainly after 2000. The ice cover most frequently begins in December and ends in January (seven times), February (ten times), or March (eight times).

4. Discussion

Understanding the dynamics of ice formation, its characteristics and duration is important for a few reasons, and its significance may go beyond the local context of one region. The occurrence of ice phenomena has a direct impact on the safety of hydrotechnical structures. Comprehending their dynamics and frequency of occurrence enables a better design of these structures and more effective management of retention reservoirs. On transboundary rivers, knowledge of the course of ice phenomena is important not only for the safety of river transport, but also for flood risk forecasting. Accurate data on when ice forms and when it disappears allows for better prediction of moments of increased risk of ice jams. In addition, ice phenomena are a sensitive indicator of climate change. Their later appearance and rapid disappearance, or complete absence, are signs of climate warming [42,43], enabling an easy comparison of these changes between regions and countries.

Between 1968 and 2016, the occurrence of ice phenomena on selected rivers in the Middle Vistula basin in central Poland shifted significantly over time, by several weeks in some cases. This trend is most evident on the Pilica River due to the hydrological data records covering a long period. In the first half of the studied period, ice phenomena began forming as early as the first week of November, but after 2000, their formation shifted to mid-December, and in recent years, even to late December. Ice phenomena were less frequent on other rivers than on the Pilica. The timing of their formation in individual years aligns across all studied cases and has shifted later over time. Similarly, the timing of the disappearance of ice phenomena has also shifted. On the Pilica, ice phenomena ended on 1 April in 1969, whereas in 2016, they ended on 29 January. On other rivers, the disappearance of ice phenomena typically occurs in the last week of January or the first week of February.

The shortening of the period during which ice phenomena occur is closely related to the temperature variation in recent decades. Taking into account the thermal classification of years for selected Polish meteorological stations, including Warsaw, it is clear that since 1999, individual years have been classified as increasingly warmer than the normal period (1971–2000) [44]. In 1971–2000, only some years were classified as slightly warm or warm at most stations (1975, 1983, 1992). The years 1989 and 1990 were recorded as very warm at most stations, while the years 2014 and 2015 were classified as abnormally warm or extremely warm, depending on the location of the station. A similar trend continues for the next normal period (1991–2020), where the years 2018, 2019, 2020, 2023, and 2024 at most stations were recorded as abnormally warm or extremely warm [45]. Years 2007 and 2008 at most stations were warm or very warm. The data are similar if we look at the temperature distribution during individual months of the winter half of the hydrological year. Over the past decade, central Poland has been dominated by warm and snowless winters. Research conducted on snow cover during the winter seasons of 1960/61–2009/10 in Poland [46] showed a slight downward trend in the frequency and thickness of snow cover and a statistically significant downward trend in the number of days with snow, especially in December and January.

Analysis of air temperature and precipitation data from the selected stations indicates that both parameters surpassed their long-term averages in 1983, 1994, 1998, 2000, 2001, 2004, 2007, 2008, 2014, and 2016. As reported by [47], only 1983 was classified as a dry year, 1998, 2001, and 2007 were considered humid, while the remaining years were normal. Based on air temperature data [47], the years 1983 and 1994 were classified as normal, 2000, 2007 and 2008 as warm, and 2014 and 2016 as very warm. Only 1998 was categorized as very cold, 2001 as cold, and 2004 as slightly cold. Precipitation in Poland is determined by atmospheric circulation [48]. Recent studies [29] indicate that the monthly distribution of precipitation is more important than the annual totals. It has been observed that the wetter conditions, previously occurring at the end of the year (November and December), have shifted to the beginning of the year (January, February). This is probably the result of climate change (warmer winters and less snow), which affects the water resources of the catchment area. It is also not conductive to the formation of ice phenomena. It is expected that as climate change intensifies in the coming years, the frequency and intensity of extreme precipitation will likely increase [49].

As the temperature increases, the probability of ice phenomena decreases, and the full cycle of their formation is also disturbed. The full cycle of ice formation on a river, including permanent ice cover, involves a series of consecutive stages that depend mainly on thermal, hydraulic and meteorological conditions. The water in the river cools down if the air temperature remains below 0 °C for an extended period, which promotes the initiation of the ice-forming process. When the water reaches a temperature of about 0 °C, the formation of frazil ice begins. As temperatures remain low, ice begins to form along the riverbanks. The current near the riverbanks is slower, which promotes the formation of ice sheets. With a further drop in temperature and slowing of the flow, small crystals and ice sheets merge into a compact ice cover. The ice cover can vary in thickness, and its duration depends on the air temperature. Sudden warming causes the ice cover to melt, become thinner, and begin to crack.

Analysis of the course of ice phenomena on rivers in the Central Vistula region clearly shows a decrease in the frequency of the full ice formation cycle, including the development of ice cover. The dominant ice phenomena across all hydrological profiles are shore ice and ice cover. On the Pokrzywianka River, the most frequently occurring ice phenomena were shore ice (150 days) and ice cover (145 days). Between 2008 and 2016, the longest-lasting ice cover occurred in 2009, lasting 44 days. The warmest winter half-years were recorded in 2007 and 2015, with fewer than 15 days with ice phenomena. The years 2012, 2013, and 2014 were the most diverse in terms of ice phenomena, with four forms of ice phenomena observed, including water on ice, which lasted 1–4 days. The peak of ice phenomena development, including ice cover, typically occurred in January and February. Over 25 years on the Kamienna River, the most frequently occurring ice phenomenon was shore ice (272 days in Michałów from 2004 to 2015 and 141 days in Czekarzewice from 1991 to 2010). After 1990, ice cover appeared only three times (in 2007, 2010, and 2011), lasting 1–5 days between late January and early February. The highest number of ice phenomena was recorded in 1988 in Czekarzewice, lasting 44 days. Between 1991 and 2015, shore ice accounted for 75.5% of all ice forms, frazil ice 7.7%, ice cover 12.8%, and combinations of shore ice with frazil or drifting ice 4%. After 1996, the diversity and duration of ice forms decreased. In 2000, 2001, 2003 (Czekarzewice), and 2007 and 2014 (Michałów), no ice phenomena were recorded. In recent years, the only phenomenon observed on the Kamienna River has been shore ice, lasting intermittently from early January to late February. At the Kazanów station, ice forms were recorded for 613 days over 26 years, with shore ice accounting for 59.4% and ice cover for 40.1%. Minor contributions came from frazil ice and water on ice. The average duration of ice cover was 9.5 days. The maximum duration was 44 days in 1996. During the 1990s, ice cover appeared several times during the winter season, but its frequency declined significantly, and in some years (2000, 2001, 2007, 2008, 2011, 2015), no ice cover was recorded. After 2000, ice cover appeared only once per winter, typically from January to early February. As ice cover occurrences decreased, the number of days with shore ice increased, with an average of 14 days per year.

The Pilica River exhibited the greatest diversity of ice phenomena. Ice forms appeared as early as October and persisted until late March. Ice cover accounted for over 32% of all phenomena, with an average of 18 days per year. In eighteen cases, the number of days with ice cover exceeded the annual average, and in six instances, it surpassed 40 days (1968, 1969, 1978, 1986, 1995, and 1996). The maximum duration of ice cover was 100 days in 1969. Shore ice was the second most common phenomenon (28.3%), lasting an average of 16 days per year. Shore ice frequently co-occurred with frazil ice or drifting ice. Between 1968 and 2016, the average duration of shore ice with frazil was 13.5 days, while shore ice with drifting ice lasted 0.3 days. Over three decades, there has been an increasing trend in the occurrence of shore ice and a declining trend in ice cover. After 1995, there was a significant drop in ice cover occurrences, with several years entirely lacking it. Drifting ice became rare, occurring mainly in the first half of the studied period, lasting an average of two days. After 1990, drifting ice occurred only five times, lasting 1–3 days. Early ice phenomena, such as frazil ice and combinations of frazil and shore ice, were observed more frequently in the past.

Over 30 years, the timeframe for all ice phenomena has narrowed, and their frequency has declined. The long duration of ice cover in Białobrzegi is linked to its location in central-eastern Poland, where the average winter temperature hovers around 0 °C. In Warsaw, the average January temperature (1981–2010) was −1.9 °C, while in Kielce, it was −2.7 °C. The first ice forms on rivers are typically frazil ice and ice cover. In some cases, frazil ice forms between two freezing periods, which is most evident on the Pilica River. Similar findings were reported by [21] in their study of ice variability on the Bug River (1901–1960 and 2001–2012), east of the presented study area. Since the late 20th century, the timeframe for ice phenomena has shifted [50]. Ice forms now appear later and disappear earlier, with some disappearing altogether. This trend aligns with findings by other researchers, such as [50].

Disparities in the occurrence of ice phenomena have also increased. For example, during the winter of 1995/1996, strong cooling resulted in ice cover persisting for 54 days in Białobrzegi, 30 days in Czekarzewice, and 44 days in Kazanów, all exceeding their respective long-term averages. Conversely, ice phenomena were nearly absent in the winters of 2007 and 2015, as warm air masses dominated in central-eastern Poland. In Puławy and Kozienice, the average winter half-year temperature in 2007 exceeded 4.5 °C, which was unfavorable for ice formation. Based on this research, a decline in the number of days with ice phenomena on Middle Vistula tributaries is evident. The primary cause of this negative trend is the rising air temperature during the winter half-year of the hydrological year, particularly after 1988. Similar trends were observed by [2] for the Warta River (central-western Poland, northwest of the presented study area) and [26] for the Vistula River near Bydgoszcz (1947–2012), in northern Poland. Research on river ice formation in the Carpathians (over 20 stations in southern Poland) [43] also showed a progressive disappearance of the full ice cycle of rivers in this region, a decrease in the frequency of ice cover, and a change in the structure of ice phenomena, mainly caused by an increase in air temperature during the winter season.

The results obtained are consistent not only with the work of Polish researchers. According to [35], over the last 30 years, the duration of ice cover on rivers has shortened worldwide—the rivers freeze later and the ice cover is thinner. Similar trends are observed in regions across Europe, North America, Russia [42], Sweden [51], Finland [52], and Lithuania [34].

5. Conclusions

This study presents a comprehensive analysis of the occurrence of ice phenomena in selected river profiles within the Middle Vistula River basin. The investigation focused on the Kamienna, Pokrzywianka, Iłżanka, and Pilica rivers, utilizing hydrometeorological data provided by the Institute of Meteorology and Water Management—National Research Institute (IMGW-PIB). For each of the analyzed profiles, the types of ice phenomena were identified, the number of days with ice phenomena presence was quantified, and the onset and cessation dates of these phenomena were established. Furthermore, the frequency and duration of ice cover were evaluated, and the temporal distribution of ice phenomena over the multi-year study period was assessed. The analysis of long-term data led to the following conclusions:

Winter half-year mean air temperature increased at Puławy (1951–2016) and Kozienice (1977–2016), while average precipitation at Annopol and Białobrzegi (1951–2016) decreased, reflecting progressive regional climate change.

On the Pilica, Kamienna, Iłżanka, and Pokrzywianka rivers, various ice forms were observed. Shore ice and ice cover were predominant in most measurement profiles, with the greatest variety recorded on the Pilica River. The timing of formation, persistence, and disappearance of individual ice phenomena varies across profiles.

Ice phenomena on the rivers of Central Vistula River basin are sensitive to ongoing climate change. The continuing warming trend is contributing to a systematic shortening of ice seasons and a reduction in ice intensity. Between 1968 and 2016, ice phenomena on the studied rivers appeared later and disappeared earlier: shifting from an initial period starting in early November and lasting until April, to forming around mid-December or even January and disappearing by late January or February.

Over the study period, a decline in the number of days with ice phenomena was observed across all monitoring profiles. The duration of ice phenomena varied widely, from only 2 days in Czekarzewice to 73 days in Białobrzegi, averaging 37 days on the Pokrzywianka and 23 days in both Kazanów and Michałów.

The occurrence of full ice formation cycles, including the development of a continuous ice cover, has decreased significantly. Such full cycles, once relatively common, have become increasingly rare, highlighting the ongoing change in climatic conditions. The ice cover now forms later, lasts shorter, and in some years does not appear at all, while the intervals between periods of full ice cover have lengthened. On some rivers it appears only sporadically, while shore ice has become more frequent. For example, on the Pilica River, average ice cover duration has dropped from over 70 days in 1951 to less than 40 days in 2016.

These observations are consistent with trends in other Polish rivers and northern hemisphere freshwater systems, signifying widespread impacts of climate change.