**Biodiversity of Macroinvertebrates in Oxbow-Lakes of Early Glacial River Basins in Northern Poland**

Krystian Obolewski *Pomeranian University in Słupsk, Department of Aquatic Ecology Poland* 

## **1. Introduction**

138 Ecosystems Biodiversity

Weisburg, W.G.; Barns, S.M.; Pelletier, D.A. & Lane, D.J. (1991). 16S ribosomal DNA

WHO - World Health Organization. *Guidelines for drinking water quality.* Second Edition, vol.

WHO - World Health Organization. *Guidelines for drinking water quality.* Second Edition, vol.

2, Health criteria and other supporting information. Geneva, World Health

2, Health criteria and other supporting information. Geneva, World Health Organization, 1996.*Water Res* 1989, 23:191-197.34. International Hydrological Decade – World Health Organization (IHD-WHO): Water quality surveys; a guide for the collection and interpretation of water quality data. In *Studies and Reports in Hydrology (UNESCO), no. 23/International Hydrological Decade, 75 – Paris (France)* 

amplification for phylogenetic study. *J Bacteriol*, 17:697-703.

Geneva: World Health Organization; 1978:57-54. Woese C.R. (1987). Bacterial evolution. *Microbiol Rev*, 51:221-271.

Organization, 1996

River basins are very complex systems which include both abiotic and biotic elements. Such a high number of components results in a situation that normal functioning of river with adjacent areas depends on a set of hydrological and geological processes. They influence biological diversity observed in river basins (Arscott et al., 2005; Marshall et al., 2006). Proper assessment of a lotic system should include not only the main watercourse but also wetlands flooded in spring and autumn. The diversity of habitats in natural river valleys increases biological diversity of aquatic ecosystems and thus the quality of environment (e.g., Boulton et al., 1992; Clausen & Biggs, 1997; Tockner et al., 1999; Gibbins et al., 2001; Sheldon et al., 2002; Arscott et al., 2003; Robinson et al., 2003, 2004; Arscott et al., 2005; Whiles & Goldowitz, 2005; Gallardo et al., 2008; Reese & Batzer, 2007; Obolewski, 2011a; Obolewski & Glińska-Lewczuk, 2011).

Each meandering, lowland River is forming its riverbed constantly. Often after rapid floods it turns out that a river flows in new riverbed and the cut-off fragments transform into oxbow-lakes (Amoros & Roux, 1988). They can be filled up during river rises and undergo succession (Junk et al., 1989; Tockner et al., 2000). Due to the diversity of river rise intensity, the connectivity between an oxbow-lake and a river can vary. Therefore, we distinguish lentic, semi-lotic and lotic oxbow-lakes. The first type is supplied by river waters under high water table level or by ground waters while the two remaining types are partly or totally connected to a river. Limited exchange of water in a river valley as well as its agricultural use causes that oxbow-lake drainage area often undergoes anthropopression which leads to quick eutrophication and massive phytoplankton blooms. As a result, water contains considerable amounts of biogenes, mineral salts but low oxygen content. Additional unfavourable factors are hydromorphological features of oxbows, i.e. small area (between a few hundreds squared meters and a few hectares) and depth which usually does not exceed 3 meters. As a result oxbows quickly react to changes in temperature and thanks to that they are perfect objects for the research on climate changes even in the global depiction (Klimaszyk, 2004).

The structure and functioning of wetland ecosystems, including oxbow-lakes, are directly and indirectly connected with the fluctuations in water table level of rivers during floods or flow pulsations (Junk et al., 1989; Tockner et al., 2000). According to Amoros & Roux (1988),

Biodiversity of Macroinvertebrates

stages reaches about 4 m.

No. of kilometres (from

Mean number of oxbows per1 km of river valley

(Smolajny), Drwęca (Rodzone) and Słupia (Słupsk)

there comparing to the remaining rivers studied in this investigation.

length

upper course of the Łyna River is at the level of 1- 2 ‰.

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 141

elevation about 160 m.a.s.l., north-east from Łyna village, by the western edge of Lasy Napiwodzkie and 10 km to the north of Nidzica. The spring of Łyna is fairly abundant due to the presence of impermeable Pliocene loam. Spring streams formed a deep and branch ravine. Łyna flows through many lakes (Persing, Kiernoz Wielki and Kiernoz Mały, Łańskie, Ustrych), among which the Łańskie Lake is the largest. At the latitude of Kiernoz Wielki one of the largest tributaries- Marózka- flows into the Łyna River. Further course of Łyna is diverse in terms of its direction through morainic elevation and rich with numerous meanders. Just below the city of Olsztyn Łyna joints with the second largest tributary-Wadąg- which is 20 km long and with drainage area of 1218 km2. The average slope of the

In the middle course of the river, between Dobre Miasto and Lidzbark Warmiński, Łyna reveals the features of a meandering river, washes away river banks and transports considerable amounts of fine sediments. In the vicinity of Sępopol the river joints with such tributaries as Sunia and Kirsna, located in the area of the studied oxbows as well as Symsarna (drainage area of 276 km2), Ekna (drainage area of 301 km2) and Guber (length of 73 km, drainage area of 1682 km2). Passing Sępopol the river crosses the border near Znamieńsk and flows into Pregoła at the altitude of 4 m.a.s.l. The slope in its lower course is around 0.4 ‰. The mean annual flow near the border is 38.5 m3/s while the spread of water

Parameter Unit River – gauging station

estuary) km 172.0 126.7 31.6 Drainage area km2 2290 2725 1450 Mean annual river flow SSQ m3 s-1 14.7 11.2 15.0 WQ10% m3 s-1 50.5 46.1 49.0 NQ10% m3 s-1 4.49 4.50 6.02 Mean low river flow SNQ m3 s-1 6.45 6.30 7.45 Flow coefficient *cv* - 0.439 0.442 0.450 Oscillation of water stages cm 207 191 171 Watercourse slope m km-1 0.32 0.30 0.18 Width of valley-bottom zone m 372 618 315 River sinuosity - 1.36±1.82 1.66±2.40 1.21±2.13

Table 1. Characteristics of the studied rivers at sections with meandering riverbed: Łyna

The most interesting section of the Łyna River Valley is its middle part, rich with oxbowlakes, which was included in this study. The mean annual flow over years 2007-2009 at the cross-section located in Smolajny (172 km) reached 14.7 m3 s-1 while the mean low river flow was almost 6.5 m3 s-1 (Table 1). Moreover, the highest water stage amplitudes were observed

Łyna-Smolajny

no/1km 3 10 4

Drwęca-Rodzone Słupia-Słupsk

hydrological connectivity determines the processes that take place both in a river and adjacent wetland systems. That concerns the transport of dissolved or suspended elements and organisms, environment reorganization, productivity and biodiversity of aquatic and land ecosystems (Amoros & Roux, 1988; Junk et al., 1989; Poff & Ward, 1989; Heiler et al., 1995; Ward & Stanford, 1995; Poff et al., 1997; Ward et al., 2002; Tockner et al., 2000). Limited hydrological connectivity as well as hydrotechnical appliances located on rivers of moderate river slope result in hydrological changes which can lead to the loss of many valuable ecosystems, including oxbow-lakes (Poff & Ward, 1989; Dynesius & Nilsson, 1994; Heiler et al., 1995; Poff et al., 1997; Tockner et al., 1999; Ward et al., 2002; Thorns & Sheldon, 2000). Consequently, the flora and fauna migration in a river valley occurs only during the periods of water exchange between wetlands and the main river channel, which deteriorates the river ecosystem biodiversity and functioning (Amoros & Roux, 1988; Heiler et al., 1995; Walker et al., 1995). Those changes can be observed by the monitoring of a selected hydrobiont group and its changes in terms of qualitative and quantitative structure that reflect the ecological state of a river valley.

Among various hydrobiont groups, benthic invertebrates are particularly attractive research objects, because their life cycles are short and they also easily adapt to environmental conditions due to, for instance, their shape and dispersion (Gasith & Resh, 1999). High abundance of benthic macroinvertebrates allows using them as indicator organisms in the monitoring of aquatic ecosystem quality (e.g. Boon, 1988; Obolewski et al., 2009). Moreover, some groups of macroinvertebrates periodically reach their maximum abundance and diversity in relation to spatial and environmental variables, however those phenomena have not been fully understood so far (e.g., Van der Brink et al., 1991; Tockner et al., 1999; Heino, 2000; Griffith et al., 2001; Arscott et al., 2005; Monaghan et al., 2005).

This study investigates the relationships between macrozoobenthos communities and the level of hydrological connectivity between wetlands and the main river channels in the river valleys of three lowland rivers located in northern Poland: Słupia, Łyna and Drwęca. The studied wetlands were selected according to the hypothesis, that the level of hydrological contact between oxbow-lakes and rivers influences the qualitative and quantitative structure of benthic macroinvertebrates inhabiting wetlands.

## **2. Study area**

#### **2.1 Characteristics of the studied river valleys in northern Poland**

The study area covered the wetlands of middle Łyna, Pasłęka, Drwęca and Słupia rivers. They were chosen because of geomorphologic and hydrological similarities. They are also located in areas of similar climatic conditions and shows hydrological regime typical of lakelands.

#### *Łyna River Valley*

The studied oxbows are located in the middle section of the Łyna River Valley- the largest tributary of Pregoła which flows into the Zalew Wiślany reservoir. The drainage area of Łyna reaches 7126 km2 and 5773 km2 is within the territory of Poland while the lower part of drainage area with the estuary are located in Russia. Total length of the river is around 290 km. Łyna is characterized by varied, natural and valuable landscape of its valley thanks to polygenetic processes that occurred during ice-sheet recession in the Baltic glaciation (Michniewska – Szczepkowska & Szczepkowski, 1969). The river begins its course at the

hydrological connectivity determines the processes that take place both in a river and adjacent wetland systems. That concerns the transport of dissolved or suspended elements and organisms, environment reorganization, productivity and biodiversity of aquatic and land ecosystems (Amoros & Roux, 1988; Junk et al., 1989; Poff & Ward, 1989; Heiler et al., 1995; Ward & Stanford, 1995; Poff et al., 1997; Ward et al., 2002; Tockner et al., 2000). Limited hydrological connectivity as well as hydrotechnical appliances located on rivers of moderate river slope result in hydrological changes which can lead to the loss of many valuable ecosystems, including oxbow-lakes (Poff & Ward, 1989; Dynesius & Nilsson, 1994; Heiler et al., 1995; Poff et al., 1997; Tockner et al., 1999; Ward et al., 2002; Thorns & Sheldon, 2000). Consequently, the flora and fauna migration in a river valley occurs only during the periods of water exchange between wetlands and the main river channel, which deteriorates the river ecosystem biodiversity and functioning (Amoros & Roux, 1988; Heiler et al., 1995; Walker et al., 1995). Those changes can be observed by the monitoring of a selected hydrobiont group and its changes in terms of qualitative and quantitative structure that

Among various hydrobiont groups, benthic invertebrates are particularly attractive research objects, because their life cycles are short and they also easily adapt to environmental conditions due to, for instance, their shape and dispersion (Gasith & Resh, 1999). High abundance of benthic macroinvertebrates allows using them as indicator organisms in the monitoring of aquatic ecosystem quality (e.g. Boon, 1988; Obolewski et al., 2009). Moreover, some groups of macroinvertebrates periodically reach their maximum abundance and diversity in relation to spatial and environmental variables, however those phenomena have not been fully understood so far (e.g., Van der Brink et al., 1991; Tockner et al., 1999; Heino,

This study investigates the relationships between macrozoobenthos communities and the level of hydrological connectivity between wetlands and the main river channels in the river valleys of three lowland rivers located in northern Poland: Słupia, Łyna and Drwęca. The studied wetlands were selected according to the hypothesis, that the level of hydrological contact between oxbow-lakes and rivers influences the qualitative and quantitative structure

The study area covered the wetlands of middle Łyna, Pasłęka, Drwęca and Słupia rivers. They were chosen because of geomorphologic and hydrological similarities. They are also located in areas of similar climatic conditions and shows hydrological regime typical of

The studied oxbows are located in the middle section of the Łyna River Valley- the largest tributary of Pregoła which flows into the Zalew Wiślany reservoir. The drainage area of Łyna reaches 7126 km2 and 5773 km2 is within the territory of Poland while the lower part of drainage area with the estuary are located in Russia. Total length of the river is around 290 km. Łyna is characterized by varied, natural and valuable landscape of its valley thanks to polygenetic processes that occurred during ice-sheet recession in the Baltic glaciation (Michniewska – Szczepkowska & Szczepkowski, 1969). The river begins its course at the

2000; Griffith et al., 2001; Arscott et al., 2005; Monaghan et al., 2005).

**2.1 Characteristics of the studied river valleys in northern Poland** 

of benthic macroinvertebrates inhabiting wetlands.

**2. Study area** 

lakelands.

*Łyna River Valley* 

reflect the ecological state of a river valley.

elevation about 160 m.a.s.l., north-east from Łyna village, by the western edge of Lasy Napiwodzkie and 10 km to the north of Nidzica. The spring of Łyna is fairly abundant due to the presence of impermeable Pliocene loam. Spring streams formed a deep and branch ravine. Łyna flows through many lakes (Persing, Kiernoz Wielki and Kiernoz Mały, Łańskie, Ustrych), among which the Łańskie Lake is the largest. At the latitude of Kiernoz Wielki one of the largest tributaries- Marózka- flows into the Łyna River. Further course of Łyna is diverse in terms of its direction through morainic elevation and rich with numerous meanders. Just below the city of Olsztyn Łyna joints with the second largest tributary-Wadąg- which is 20 km long and with drainage area of 1218 km2. The average slope of the upper course of the Łyna River is at the level of 1- 2 ‰.

In the middle course of the river, between Dobre Miasto and Lidzbark Warmiński, Łyna reveals the features of a meandering river, washes away river banks and transports considerable amounts of fine sediments. In the vicinity of Sępopol the river joints with such tributaries as Sunia and Kirsna, located in the area of the studied oxbows as well as Symsarna (drainage area of 276 km2), Ekna (drainage area of 301 km2) and Guber (length of 73 km, drainage area of 1682 km2). Passing Sępopol the river crosses the border near Znamieńsk and flows into Pregoła at the altitude of 4 m.a.s.l. The slope in its lower course is around 0.4 ‰. The mean annual flow near the border is 38.5 m3/s while the spread of water stages reaches about 4 m.


Table 1. Characteristics of the studied rivers at sections with meandering riverbed: Łyna (Smolajny), Drwęca (Rodzone) and Słupia (Słupsk)

The most interesting section of the Łyna River Valley is its middle part, rich with oxbowlakes, which was included in this study. The mean annual flow over years 2007-2009 at the cross-section located in Smolajny (172 km) reached 14.7 m3 s-1 while the mean low river flow was almost 6.5 m3 s-1 (Table 1). Moreover, the highest water stage amplitudes were observed there comparing to the remaining rivers studied in this investigation.

Biodiversity of Macroinvertebrates

Węgorzyno, Żukowskie.

15 and 20 of March.


outflow is the highest (Table 1).

oxbows can be twofold different.

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 143

The characteristic features of Słupia formed around 15 000 - 10 000 lat BP. Melting icesheet created a net of sub-glacial gullies, marginal and riverine valleys. The Słupia's tributaries are strongly meandering water-courses. In the upper section the river flows through lakes Tuchlińskie, Pręgożyno, Skrzynka, Trzebocińskie, Gowidlińskie,

The tributaries of the Słupia are asymmetrical, mostly located on the left side of the river (Skotawa- the largest tributary, Glaźna, Gnilna). On the right side there are Parchowska


The average outflow in spring constitutes around 130-180% of average monthly outflow. The lowest outflow occurs in summer (mainly July). The floods are small, mostly in December, January or March and connected with melting snow. The ice phenomena in the Słupia River basin start in late November or early December. Due to the presence of fast currents the ice covers only part of the river (channels, still water bodies) and melts between

Numerous oxbow-lakes are located in the middle part of the Słupia River and they were mainly formed after river regulation at the beginning of XXth century. Nowadays they are within the borders of "Dolina Słupi" Landscape Park. Its area is the lowest but water

The research covered 14 oxbow-lakes (four in the Drwęca river valley, five in Łyna and five in the Słupia river valleys)- 8 of them were cut-off, 4 were open and 2 were semi-open reservoirs.

The oxbow-lakes in the Drwęca river valley were formed during regulation works and all of them are cut off from the main river channel (lentic). OLD 1, OLD 2 and OLD 4 are located on the right side of the river while OLD 3 is situated on the left side (Fig. 1). Their length varies from 140 to 1200 m (Table 2). The main shape of those reservoirs is determined by one river bend of sinuosity S around 5.5. In the valley there are also oxbows almost parallel to the riverbed (S=1.5) as well as better developed (S=9.8). Water table area of the studied reservoirs ranges from 0.1 to 1.2 ha and their volume from 5.3 to 16.7 thousand m3 (in relation to the Drwęca water level H=100 cm on the watermark in Rodzone). Morphometric features change over time for each reservoir individually. The flat fragment of the river valley in the vicinity of Bratian does not limit the changes in water table level. The fluctuations in Drwęca water level cause that the volume and area of

The oxbow-lakes in the Łyna river valley are located near the Smolajny village and were also formed as a result of regulation works. Three of them (OLŁ 2, OLŁ 3 and OLŁ 5) are connected with the river on both sides (lotic) and OLŁ 1 through one arm (semi-lotic) while OLŁ 4 is cut off from the river (lentic). The length of those reservoirs ranges from 420 to 700 (Table 2) but most of them reach 200-400 m (43% of all objects). Their main shape is determined by one bend of sinuosity S around 3.0. In the valley there are also

**2.2 Selected oxbow-lakes located in lowland river basins of northern Poland** 

Those oxbows differed in habitat conditions and morphometric features (Table 2).

Struga, Stropna, Bytowa, Kamienica, Brodek, Kamienna, Żelkowa Woda, Kwacza. Hydrological characteristic of the Słupia River is typical of the Pomeranian Region: - domination of ground water supply (70-75% according to Paszczyk, 1976);

#### *Drwęca River Valley*

The Drwęca River is a right-side, large tributary of Vistula, with the length of 207.2 km and drainage area of 5343.5 km2. The spring of Drwęca is localized at the northern foot of the Dylewskie hills, to the south of Drwęck. The river flows into Vistula near Złotoria (vicinity of the city of Toruń). The upper section of the riverbed is a ravine, 20-30 m deep and 8 km long, called the "Czarci Jar". First the river flows to north-west as a small stream in a valley covered with forest and surrounded by agricultural areas, crosses the Ostrowin Lake and then joins with its tributary- the Grabiczek River. Further, the river changes its direction into west and flows into the Drwęckie Lake. Passing that lake the river is regulated and flows to south-west, receiving the waters of Poborska Struga, Gizela, Elszka (left-sided tributaries), Ruda and Iławka (right-sided tributaries). In the town of Bratian Drwęca joins with its largest tributary- the Wel River.

Starting from the town of Bratian the river is meandering and flows in a deep valley. After joining with the left-sided tributary- the Brybica River- Drwęca flows to north-west direction. Near Brodnica the river joins with its tributary- Skarlanka, then Rypienica and Struga Wąbrzeska. In the vicinity of Młyniec the river changes its characteristics. After building a dam in 1997 in Lubicz the river formed a reservoir with the area of 50 ha, where the water flow decreased to 0.2-0.3 m3 s-1. In the lower section of the Drwęca River the most important tributaries are: Struga Kowalewska, Struga Rychnowska and Struga Lubicka. After passing the town of Nowa Wieś the river turns to north-west and flows into the Vistula River in Złotoryja (Mileska, 1992).

The drainage area of Drwęca is localised on clay, sand and gravel of glacial origin with peatlands in depressions. It was formed during the Baltic glaciation in the Toruń-Eberswald pra-valley. The studied part of valley is located 127 km from the estuary on ground moraine and 6 km wide. Typical of this section is a considerable number of oxbow-lakes (Table 1). The river flow of Drwęca is relatively stable thanks to the retention ensured by numerous lakes and forests. In general no catastrophic floods or low water stages are observed in the valley (Bralczyk, 1996). The average annual river flow in years 2007–2009 at Rodzone (127 km) was 11.2 m3 s-1 while the low water flow slightly exceeded 6 m3 s-1 (Table 1).

The Drwęca River Valley is under various forms of legal protection. In 1961 the whole river with lakes Drwęckie and Ostrowin, some of the tributaries and 5 m wide strip of river bank became a reserve in order to protect breeding ground for salmon fish. Moreover, the Area of Protected Landscape in the Lower Drwęca River Basin has been created with the area of 17 472.4 ha as well as the Area of Protected Landscape of the Upper Drwęca River Valley (area 8 039.5 ha). It is planned to include the river with some of its tributaries (area of 2 162.1 ha) to the European Ecological Network Nature 2000.

#### *Słupia River Valley*

The Słupia River flows into the Baltic Sea. The whole river basin is located in the Pomeranian Voivodeship. It borders the Wieprza River Basin (west), the Brda River Basin (south) as well as the Łeba and Łupawa river basins (east). The length of Słupia is 138.6 km (according to the "Hydrographic division of Poland", IMGW, 1983) while its basin covers the area of 1310 km2. The spring of Słupia is in the Kaszuby Lake District near Sierakowska Huta at the height of 178 m.a.s.l. The river estuary is in Ustka and its average slope is around 1.3 per mill.

The characteristic features of Słupia formed around 15 000 - 10 000 lat BP. Melting icesheet created a net of sub-glacial gullies, marginal and riverine valleys. The Słupia's tributaries are strongly meandering water-courses. In the upper section the river flows through lakes Tuchlińskie, Pręgożyno, Skrzynka, Trzebocińskie, Gowidlińskie, Węgorzyno, Żukowskie.

The tributaries of the Słupia are asymmetrical, mostly located on the left side of the river (Skotawa- the largest tributary, Glaźna, Gnilna). On the right side there are Parchowska Struga, Stropna, Bytowa, Kamienica, Brodek, Kamienna, Żelkowa Woda, Kwacza.

Hydrological characteristic of the Słupia River is typical of the Pomeranian Region:


142 Ecosystems Biodiversity

The Drwęca River is a right-side, large tributary of Vistula, with the length of 207.2 km and drainage area of 5343.5 km2. The spring of Drwęca is localized at the northern foot of the Dylewskie hills, to the south of Drwęck. The river flows into Vistula near Złotoria (vicinity of the city of Toruń). The upper section of the riverbed is a ravine, 20-30 m deep and 8 km long, called the "Czarci Jar". First the river flows to north-west as a small stream in a valley covered with forest and surrounded by agricultural areas, crosses the Ostrowin Lake and then joins with its tributary- the Grabiczek River. Further, the river changes its direction into west and flows into the Drwęckie Lake. Passing that lake the river is regulated and flows to south-west, receiving the waters of Poborska Struga, Gizela, Elszka (left-sided tributaries), Ruda and Iławka (right-sided tributaries). In the town of Bratian Drwęca joins

Starting from the town of Bratian the river is meandering and flows in a deep valley. After joining with the left-sided tributary- the Brybica River- Drwęca flows to north-west direction. Near Brodnica the river joins with its tributary- Skarlanka, then Rypienica and Struga Wąbrzeska. In the vicinity of Młyniec the river changes its characteristics. After building a dam in 1997 in Lubicz the river formed a reservoir with the area of 50 ha, where the water flow decreased to 0.2-0.3 m3 s-1. In the lower section of the Drwęca River the most important tributaries are: Struga Kowalewska, Struga Rychnowska and Struga Lubicka. After passing the town of Nowa Wieś the river turns to north-west and flows into the

The drainage area of Drwęca is localised on clay, sand and gravel of glacial origin with peatlands in depressions. It was formed during the Baltic glaciation in the Toruń-Eberswald pra-valley. The studied part of valley is located 127 km from the estuary on ground moraine and 6 km wide. Typical of this section is a considerable number of oxbow-lakes (Table 1). The river flow of Drwęca is relatively stable thanks to the retention ensured by numerous lakes and forests. In general no catastrophic floods or low water stages are observed in the valley (Bralczyk, 1996). The average annual river flow in years 2007–2009 at Rodzone (127

The Drwęca River Valley is under various forms of legal protection. In 1961 the whole river with lakes Drwęckie and Ostrowin, some of the tributaries and 5 m wide strip of river bank became a reserve in order to protect breeding ground for salmon fish. Moreover, the Area of Protected Landscape in the Lower Drwęca River Basin has been created with the area of 17 472.4 ha as well as the Area of Protected Landscape of the Upper Drwęca River Valley (area 8 039.5 ha). It is planned to include the river with some of its tributaries (area of

The Słupia River flows into the Baltic Sea. The whole river basin is located in the Pomeranian Voivodeship. It borders the Wieprza River Basin (west), the Brda River Basin (south) as well as the Łeba and Łupawa river basins (east). The length of Słupia is 138.6 km (according to the "Hydrographic division of Poland", IMGW, 1983) while its basin covers the area of 1310 km2. The spring of Słupia is in the Kaszuby Lake District near Sierakowska Huta at the height of 178 m.a.s.l. The river estuary is in Ustka and its average slope is

km) was 11.2 m3 s-1 while the low water flow slightly exceeded 6 m3 s-1 (Table 1).

2 162.1 ha) to the European Ecological Network Nature 2000.

*Drwęca River Valley*

with its largest tributary- the Wel River.

Vistula River in Złotoryja (Mileska, 1992).

*Słupia River Valley* 

around 1.3 per mill.

The average outflow in spring constitutes around 130-180% of average monthly outflow. The lowest outflow occurs in summer (mainly July). The floods are small, mostly in December, January or March and connected with melting snow. The ice phenomena in the Słupia River basin start in late November or early December. Due to the presence of fast currents the ice covers only part of the river (channels, still water bodies) and melts between 15 and 20 of March.

Numerous oxbow-lakes are located in the middle part of the Słupia River and they were mainly formed after river regulation at the beginning of XXth century. Nowadays they are within the borders of "Dolina Słupi" Landscape Park. Its area is the lowest but water outflow is the highest (Table 1).
