Is it possible to use beaver building activity to reduce lake sedimentation? Dmitry Gorshkov Volga-Kama National Nature Preserve, Sadovy, P.O. Raifa, Zelenodolsk District, Tatarstan, 422537 Russia, e-mail: [email protected] Abstract: Erosion processes and lake sedimentation are among the major environmental problems in Tatarstan Republic. Ploughed soils from the agricultural lands are easily washed away from the fields during flooding, and carried through rivers and relief depressions to lakes and bogs, which then fill with the sediment. Raifa Lake is an example of such lake degradation. Due to erosion from agricultural lands upstream from the lake its length has de- creased from 6 to 1.3 km since 1650, its maximum depth has decreased from 36 to 19 m, and its area has decreased from 150 to 32 ha. The possibility of “harnessing” beavers (Castor fiber) to stop sedimentation by building dams is of great interest. We hoped that dams constructed by beavers would reduce the volume of solid particles that flow into Raifa Lake. With this objective in mind, we reintroduced 21 beavers between 1996 and 2000 on the Sumka River, which runs through Raifa Lake. Investigation was done during the annual flooding periods of 1999-2001 when 115 water samples were taken. The main factor that affects sedimentation is the volume of water that can be stopped by beaver ponds. During the flooding period of 2001, 4,250 tons of solid particles were stopped by three beaver dams in the settlement on Sumka River amounting to a cumulative area of 5.2 ha. Sediment mass per litre of water decreased by 53% (from 0.49 to 0.26 g/l) after water had passed the cascade of three dams. Keywords: beaver, Castor fiber, Volga-Kama Nature Preserve, water quality, sedimentation, reintroduction, dams, building activity. Introduction Nature Preserve (VKNNP). No human settle- ments were present, and the watershed was Today the role of the beaver (Castor fiber) as a covered by forest until 1660, when a monastery hunting-trade species has largely disappeared, at was built on the bank of Raifa Lake. Settlements least in the Tatarstan Republic, and emphasis appeared and the forest was cleared for agricul- is now placed on researching its landscape- tural lands. A reduction in the percentage of land creation role in planning the restoration of ripar- covered by forest and irrational agricultural ian zones disturbed by humans. Landscapes in practices (ploughing up slopes and territories the Tatarstan Republic are influenced by inten- near rivers) have resulted in intensive soil sive economic activity, such as agriculture (cere- erosion on land upstream of the preserve. Sedi- als, grain crops, root crops and fodder crops), oil ment that is washed away from fields during production and cattle grazing. In particular, there spring flooding enters the Sumka River and is is a high anthropogenic pressure on the hydro- carried to lakes and bogs of the preserve, which logical system because of the development of has decreased their area. For example, the area of erosive processes that cause sedimentation in Raifa Lake has decreased from 150 ha to 32 ha lakes and bogs. during the last 400 years. Without in the intro- Detailed research of this problem was carried duction of less erosive agricultural techniques in out in the Raifa portion of Volga-Kama National the Sumka River basin, some lakes will dis- appear or turn to bogs (Taisin 1969, Taisin © 2003 Vereniging voor Zoogdierkunde en Zoog- 1996). dierbescherming. Lutra abstracts on the internet: We have attacked the problem on two fronts. http://www.vzz.nl One involves the re-establishment of beavers, Gorshkov / Lutra 2003 46 (2): 189-196 189 whose dams may catch sediment and lengthen term tactic is to work directly on sediment the life of lakes and bogs. Beaver dams affect sources, as beaver ponds will only hold a finite sedimentation through (1) reducing sediment quantity of sediment. loads per cubic metre, and (2) stopping big volumes of water thereby causing a reduction in the absolute amount of sediment carried down- Study area stream. Later water evaporates and soaks into the ground and sediment accumulates on the bottom The Raifa part (5,921 ha) of the VKNNP of the pond. The last of the beavers in the is located 800 km east of Moscow and 600 Tatarstan Republic were killed in 1802. Begin- km west of the Ural Mountains near the city ning in 1949, beavers were reintroduced to many of Kazan, Tatarstan Republic, Russian Federa- parts of the region and now number about 3,000. tion. Much of the VKNNP forest is 250-300 But only 16% of the republic is forested and the years old. Western and eastern sections of the island of forest in which the preserve is located preserve are bordered by forests; the northern would take a long time to recolonise, if it hap- and southern sections border agricultural lands pened at all, without human intervention. So, our (figure 1). first step was to reintroduce beavers to the The Sumka River (length 37.5 km) and its preserve (Gorshkov et al. 1999, Gorshkov et al. main tributary, the Ser-Bulak River (length 11.5 2002). A second and longer-term front on which km; figure 1), are connected to ten lakes. The to solve the erosion problem is to find the Sumka River watershed contains 46% forest in a sources of sediment and implement less erosive patch distribution. The upper part of the Sumka agricultural practices in those areas. Our long- River is agricultural land which causes intensive Figure 1. Map of Raifa, part of Volga-Kama National Nature Preserve. 190 Gorshkov / Lutra 2003 46 (2): 189-196 erosion. The watershed of the Ser-Bulak River is almost completely covered by forest. Both rivers flow intermittently, with 80% of the annual runoff in April (Unkovskaia et al. 2002). Methods and materials Beavers were relocated from tributaries of the Vyatka River to the Raifa portion of the VKNNP. Animals were released in 1996, 1997, 1999, and 2000, and numbered 6, 7, 3 and 5 indi- viduals, respectively. To study the role of beaver dams in sediment retention, we took water sam- ples during the spring floods of 1999-2001. To perform the investigation we chose two beaver settlements: A and B. Site A, with three dams, was located in the upper part of the Sumka River, Figure 2. Scheme of hydrometric plots on an observa- in the preserve’s buffer zone. Plot A1 was a tion site on the Sumka River (A) and the Ser-Bulak control plot upstream of all the ponds (figure 2). River (B). Plot A2 was in the upper pond and A3 was in the middle pond 50 metres below the upper dam. collected on the Sumka River and 48 on the Ser- Plot A4 was downstream from the whole cascade Bulak River. Water samples were taken around of dams. Site B was located at a beaver settle- midday at each plot (time of sampling was ment in the Ser-Bulak River, which contained recorded to within a minute accuracy), once plots B1, B2, and B3, respectively above, in and every 2-4 days depending on the intensity of below a single beaver pond (figure 2). Settle- flooding. ments were active throughout the study and the location of the dams did not change. If the depth was less than 50 cm we took a Results water sample at half the depth. If the depth was more than 50 cm we took a water sample 20 cm Beavers built 26 dams during the seven years from the bottom and 20 cm from the surface of they occupied the Raifa portion of VKNNP and the river. Water samples were filtered to deter- its buffer zone. Dam height varied from 0.3 to mine the mass of sediment per litre. We calculat- 1.6 m, and maximum length was 8 m. The total ed water flow using water velocity and area of area of active beaver ponds in the Raifa portion wetted cross-section, which were measured of the preserve was about 14 ha, and 0.54 ha on using standard methods (Potapova 1975, Luch- average (table 1). sheva 1983). Using the amount of sediment per The Sumka River within site A (figure 1) was litre and water flow rates, we calculated the 1-2 m wide during mean flow rates of a year and amount of sediment that passed through the about 6-8 m during the flooding period. The stream projection in a time period. The differ- depth varied from 0.15-0.20 up to 1 m. During ence between total amounts of sediment that the spring flooding of 2001 the total sediment passed through plots A1 and A4 during spring mass at site A reached 4,600 tons (figure 3). The flood was the amount of sediment that was dams decreased the amount of sediment in a litre retained by beaver dams during flooding. of water (there was 53% less (reduced from 0.49 Using these methods, 115 water samples were to 0.26 g/l) below the cascade of ponds) and also Gorshkov / Lutra 2003 46 (2): 189-196 191 Figure 3. Sediment mass characteristics at hydro- metric plots above, in and below beaver ponds on the Sumka River during a spring flooding of 2001. stopped water that contained sediment. The mean width of 0.5 m during low water and about difference of the parameters of plots A1 and A4 2 m during flooding. The depth varies from 0.05 (figure 3) shows that during the flooding period to 1 m.