The intensity of siltation of a small reservoir in Poland and its relationship to environmental changes

Siltation of reservoirs is an indicator of the intensity of erosion processes and sediment yields of river catchments. An estimation of sediment yield is significant for hydroengineering practices, environmental prediction and modelling. It also reflects environmental changes in the form of climate and land use. The present study aimed to compare the amount of sediment deposited in a small reservoir in Poland, with the estimate of sediment yield from its catchment, using the Universal Soil Loss Equation coupled with a sediment delivery ratio (USLE-SDR) and an estimate of annual bed load transport, provided by a regional bed load formula. The estimate of the annual amount of bed load was based on the flow frequency graph and bed load rating curve of Skibinski. The dam of the reservoir was constructed in 1976. The first reservoir survey was carried out between 1979 and 1980, when the reservoir volume was estimated to be 252,000 m(3). Subsequent surveys were carried out in 1991, 2003, 2009 and in the Spring of 2020. These surveys enabled the amount of sediment, deposited in the reservoir, to be measured across four different periods: 1980-1991, 1992-2003, 2004-2009 and 2010-2019. For the same periods, the USLE with a constant SDR (calculated using the approach provided by Vanoni) were used to estimate the quantity (mass) of suspended sediment yield from the contributing area of the catchment. The volume of suspended sediment, deposited in the reservoir, was obtained, taking into account the reservoir trap efficiency and assuming a density of suspended sediment deposits. Hydrological investigations of the Plachty gauging station on the Zagozdzonka River, the left tributary of the Vistula River, have been carried out by the Department of River Engineering at Warsaw University of Life Sciences since 1962. The catchment area at the Plachty gauging station, located just upstream of the reservoir, is 82.4 km(2), whereas the catchment area above the dam of the Staw Gorny reservoir, is 91.4 km(2). The estimated annual sediment yields for the catchment, based on reservoir surveys across four periods between 1980 and 2020, suggest a general decrease over time. This study was also undertaken to examine long-term variability in sediment yield, both in suspended sediment and bed load, related to rainfall-runoff erosivity, river discharge and land use changes, which could influence changes in catchment sediment yield. An analysis of local temperature and catchment runoff revealed a long-term increase in the mean annual temperature and a corresponding decrease in annual discharge. The annual values of total sediment input to the reservoir (suspended and bed load), estimated using the USLE-SDR and a bed load formula with sediment trap efficiency, are very close to the measured amounts of deposits in the reservoir. Annual deposits in the reservoir are decreasing with time. This is presumably caused by the decrease in arable land in the catchment. From the mean annual deposits in the reservoir, around 35% is bed load and 65% is suspended load. The average annual specific suspended sediment yield in the catchment, is ca. 9 Mg/km(2).

Automated summary

The study compared the sediment deposited in a small reservoir in Poland with estimates of sediment yield from its catchment. Results showed a decreasing trend in sediment deposition in the reservoir over four different periods.