Effect Sediment Accumulation In The Reservoirs Of Two Contrasting Lithologic Small Catchments In Yimeng Mountain Region Over The Past 50a

Construction of many small reservoirs has changed the process of surface sediment transport.Yimeng Mountain region is a typical representative of northern rocky mountain area.Granite and limestone are distributed in multiple areas.Erosion and sediment yield characteristics of two types of lithologic areas are different.In this study,one of the typical granite and limestone basins in Yimeng Mountain region,namely the Jiangzhuang and Quananzi catchment,was selected to compare and analyze the effect sediment accumulation in the reservoirs of two contrasting lithologic small catchment over the past50a.Three holes of sediment cores,A,B,C,D,E and F,were taken at the center of the Jiangzhuang and Quananzi catchment reservoirs along the main channel direction.According to the evolution history of human activities and environment in the area,the variation characteristics of¹³⁷Cs,²¹⁰Pb_ex,clay content and bulk density depth of each pore were investigated.The sediment dating was carried out by¹³⁷Cs and²¹⁰Pb_exx tracer method,and the average annual silt amount of the pond in different periods since the construction of the dam was calculated by referring to the original reservoir capacity curve.It is of great theoretical and practical significance to understand the characteristics and regular of the impact of reservoir construction on sediment transport process under two types of lithologic conditions,and to take targeted catchment management measures.The main research conclusions are as follows:?1?The activity of ¹³⁷Cs and ²¹⁰Pb_ex,clay content and bulk weight of reservoirsediment profile are complex with depth in the small catchment of Jiangzhuang,reflecting the general history of environmental evolution in the basin under the influence of strong human disturbance..The A^C of¹³⁷Cs came from the bottom to the bottom,and reached the peak in 1963.The distribution regular of²¹⁰Pb_exx in three boreholes shows A decreasing trend from top to bottom,while the distribution regular of B and C holes is more complicated.?2?The variation patterns of ¹³⁷Cs and ²¹⁰Pb_exx activity,clay content and bulk weight with depth were similar in the reservoir of Quananzi catchment,which is difficult to reflect the basic history of human activities and environmental evolution of the basin.The¹³⁷Cs distribution regular of D^F is roughly similar.As a result of the construction of the reservoir in 1959,the¹³⁷Cs of the three boreholes did not come from the bottom to the bottom from nothing,but all the bottom of the three boreholes quickly reached the maximum corresponding to 1963.The activity of²¹⁰Pb_exx in three boreholes decreases exponentially with depth.From top to bottom,there was no significant change in the content of grains in the three holes,and the bulk density was slightly increased.The differences of sediment profiles between the two reservoirs reflect the differences of lithologic background,soil properties,human activities and response to erosion and sediment yield in the two small basins.?3?Effect sediment accumulation of A,B,and C was between 394±142¹21±43 t·a^-1,with an average of 208±75 t·a^-1in Jiangzhuang catchment.The total sediment yield since the construction of the dam in 1953 is about 8902,11129,11395 t,with an average value of10475 t.?4?Effect sediment accumulation of D,E and F was between 22±7⁵9±21 t·a^-1,with an average of 41±15 t·a^-1in Quananzi catchment.The total sediment yield since the construction of the dam in 1959 was about 1975,2079,2237 t,with an average value of2097 t.?5?Effect sediment accumulation of Jiangzhuang reservoir is greater than that of Quananzi reservoir,which is caused by the difference of sediment yield strength under the influence of human activities.Brown soil is developed in the granite area,with thick soil layer,loose structure,easy to be disturbed,and strong human activity.In limestone areas,thin layer of brown soil is developed with strong resistance to erosion.It is interlaced with bare rock and is not easily disturbed.Human activities are limited.The variation trend of sediment retention effect in the near 50a liangtang reservoir reflects the recent characteristics and laws of human activity and sediment yield intensity response in two small lithologic basins.