A study on soil erosion induced sediment yield by reservoir and pond deposits dating with 137Cs in small catchments of the hilly Sichuan Basin and the Three Gorges Region
川中丘陵区和三峡地区小流域侵蚀产沙的塘库沉积137Cs断代

There are a huge number of small reservoirs and ponds with quite high deposition rates in the hilly Sichuan Basin and the Three Gorges Region.Four small catchments with a drainage area of less than 1km2 each in Yanting and Nanchong of Sichuan Province and Kaixian of Chongqing were selected for this study.Incremental samples of sediment deposit profiles for 137Cs dating were collected in the four ponds to estimate the deposition volumes since 1963,and to analyse the specific sediment yields and the average erosion rates of those catchments.The highest specific sediment yields for deposition in the ponds is 1869 t·km-2·a-1for the Chunqiu Gully in Kaixian,and 701 t·km-2·a-1 and 710 t·km2·a-1for Wujia and Jiliu gullies in Yanting,respectively.The yield in the pond is about 566 t·km-2·a-1 for the Tianmawan Gully in Nanchong.Being small drainage areas,the four study catchments have little valley areas and relatively high channel gradients.The 137Cs distribution depths are close to the plough depth of 25 cm in the paddy field of the Tianmawan Gully.As the original valley areas above the dam in the three catchments in Yanting and Kaixian were predominantly occupied by ponds,little sediment has accumulated in the upstream valley of the ponds since the dam construction in the 1950s.The small gullies on slopes above the valleys are steep and the bedrock bottoms of the gullies indicate no sediment accumulation occurring there.The sediment delivery ratio should be close to 1 for the slopes.By the above analyses of the topographic characteristics and the 137Cs depth distribution in paddy fields of the valleys,no significant sediment accumulation occurs in the valley areas of the study catchments.So it is reasonable to use the specific sediment yields for deposition in ponds to represent the specific sediment yield and soil erosion rate of the study catchments excluding the Tianmawan Gully in Nanchong where the current water surface area of the pond used for estimating the yield is underestimated.Clearly,the annual precipitations and the land use conditions,especially the cropland ratios are the important erosion factors of the study catchments.On the other hand,the landform,soil erodibility,and bedrock bedding conditions are the significant erosion factors too.The Chunqiu Gully is underlain of the Jurassic Shaximiao Group with medium erosion resistance and high soil erosion rate.The Wujia and Jiliu gullies in Yanting are underlain of the Jurassic Penglaizhen Group with high proportion of sandstones so they have high erosion resistance and low soil erosion rate.