Tangjiabo River valley，the terrain sloping gently，is a vast area of land between gullies with average slope 110 45' amounts to 85.400 of the drainage area;Gullies there are cut slightly，the ditch density in the greater gullies being only 0.7一0.8km/km2.With respect to climate，there is smaller rainfall and less strong rainstorm.Under the conditions of landforms and climate described above，there would be no strong soil erosion. However，the hydrological data showed that eroded modulus per year in this valley is up to 4，500t/km2.It has been found through intensive investigation of soil erosion and calculation of eroding capacity in different ways of erosion that greater eroded modulus in this valley is caused by geological-geomorphological and other factors. In this valley，there is a soil erosion way切produce a great quantity of sand一seasonal congelifraction. It is calculated that the congelifraction of this kind may produce a sand amount being equal to 36.5一41 .8%of the total sand produced in this valley.Such facts are common in a number of counties near Dingxi County.