This paper aimed at the problem that the calculation of siltation amount of silt dam caused by gravity erosion because of gully bank collapse and slope collapse during the operation of 5th sub-region silt dam on the Loess Plateau. By the method of actual measurement and 3S technique generation and original design of reservoir capacity curve

the silt blocking amount and soil erosion mode of silt dam during running period were checked and analyzed. The results showed that

(1) The morphological discriminant coefficient (α')

which reflects the silt form of silt dam

is between 0.711 and 208.517

and the mean value is 48.238. it indicated that the other silt dams were Delta deposition except Yangshan

which were the Vertebral siltation because the critical number were less than 2.2; (2) The shape parameters of dam controlling area

such as area of silt surface

circumference

silt volume and discriminant coefficient of silt form

had great influence on gravity erosion. The calculation relation of gravity erosion amount and erosion modulus were given; (3) The range of the modulus of gravity erosion is 184.79~812.80 t/(km2·a)

the mean value is 360.68 t/(km2·a)

and the range of modulus of siltation and sediment production in dam controlling area was 225~2 719 t/(km2·a)

with a mean value of 1 382 t/(km2·a).The modulus of gravity erosion accounts for 26.1% of the modulus of sediment production. The above results showed that the reservoir sediment came from the hydraulic erosion and the gravity erosion of the reservoir area in the 5th sub-region of the Loess Plateau. The operation and management of the silt dam should prevent the gravity erosion from destroying the existing cultivated landand silt of channel base floweroded dam land. These could result in the silt dam being not able to be used efficiently.