Abstract:River reaches in the mountain area are often in the alternation between bedrock exposure and alluvial covered.The field survey found that at least several rivers in the transition zone between Qinghai-Tibet Plateau and Sichuan Basin have the abnormal phenomena that bed rock is exposure in gentle slope reaches and alluvial deposits are covered in the steep slope reaches.In order to study the abnormal phenomena

taking the Longxi River

which is the secondary tributary of Minjiang River as a case study

field observation and data collection were carried out many times.By extracting basin’s terrain data

the longitudinal profile of the truck stream were obtained and analyzed.Moreover

combined with the pervious flume experiment results

sediment transport and distribution characteristics in Longxi River was further illustrated

and the conclusions were obtained as the lithology of Longxi River basin is mainly sandstone

from the head waters

the slopes of truck stream have four distinguish reaches with different slopes and bed surface characteristics

namely

steep

gentle

steep and gentle successively

with average slope of 8.1%

0.2%

6.2% and 1.2% respectively

the two steep reaches are alluvial covered but the two gentle reaches are bed rock exposure

The steep slope reaches are supplied from colluvially derived sediment of the hillslopes

particles from sand to large boulders will gradually form relatively stable interlock structures with high resistance

and the bed roughness of narrow and deep channel enhances the erosion resistance of the bed surface

which will inhibit bedrock exposure.The interlocked structures also result in very low sediment transport rate to the downstream gentle slope reaches;moreover

the rare sandstone grains transported downstream are easily broken into smaller ones

which are more mobile

generating the bedrock exposure phenomenon to be prominent; steep slope reaches covered by alluvial deposits will suppress the erosion rate of narrow and deep bedrock and further affect the evolution of basin geomorphology

for e.g.

promote stream piracies.