We have investigated the number of taxa and individuals in riffle both in the upstream and downstream of dam since before the bam operation started in 2005 in order to examine the biological response to impoundment of the dam. Stenopsyche marmorata and Hydrosyche orientalis (both net-spinning caddisfly) was the most commonly dominated at the downstream of a dam. S. marmorata was not found in upstream both before and after impoundment. On the other hand, H.orientalis was found before the impoundment.

Averaged wet weight of individuals for the fifth instar of Stenopsyche was larger than 350 mg and it was more ten times larger than Hydrosyche. It is well known that the net-spinning caddisfly dominate at the outlet of a lake and dam. We considered the origin of organic matter as food for them was not only phytoplankton but mainly periphyton for  S. marmorata.

During impoundment, water temperature at downstream site was 3-7 degrees higher than at upsteam site in all season. The substrate at the downstream site are composed by mostly larger size of stone. Therefore, the periphyton growth rate was fast and high primary productivity was maintained. This situation supports high secondary productivity and mechanism of ecological proccess at downstream is different from at upstream.