Physicochemical data, adult caddisflies, and benthic macroinvertebrates were sampled in February 2008 from two sites upstream and two sites downstream of Mae Ngat Dam in northern Thailand. Water quality differed among sites. Downstream water velocity was variable depending on water release from the dam. Conductivity and total dissolved solids were higher at upstream sites than downstream sites. High filamentous algal growth just below the dam produced high dissolved oxygen, pH, and BOD. Dissolved oxygen and pH decreased rapidly downstream. The species richness of adult caddisflies was similar at upstream and downstream sites; however species composition differed. Upstream sites had more net-spinning filter feeders that require running waters. Downstream caddisflies included some grazing species that are tolerant of still water. Macroinvertebrate diversity was much higher at upstream sites than at downstream sites. Upstream sites had a more diverse insect fauna, whereas non-insects, such as snails, were more common at downstream sites.