AbstractRiverine trichopteran (caddisfly) communities were sampled seasonally at eight locations over a two‐year period to determine if a multiple‐level release hydroelectric dam created a faunal discontinuity along the longitudinal profile of the Clearwater River in northern Idaho. The confluence of a non‐regulated river with a regulated reach provided an unusual opportunity to examine the recovery of community parameters in a semi‐regulated river.A marked discontinuity in the caddisfly community occurred in the regulated reach immediately below the dam, reflected by greatly reduced diversity and abundance values, and by shifts in functional relationships. These faunal changes are primarily attributable to the modified temperature, discharge, and food resources induced by river regulation. The non‐regulated river exerted a major mitigative effect on the trichopteran fauna below its confluence with the regulated reach, despite maintenance of semi‐regulated conditions. Trichopteran community parameters approached near normal levels concomitant with partial reductions in flow fluctuations, and partial recovery of temperature and food resources. In addition, the non‐regulated river provided an important colonization pathway for riverine species to enter the semi‐regulated lower reaches.