The phytoplankton, zooplankton, and the macroinvertebrate snag communities of Black Creek, South Carolina, upstream and downstream of an impoundment were Investigated during 1982. Phytoplankton and zooplankton densities upstream of the impoundment were very low. The reservoir outflow contributed a large part of the zooplankton of all sizes to the stream immediately below the impoundment. Large copepods and cladocerans were almost absent from the community 2.2 km downstream leaving mainly nauplii, small cladocerans, and rotifers. The reservoir also released a moderate amount of phytoplankton into the stream which was only slightly reduced further downstream.