The aim of this study was to determine the ability of crustacean zooplankton outflowing from the dam reservoir to colonize new habitats of a river valley. Siemianówka Reservoir is a huge source of crustaceans for the outflowing Narew River and results in a 33- to 307-fold increase in summer biomass of potamozooplankton. Zooplankton samples were collected from a permanently connected oxbow lake located directly behind the reservoir and from 18 oxbow lakes 10 to 180 km from the dam. The results of our study showed that crustacean zooplankton of oxbow lakes generally showed low similarity to Siemianówka Reservoir. However, a high level of zooplankton similarity was observed among oxbow lakes themselves. This could suggest that the crustacean zooplankton of the oxbow lakes in the river valley show a metacommunity structure.