The invasive New Zealand mudsnail, Potamopyrgus antipodarum, can reach high densities and reduce or displace native North American invertebrates. It was first documented in the Portneuf River in 2000 during a 5-year drought when peak flows were roughly half of long-term mean peak flows. Benthic invertebrates were compared at 10 sampling locations during 2004 and following a 2006 spring flood when peak flows were 3 times greater than the previous 10-year mean. A data set collected in the same manner during 1969 allowed for comparison before the P. antipodarum invasion. In fall 2004, P. antipodarum densities reached over 400,000/m² and comprised up to 97% of the invertebrate community. Sites dominated by P. antipodarum had reduced species richness and lost between 9 and 20 invertebrate taxa that were present historically, mostly EPT taxa and native gastropods. Following the 2006 spring flood, P. antipodarum densities were reduced 22-80% with a similar reduction in macrophyte abundance. The 2006 invertebrate composition was more similar to that recorded in 1969 due to return of Trichoptera taxa Helicopsyche and Brachycentrus occidentalis, and increased densities of Hydropsyche (Trichoptera) and Simulium (Diptera). High flows may help decrease the effect of P. antipodarum in regulated river systems.