As invasive nonindigenous plants colonize streamside ecosystems, they can transform ecological functioning by replacing native species, altering interspecific interactions, and reducing biodiversity. As a result of the intimate relationship between the riparian areas and their associated freshwater ecosystems, alteration of the riparian community may be reflected in the freshwater ecosystem. Research conducted in arid climates repeatedly has demonstrated that invasive nonindigenous plants can reduce water quantity, but far fewer studies have investigated the effects on water quantity and aquatic ecosystem functions when nonindigenous plants invade mesic temperate riparian areas. We identified 35 studies published since 1981 documenting impacts to freshwater ecosystems in mesic temperate climates attributable to invasions of riparian areas by nonindigenous plants. These studies revealed impacts on water quantity and on ecosystem function related to water temperature, edaphic characteristics, and physical and chemical alteration in allochthonous inputs. However, our assessment of the results found great variability in hydrological and ecological responses, with the strongest evidence for changes to (1) the timing of allochthonous organic matter inputs, (2) organic matter decomposition rates in aquatic ecosystems, and (3) aquatic community structure. In addition, in watersheds densely colonized by nitrogen-fixing invasive species, stream eutrophication is likely to occur. Because of the variability in the existing research results, more studies need to be focused specifically on invaded mesic temperate riparian ecosystems. Furthermore, more research is needed to consider the impact of additional problematic plant species that have yet to be studied, the correlation between invasive plant density and aquatic ecosystem response, the relationship between the length of time a plant community has been invaded and the aquatic community's reaction, and if invasive plant management and native species reintroduction can help restore the aquatic ecosystem.