Freshwater mussels (family Unionidae) form dense, species-rich assemblages which are often the dominant macroinvertebrate biomass in streams. Since Unionids are long-lived species, shells and the soft tissue can be important nutrient storage sites in eutrophic systems. Urban streams often have elevated nutrients which impacts stream health and can negatively impact downstream ecosystems. The retention of nutrients such as carbon (C), nitrogen (N), and phosphorus (P) in Unionid mussels, however, are not well documented in urban streams. In July 2023, two restored urban streams (Glade and Snakeden) in Reston, VA were stocked with adult Eastern elliptio (Elliptio complanata) across a ~200 m reach in each stream. We sampled these populations in August 2023 and 2024 to measure changes in nutrient content one year post-restoration. Mussel tissue mass significantly increased at Glade but not at Snakeden. Both sites, however, had similar changes in tissue N and P content. N content decreased from 2023 to 2024 while P content increased. Changes in elemental composition after one year post restoration may be due to acclimation to elemental composition of food resources in these urban streams. Adding mussels to the streams increased C,N, and P storage in the benthos as compared to stream reaches with no mussels. Collectively, Unionid assemblages can be a significant source of C, N, and P retention in urban streams, which may have important implications for nutrient management and mussel restoration efforts.