In most streams, assimilatory uptake dominates nitrogen (N) uptake. Assimilatory uptake is a temporary removal, and so understanding the fate of assimilated N is important to an overall understanding of stream N cycling patterns. In plant-dominated river reaches, N assimilation rates tend to be high, and the assimilated N is typically retained for longer than N assimilated by biofilm and other compartments. However, plants typically senesce at the end of the growing season and much of the biomass drifts downstream, potentially releasing a pulse of assimilated N as particulate organic N (PON). We hypothesized that a plant-dominated river reach would have N assimilation and inorganic N retention rates that exceeded typical rates in reaches dominated by biofilm, but that PON losses would offset much of the total assimilation. We measured assimilation, retention, and particulate N losses in the lower Yakima River, in Washington state, USA, where nearly the full river width contains water stargrass (Heteranthera dubia). We estimated gross assimilatory uptake as plant biomass from quadrat samples multiplied by N content. Next, we estimated annual inorganic N retention by doing a nitrate mass balance of a 35 km reach with no tributaries or groundwater inputs. To complete the nitrate mass balance, we used continuous nitrate and discharge data from USGS sensors. Inorganic N retention calculations do not include PON, and so we measured PON losses by deploying a drift net during plant senescence and measuring biomass and N content of drifting plant material. The annual gross plant N assimilatory uptake rate was 20 g N m-2 yr-1, with half subsequently released as nitrate, for an N retention rate of 10 g N m-2 yr-1. During senescence and drift, 10-20% of the retained N was released as PON, suggesting that PON losses are a meaningful portion of N loss, but even so, this plant-dominated river reach retained more N than a typical biofilm-dominated reach.