Geochemical composition of beach groundwater, surface backwaters in return-current channels, and mainstem waters were measured before, during, and after the 1996 controlled flood in Grand Canyon. Similar, distinctive geochemical environments in return-channel sediments, back beach, and beach front positions within a reattachment-bar complex occurred at all study bars. The flood buried autochthonous vegetation at the study sites, resulting in significant increases in dissolved C, N, and P in beach groundwater and in surface backwaters. The trends observed in individual wells and site averages include measurable increases in dissolved ammonium, non-purgeable organic carbon (NPOC), and orthophosphate. Large standard errors resulted from large variation in concentrations between wells produced by their differing geomorphic settings. When comparing changes between pre-and post-flood samples from the same site, only DO, NPOC, and dissolved orthophosphate were significant. The largest pre-to post-flood changes occurred on a seasonal time scale. The magnitudes of seasonal changes following flooding were large compared to natural seasonal changes in nutrient concentrations previously observed. The nutrient composition changes occurring in groundwater and backwaters were not directly reflected in mainstem water compositions, although increases in mainstem primary productivity during high flows following the flood did correlate with periods of high nutrient concentrations in beach groundwater.