The Rio Grande supplies water for endangered biota and a burgeoning human population, but little is known about sources and cycling of nutrients through the food web of this dryland river ecosystem. We described aquatic foodweb structure at 4 sites in the Rio Grande (New Mexico, USA) based on analysis of stable isotopes of C and N as tracers of sources of primary production and trophic positions of consumers, respectively. δ¹³C values obtained from primary producers (e.g., benthic algae and vascular plants) and consumers (macroinvertebrates and fishes) indicated that autochthonous (instream) production predominated as an energy source for fishes, but not for macroinvertebrates. Statistical comparisons of isotopic niche space among functional groups suggested that macroinvertebrates obtained dietary C from both instream and riparian (terrestrial) sources and were more broadly dispersed in isotopic space than were fishes. Macroinvertebrates that fed on riparian sources were less likely to be consumed by fishes. Baseline isotopic ratios changed in nonlinear fashion across sites, as did isotopic values of local consumers, and maximum food-chain lengths. The Rio Grande food web appears to be controlled predominantly at the reach scale by point-source effects related to inflow from tributaries, dams, and wastewater return.