Stable isotope tracers provide unique insights for understanding complex ecological interactions in watersheds. Stable isotope approaches are advancing rapidly due to improved analytical methods, lower costs, and increased knowledge of the factors that regulate isotopic variations in ecosystems. In this talk, we present results from 15 years of investigations of energy flow and trophic interactions using stable isotopes in streams and riparian zones of a forested watershed in northern California, USA. Combined use of mercury and light stable isotopes reveals information that illustrates how physical features of channel networks regulate the sources of energy and nutrients for consumers within stream networks. In our work, identification of the biogeochemical drivers of isotopic variations at the base of food webs has been useful for application of isotope approaches to food web analysis. All stable isotopes tracers in the study watershed are responsive to changes in stream productivity, but the differences in the scale of responses allows differentiation of multiple carbon and nutrient sources to aquatic and riparian consumers.