Lakes can play a large role in structuring the chemical and physical templates of streams, yet they often are overlooked when stream function is evaluated. We examined how lakes within stream networks affect organic matter decomposition. We used a cotton-strip assay to evaluate cellulose decomposition potential (CDP) as loss of cotton-strip tensile strength in streams upstream and downstream of 4 mountain lakes in a lake district in the Sawtooth Mountains, Idaho. In addition, we used this assay to determine decomposition potentials along a longitudinal gradient in 1 stream–lake network. Decomposition coefficients (k) were 43% greater in lake-outflow streams than lake-inflow streams. Temperature explained 65% of the variability in decomposition rates across lake inflows and outflows, and decomposition rates in inflow streams were highly correlated with mean daily temperature (r  =  0.99). Conversely, decomposition rates in outflow stream were strongly correlated with total N/total P ratios (TN/TP; r  =  0.94). When temperature ranges were small, as was observed across 4 lake outflows and along a longitudinal gradient, decomposition rates appeared to be driven by differences in nutrient availability, mainly TP and, to a lesser extent, TN. Together, these results demonstrate that although temperature is a primary driver of decomposition, when temperature is similar across locations, nutrient availability can drive decomposition rates. Our results indicate that lakes within fluvial networks can modify stream function by altering both temperature and nutrient availability.