Mercury (Hg) is a global contaminant with its toxic organic form (methylmercury; MeHg) biomagnifying in food webs. Inputs of Hg to aquatic systems are predominately atmospheric, and ecotoxicologists have identified factors influencing river food web sensitivity to MeHg biomagnification. However, the relative strength of factors in determining MeHg contamination across broad spatial scales remains unknown. We assessed spatial variation and environmental factors driving MeHg concentrations in caddisfly genera Hydropsyche and Cheumotopsyche across a complex river basin (Colorado River basin, Texas, USA) exhibiting pronounced spatial gradients in land use and land cover (LULC) and water chemistry. Invertebrates and water chemistry were collected from 20 reaches from five rivers in the basin, and LULC composition for each reach was determined at riparian and whole watershed scales. Invertebrates were analyzed for total Hg (THg), MeHg, and stable isotopes of carbon and nitrogen. There were pronounced gradients in LULC and water chemistry across the basin, with reaches in two rivers (Concho and Pedernales) having greater forest cover. In addition, the Concho and upper Colorado river study reaches exhibited higher nutrients and salinity when compared to other study reaches. Preliminary analysis indicates a significant difference in caddisfly THg among rivers (p<0.001), with a marginally non-significant effect of reaches nested within rivers (p=0.0548). However, there was a significant river x reach interaction (p<0.001), indicating that caddisfly THg between upper and lower portions of rivers were not consistent among rivers. Elevated THg was found in the San Saba, Llano, and Pedernales compared to the Concho and Colorado. The Colorado and Llano rivers had greater THg concentrations in the lower reaches than upper, opposite from the remaining three rivers. This study provides a better understanding of the relative strength of local- and watershed-scale factors influencing MeHg contamination of food webs in spatially complex drainages and provides a foundation to assess sensitivity of specific river reaches to MeHg contamination.