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The chemical composition of surface waters of nine streams with drainage sizes ranging from 44 to 8900 km(2) within a semiarid catchment in the southwestern United States was examined over the monsoonal season of 2006. Primary goals were to determine the impact of monsoonal rains on linkages between landscape cover features and water chemistry in a semiarid river network and to identify whether drainage size influences the temporal variability in water chemistry of streams. Landscape cover features (geology, soil, and vegetation types) were quantified for the subcatchment upstream of each study site and the riparian ecotone. Processes of binary mixing, dilution, and concentration were identified by end member mixing analysis (EMMA). Results showed that most chemical constituents corresponded to geological features at the basin scale, but other constituents (TSS and PO(4)(3-)) corresponded more closely to riparian features. Importantly, statistical relationships between land cover and water chemistry differed between baseflow and monsoonal conditions, suggesting that seasonal changes in hydrologic routing and water sources ( e. g., overland flow versus groundwater sources) have important consequences for stream chemistry. In contrast with mesic drainages, temporal variability in water chemistry increased with spatial scale. The variety of contributing water sources and the relative effect of dilution and concentration in each spatial scale may be underlying processes that explain differences across spatial scales. Observed patterns in the Gila basin suggest that climate variability interacts with hydrologic routing to influence spatial patterns of stream water chemistry in arid basins. Observations made here may be similar to other systems with strongly seasonal climate patterns, and the underlying determinants of stream chemistry change with drainage size.