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Pathways of streamflow were investigated using geochemical tracers in ungauged headwater and higher-order streams in the semi-arid, forested, Valles Caldera, New Mexico. Combining diagnostic tools of mixing models and endmember mixing analysis (EMMA) shows that streamflow at East Fork Jemez River and San Antonio West (both > 120 km(2)) was controlled by mixing of three endmembers: near-surface runoff, subsurface flow and groundwater. The contribution of near-surface runoff was primarily controlled by soil moisture and evapotranspiration, and was much higher in wet years (2005 and 2007) than a dry year (2006) and during the spring snowmelt than in the summer monsoon. Streamflow at the San Antonio Toledo headwater catchment (55 km(2)) was dominated by subsurface flow alone, consistent with the distribution of coarse-textured, well-drained soils at higher elevations. During the snowmelt period and at catchments with higher near-surface runoff contributions, nutrient concentrations in streams were found to be higher, as a result of nutrient leaching from the upper organic-rich soil horizons by near-surface runoff. These results highlight the importance of seasonal and interannual variability of hydrology in semi-arid regions, and the role of flowpaths in controlling sources of nutrients in streams. Copyright (C) 2008 John Wiley & Sons, Ltd.