A geochemical and end-member mixing analysis (EMMA) is undertaken in Devil Canyon catchment, located in southern California, to further understanding of watershed behaviour and source water contributions after an acute and extensive wildfire. Physical and chemical transformations in post-fire watersheds are known to increase overland flow and decrease infiltration, mainly due to formation of a hydrophobic layer at, or near, the soil surface. However, less is known about subsurface flow response in burned watersheds. The current study incorporates EMMA to evaluate and quantify source water contributions before, and after, a catchment affected by wildfires in southern California during the fall of 2003. Pre- and post-fire stream water data were available at several sampling sites within the catchment, allowing the identification of contributing water sources at varying spatial scales. Proposed end-member observations (groundwater, overland flow, shallow subsurface flow) were also collected to constrain and develop the catchment mixing model. Post-fire source water changes are more evident in the smaller and faster responding sub-basin (interior sampling point). Early post-fire storm events are dominated by overland flow with no significant soil water or groundwater flow contribution. Inter-storm streamwater in this smaller basin shows an increase in groundwater and a decrease in soil water. In the larger, baseflow-dominated system, source water components appear less affected by fire. A slight increase in lateral flow is observed with only a slight decrease in baseflow. Changes in the post-fire flow regimes affect nutrient loading and chemical response of the basin. Relatively rapid recovery of the chaparral ecosystem is evidenced, with active re-growth and evapotranspiration evidenced by the fourth post-fire rainy season. Copyright © 2008 John Wiley & Sons, Ltd.