An outstanding question for snowmelt-dominated watersheds of the western US is the response of stream flow to changes in climate. We know little about mountain aquifers because they commonly involve structurally complicated rocks, extreme head gradients (ground slope angles 10-40°), and dramatically fluctuating recharge due to seasonal snow-melt. In general, the western United States is predicted to face warmer temperatures and more frequent and prolonged droughts, and we can expect to see a decrease in annual snowpack, earlier onset of snowmelt, and increased evaporation. Understanding streamflow generation under these climatic conditions will become increasingly important as hydrologic inputs change drastically and outputs are increasingly needed for human consumption. To improve our understanding of surface/groundwater interactions, we are simultaneously collecting surface water, subsurface, and precipitation samples at four gauged headwater catchments along a 1,500-m elevational gradient: (1) Green Lakes Valley (3,500 m); (2) Como Creek (2,900 m); (3) Gordon Gulch (2,400 m); and Betasso (1,830 m). All water samples are analyzed for geochemical and isotopic (δ18O, δD) composition. The average residence time for subsurface flow is calculated by comparing the smoothing of the δ18O input (precipitation) and output (streamflow) using a convolution algorithm. The calculation of residence times is also constrained by measuring concentrations of tritium (3H), a naturally occurring radioisotope, to better understand sub-surface transit times. A two-component mixing model will be used to determine source waters from old (reacted) waters and new (unreacted) waters. End member mixing analysis (EMMA) is a statistically unbiased technique that will also be used to identify the most important end members contributing to stream flow. The application of Principle Component Analysis (PCA) using all of the isotopic and geochemical measurements collected improves the accuracy of end member selection. Successful application of hydrograph separation and EMMA determines the proportions of sources contributing to stream flow and groundwater recharge.