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Our current understanding of water fluxes and flow paths within the mountain block is limited, and improved understanding is necessary to assess hydrology more accurately above the mountain front. Source waters and the processes controlling their mixing were characterized in the Merced River basin within Yosemite National Park, California, using 36

Both concentration‐discharge relation and end‐member mixing analysis were explored to elucidate the connectivity of hydrologic and hydrochemical processes using chemical data collected during 2006–2008 at Happy Isles (468 km2), Pohono Bridge (833 km2), and Briceburg (1873 km2) in the snowmelt‐fed mid‐Merced River basin, augmented by chemical data collected by the USGS during 1990–2014 at Happy Isles. Concentration‐discharge (C‐Q) in streamflow was dominated by a well‐defined power law relation, with the magnitude of exponent (0.02–0.6) and R2 values (p < 0.001) lower on rising than falling limbs. Concentrations of conservative solutes in streamflow resulted from mixing of two end‐members at Happy Isles and Pohono Bridge and three at Briceburg, with relatively constant solute concentrations in end‐members. The fractional contribution of groundwater was higher on rising than falling limbs at all basin scales. The relationship between the fractional contributions of subsurface flow and groundwater and streamflow (F‐Q) followed the same relation as C‐Q as a result of end‐member mixing. The F‐Q relation was used as a simple model to simulate subsurface flow and groundwater discharges to Happy Isles from 1990 to 2014 and was successfully validated by solute concentrations measured by the USGS. It was also demonstrated that the consistency of F‐Q and C‐Q relations is applicable to other catchments where end‐members and the C‐Q relationships are well defined, suggesting hydrologic and hydrochemical processes are strongly coupled and mutually predictable. Combining concentration‐discharge and end‐member mixing analyses could be used as a diagnostic tool to understand streamflow generation and hydrochemical controls in catchment hydrologic studies.

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Stable isotopes of the water molecule (δ 18 O and δD) for groundwater, lake water, streams, and precipitation were coupled with physical flux measurements to investigate groundwater-lake interactions and to establish a water balance for a structurally complex lake. Georgetown Lake, a shallow high-latitude high-elevation lake, is located in southwestern Montana, USA. The lake is situated between two mountain ranges with highlands primarily to the east and south of the lake and a lower valley to the west. An annual water balance and (δ 18 O and δD) isotope balance were used to quantify annual groundwater inflows of 2.5 × 10 7 m 3 /year and lake leakage outflows of 1.6 × 10 7 m 3 /year. Roughly, 57% of total inflow to the lake is from groundwater, and 37% of total outflow at Georgetown Lake is groundwater. Stable isotopes of groundwater and springs around the lake and surrounding region show that the east side of the lake contains meteoric water recharged annually from higher mountain sources, and groundwater discharge to the lake occurs through this region. However, springs located in the lower western valley and some of the surrounding domestic wells west of the lake show isotopic enrichment indicative of strong to moderate evaporation similar to Georgetown Lake water. This indicates that some outflowing lake water recharges groundwater through the underlying west-dipping bedrock in the region.

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