The mountain watersheds of the Sierra Nevada supply the majority of California's water, but this supply has always been highly variable. The 2012-2016 drought in California has demonstrated that this water supply is also highly vulnerable to increasing temperatures and/or reduced precipitation. Not only did the 2012-2016 drought reduce water supply for human use, but it also led to unprecedented forest mortality and fire damage. Unfortunately, the fire suppression strategy that was nearly uniformly applied to mountain forests during the 20$^{th}$ century may have exacerbated the effects of drought by increasing vegetation density and thus increasing evapotranspiration and precipitation interception. Could restoring fire regimes to their pre-European settlement condition increase water yield from these forested catchments? Such a policy would also have the potential to restore the ecological function of landscapes and reduce the risk of catastrophic fires (such as the 2013 Rim Fire) by reducing fuel loads.

This dissertation studies the hydrological and landscape-level ecological effects of restoring a frequent, mixed severity fire regime to the Illilouette Creek Basin in Yosemite National Park. A combination of field measurements, historical data analysis, remote sensing, and modeling approaches are employed to strengthen the argument by providing multiple lines of evidence. There is limited data available for Illilouette Creek Basin during much of the four decades in which the new fire regime became established, inhibiting direct evaluation of the fire regime's effects. Nevertheless, a variety of different metrics and analyses indicate a number of important changes that can be attributed to the restored fire regime: increased landscape diversity (including reduced forest cover), increased soil moisture and streamflow (both according to measurements and hydrological modeling), and decreased drought stress (both according to observations and from hydrological modeling).