| Mountain springs are commonly treated as stable groundwater inputs that sustain flow and buffer stream temperature in headwater catchments, but their hydrologic persistence and thermal behavior remain poorly constrained. This thesis evaluated hydrologic intermittency and thermal heterogeneity of mountain springs in the Gibson Jack Watershed, a semi-arid headwater catchment in southeastern Idaho, USA. I combined continuous wet/dry and temperature records, repeated discharge measurements, sinusoidal temperature modeling, generalized linear and additive models, paired air-stream temperature analysis, stable isotopes, Bayesian mixing models, and chlorofluorocarbon groundwater age tracers to characterize spring persistence, source water, residence time, and thermal behavior. Springs varied substantially in both hydrologic and thermal function. Discharge changed through time at approximately 70% of sites, and about 45% of monitored springs dried during the study period. Measured springs accounted for a large fraction of outlet flow during summer baseflow, but not all downstream discharge, indicating additional groundwater inputs, unmeasured springs, channel storage, or other sources. Stable isotope results showed overlapping spring, stream, and snow signatures, suggesting snowmelt-influenced recharge and strong mixing across the spring-stream network. Apparent groundwater ages ranged from approximately 42 to 48 years but were not reliably predicted by elevation or distance to mapped faults. Spring temperatures were consistently damped relative to air temperature, yet differed in mean temperature, seasonal amplitude, and timing. Together, these results show that Gibson Jack springs are dynamic groundwater features whose resilience depends on subsurface storage, flow-path mixing, residence time, and stream-network connectivity. Keywords: Mountain Springs, Water Temperature, Hydrologic Intermittency, Groundwater, Gibson Jack Watershed |