Wild trout and hydrologic response to restoration of incised streams with improved water use for ranching and fisheries

Abstract

AbstractIn the face of climate change and dwindling water supplies, this long‐term monitoring study examines the effectiveness of restoring two incised and degraded streams (Crystal Creek and Black Slough) to benefit livestock ranching and wild trout populations in a spring creek and wetland complex, central Idaho, USA. By raising the incised streambeds and connecting to historical floodplains, the restoration aimed to raise groundwater levels to create subsurface saturation conditions to sustain grazing forage and hay production without flood irrigation; diverted flows from the Big Wood River previously used for flood irrigation were instead routed through the restored systems. The restoration used natural channel design methods to reconstruct 5.2 km of Black Slough and 8.4 km of Crystal Creek to emulate a downstream reference reach (Willow Creek); within the relic incised channels, off‐channel oxbow ponds and wetlands were created while filling other segments. Additionally, screened gravels were placed in the reconstructed channels. We monitored before–after restoration changes in channel morphology, groundwater levels, wetlands, instream flows, spawning substrates and redds, fish populations, and invertebrates for the restored creeks and reference reach over 10+ years. Following restoration, average groundwater levels increased by 0.35 m, mean annual flows at Willow Creek (receiving stream) increased from 0.64 to 1.11 m3/s, and average days per year that baseflows were below 0.28 m3/s decreased from 44.5 to 5.4 despite a decrease in average annual precipitation (16.97 to 11.53 cm). Moreover, residual pool depths increased (<0.18 to >0.90 m), spawning substrate sizes increased (0.1 to 19.0 mm), and egg‐to‐fry trout survival rates increased from <20% to >90%. Six years after restoration, fish diversity increased from three to seven species, matching reference conditions. We also found dramatic increases in Brown Trout and Rainbow Trout redd counts (7 to 161 and 17 to 143, respectively) and relative abundance (0.013 to 1.166 trout/m and 0.029 to 0.222 trout/m, respectively). Invertebrate indices of restoration effectiveness were positive for both restored streams but negative for the reference reach. Overall, this study exemplifies how restoration reduced consumptive water use, created sustainable streams with hydrological connections and ponds and wetlands, and improved trout populations.