Hydrologic Alteration in the Upper Teton Watershed and its
Implications for Cutthroat Trout Restoration

Rob Van Kirk
Department of Mathematics
Idaho State University
and
Amy Jenkins
Idaho Association of Soil Conservation Districts

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Yellowstone cutthroat trout (YCT) abundance in the upper Teton River declined 95% between 1999 and 2003, while nonnative rainbow trout increased significantly. We investigated relationships between hydrologic alteration and YCT populations there. Hydrologic regimes in the Teton watershed are altered by withdrawal of irrigation water from tributary streams and by return via ground water pathways of a portion of this water. We estimated natural flow in the Teton River at the South Leigh gage and above the Crosscut Canal for water years 1989-2000 by adding surface diversions to daily regulated flow and subtracting inflow to the river resulting from recharge due to irrigation. We estimated the latter with an analytical model of ground water flow that incorporated recharge from direct precipitation, tributaries, and irrigation under current (sprinkler) and historic (flood) practices. Model hydraulic parameters agreed well with observed values, and mean absolute relative error between modeled and observed ground water discharge was 13%. Modeled hydrology of the Teton River under the flood irrigation scenario was very similar to that observed in the river prior to conversion from flood to sprinkler irrigation. Currently, diversion takes about 50% of combined tributary discharge during early summer, when water left in stream channels would remain as surface flow, provide YCT spawning habitat, and drive geomorphic processes. Altered peak flows in the upper Teton River are about 35% lower than natural, and altered winter flows are about 20% greater than natural under current irrigation practices and 30% greater under flood irrigation. Median annual alteration values were 21% at South Leigh and 13% at the Crosscut under current conditions and 27% at South Leigh under flood irrigation. These are relatively low when compared to the river’s natural flow variability and with other regulated streams in the Snake River basin. However, decreased peak flows and increased winter flows have significantly increased baseflow and decreased maximum/minimum discharge ratio in the upper Teton River. Linear regression showed that trout populations in the upper Teton River have responded to annual hydrologic variability, with YCT increasing in abundance relative to rainbow trout following years in which maximum/minimum ratios were high and the hydrologic regime was dominated by surface runoff rather than ground water. An analysis of six stream sites in the upper Snake basin showed that YCT do not persist in the presence of nonnative species where mean maximum/minimum ratio is below 10 and that rainbow trout do not successfully invade streams in which this value exceeds 14. Under current conditions, mean maximum/minimum ratios are about 11 and 14, respectively, in the upper and middle Teton. The Teton River between Bitch Creek and the Crosscut Canal appears to provide the greatest potential for immediate restoration of fluvial YCT in the Henry’s Fork watershed. The transformation of the upper Teton River from a runoff-dominated system to a ground water-dominated system has favored nonnative species and makes it unlikely that YCT can be restored there without restoring the natural hydrologic regime, although YCT numbers may rebound in the short term if climatic patterns result in years with high maximum/minimum discharge ratios. In the long term, restoration of the natural hydrologic regime would result in lowering of current ground water levels, some loss of wetland and riparian habitat associated with spring-fed streams, and decreased fall and winter flows. Return to flood irrigation would increase ground water levels and late summer river flows but would also increase hydrologic alteration and move the system further away from one that would give YCT an advantage over nonnative trout. A component critical to any effort to restore YCT in Teton Valley is restoration of the natural hydrologic and geomorphic processes in the Teton Range tributaries between the base of the mountains and the river, where remnant cottonwood forests indicate that aquatic productivity and habitat diversity was high prior to regulation.