EFFECTS OF GEOLOGY AND MODERN WATER MANAGEMENT ON HYDROLOGY AND FLUVIAL GEOMORPHOLOGY IN THE HENRY'S FORK WATERSHED, EASTERN IDAHO AND WESTERN WYOMING

The Henry's Fork of the Snake River watershed contains several hydrologically distinct types of streams. Some are fed predominantly by groundwater and show little variability in discharge across the water year. Others are fed predominantly by surface runoff from snowmelt and thus display much more variability in daily discharge throughout the year than the groundwater-fed rivers. In addition, hydrologic regimes throughout the watershed have been altered by irrigation storage and delivery. Because natural hydrologic regimes provide the water supply available for human use and because hydrologic regime is the primary physical driver of ecological processes in stream channels and floodplains, sound management of the watershed's resources depends on explicit knowledge of the relationships among basin geology and morphology, natural and regulated hydrologic regimes, and stream channel morphology. Thus, this project addresses the questions:

1. What drainage-scale lithologic and geomorphic factors determine variability in hydrologic regime across the watershed?

2. How do drainage- and local-scale geologic and geomorphic factors interact with hydrologic regime to influence stream channel characteristics?

3. How has alteration of hydrologic regimes affected channel characteristics, if any?

For the first objective, we find that basin morphology, but not lithology, affects the hydrologic regime. Smaller basins have a higher maximum / minimum flow ratio, as well as an earlier peak and a shorter low pulse. Larger basins have a lower maximum / minimum flow ratio, and a longer low pulse. The higher basins had the latest peak timing.

For objective two, we see that the channel width / depth is adjusted to the modern hydrologic regime, and is determined by stream size and basin and reach scale geology. Channel capacity, however, is explained by the natural hydrologic regime. Channels are less well adjusted to the natural hydrologic regime where there is little local alluvium.

For the third objective, we see 10 significantly altered streams, all of which are altered in different ways. General effects of hydrologic alteration are to decrease channel capacity and increase cross-sectional complexity.