Hydrology and geomorphology of the Snake River in Grand Teton National Park

Abstract

The influence of significant tributaries that join the Snake River within 10 km of Jackson Lake Dam (JLD) mitigate some impacts resulting from nearly 100 years of flow regulation in Grand Teton National Park. I analyzed measured and estimated unregulated flow data for all segments of the study area by accounting for tributary flows. The magnitude of the 2-yr recurrence flood immediately downstream from JLD decreased 45% since 1958 relative to estimated unregulated flows, whereas that downstream from Buffalo Fork, the largest tributary, decreased 36%. There has been no long-term progressive geomorphic change on the Snake River resulting from dam regulation. I mapped the bankfull channel on four series of aerial photographs taken in 1945, 1969, 1990/1991, and 2002 and analyzed channel change in a geographic information system. Periods of low-magnitude floods (1945 to 1969) resulted in widespread deposition whereas periods of high-magnitude floods (1969 to 1990/1991 and 1990/1991 to 2002) resulted in widespread erosion; channels narrowed and widened by as much as 31%. I mapped three distinct deposits within the Holocene alluvial valley. The lower floodplain covers 3.5% of the mapped area in the form of abandoned channel and inset, channel-margin facies and has inundating recurrence intervals of one to two years. The upper floodplain covers 36% of the mapped area, is composed of abandoned channels and bars, is higher in elevation than the lower floodplain, and is inundated by floods with recurrence intervals greater than 10 years. The lowest Holocene terrace covers 35% of the mapped area and is approximately 1 m higher in elevation than the upper floodplain. Though the lowest terrace has not been inundated or built since 1945, the two floodplain deposits have been developing since before 1945. Flood magnitudes have decreased throughout the study area as a result of regulation, but these decreases are mitigated downstream from tributaries. Dam operations have not resulted in long-term progressive channel change or the development and abandonment of floodplain deposits. However, channel change is now dependant on the frequency of high-magnitude floods, and the frequency with which the two floodplains are inundated has been reduced.