Event Title

Effects of Jackson Lake Dam and Tributaries on the Hydrology and Geomorphology of the Snake River, Grand Teton National Park, Wyoming

Presenter Information

Nicholas C. Nelson
John C. Schmidt

Location

Eccles Conference Center

Event Website

http://water.usu.edu/htm/conference/past-spring-runoff-conferences

Start Date

27-3-2006 11:30 AM

End Date

27-3-2006 11:45 AM

Description

Geomorphic and hydrologic analyses of the Snake River in Grand Teton National Park (GTNP) indicate that flow contributions of tributaries mitigate impacts of regulation. Since a flow regime change in 1958, regulation resulted in a 43 and 35% decrease in estimated unregulated flows immediately downstream of Jackson Lake Dam (JLD) and at Moose (43 km and 5 tributaries downstream of JLD), respectively. Geomorphic evidence indicates that some channel characteristics are more sensitive than others to this decreasing influence of flow regulation. First, entrainment of tracer rocks suggests that the ability of the Snake River to mobilize its bed increases downstream. A greater proportion of the bed became active, and the mobilized clasts moved further, in the two study reaches furthest downstream. Second, repeat mapping from aerial photographs suggest that some changes in channel form are the result of flow regulation and some are the result of climatically driven changes in runoff determined by tributaries. Initial decreases in flows due to regulation may have caused the observed channel narrowing between 1945 and 1969, and greater precipitation causing greater natural flows may have resulted in the subsequent channel widening between 1969 and 1990. Third, flow models were used to obtain the magnitudes of flows necessary to inundate two floodplain surfaces in 4 reaches from JLD to Moose. Recurrence intervals and inundation periods were similar for a narrow, inset floodplain in all 4 reaches, suggesting that this surface developed due to regulation. Recurrence intervals for a much broader and higher floodplain decreased downstream from 9 to 3.2 years and inundation periods increased downstream from 1.1 to 3 days immediately below JLD and at Moose, respectively. This suggests the upper floodplain was formed prior to regulation of the Snake River. Thus, the effects of flow regulation on bed mobility and connectivity between the channel and the upper floodplain decrease downstream as tributaries supply additional streamflow. However, the development of the inset floodplain associated with regulated flows has occurred throughout the study area. These studies indicate that tributaries may reduce most but not necessarily all of the impacts of flow regulation on the geomorphology of the Snake River in GTNP.

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Mar 27th, 11:30 AM Mar 27th, 11:45 AM

Effects of Jackson Lake Dam and Tributaries on the Hydrology and Geomorphology of the Snake River, Grand Teton National Park, Wyoming

Eccles Conference Center

Geomorphic and hydrologic analyses of the Snake River in Grand Teton National Park (GTNP) indicate that flow contributions of tributaries mitigate impacts of regulation. Since a flow regime change in 1958, regulation resulted in a 43 and 35% decrease in estimated unregulated flows immediately downstream of Jackson Lake Dam (JLD) and at Moose (43 km and 5 tributaries downstream of JLD), respectively. Geomorphic evidence indicates that some channel characteristics are more sensitive than others to this decreasing influence of flow regulation. First, entrainment of tracer rocks suggests that the ability of the Snake River to mobilize its bed increases downstream. A greater proportion of the bed became active, and the mobilized clasts moved further, in the two study reaches furthest downstream. Second, repeat mapping from aerial photographs suggest that some changes in channel form are the result of flow regulation and some are the result of climatically driven changes in runoff determined by tributaries. Initial decreases in flows due to regulation may have caused the observed channel narrowing between 1945 and 1969, and greater precipitation causing greater natural flows may have resulted in the subsequent channel widening between 1969 and 1990. Third, flow models were used to obtain the magnitudes of flows necessary to inundate two floodplain surfaces in 4 reaches from JLD to Moose. Recurrence intervals and inundation periods were similar for a narrow, inset floodplain in all 4 reaches, suggesting that this surface developed due to regulation. Recurrence intervals for a much broader and higher floodplain decreased downstream from 9 to 3.2 years and inundation periods increased downstream from 1.1 to 3 days immediately below JLD and at Moose, respectively. This suggests the upper floodplain was formed prior to regulation of the Snake River. Thus, the effects of flow regulation on bed mobility and connectivity between the channel and the upper floodplain decrease downstream as tributaries supply additional streamflow. However, the development of the inset floodplain associated with regulated flows has occurred throughout the study area. These studies indicate that tributaries may reduce most but not necessarily all of the impacts of flow regulation on the geomorphology of the Snake River in GTNP.

http://digitalcommons.usu.edu/runoff/2006/AllAbstracts/8