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United States Department of Agriculture

Agricultural Research Service

Title: The Northwest Flood of February 1996 in Southwestern Idaho

Authors
item Huber, A
item Johnson, Gregory
item Cooley, Keith
item Hanson, Clayton

Submitted to: American Institute of Hydrology
Publication Type: Proceedings
Publication Acceptance Date: October 7, 1996
Publication Date: N/A

Interpretive Summary: Major winter floods of the interior Pacific Northwest typically result from strong jet streams transporting warm, moist air northeast from the Pacific across the Coast Range to the interior. The December 1964 storm which produced the flood of record for Reynolds Creek Experimental Watershed (109 m3/sec) was a prime example. Three decades later,a February 1996 storm system moved east from the Pacific on a course which narrowly missed southwestern Idaho. While streams draining the Blue Mountains and western and northern Idaho experienced major flooding, the Owyhees and RCEW had high within-bank flows; the missing ingredient for RCEW was precipitation. Rapid warming produced virtually complete snowpack melt up to 1600 m elevation in RCEW. The meltwater produced a sharp flow increase, peaking on February 8 at the RCEW Outlet Weir. Perma-nent research installations such as RCEW will inevitably be impacted by some events and missed by others. Knowledge of the watershed's physical and hydrologic characteristics and ongoing hydrologic monitoring concurrent with the nearby storm allows estimation of potential con- sequences of this event. A minor southeast shift of the February 1996 storm track could have contributed over 200 mm of warm precipitation to the extant snowpack, which would have resulted in record high streamflows in southwestern Idaho.

Technical Abstract: Understanding extreme hydrologic events can be aided by long-term obser- vation sites. Reynolds Creek Experimental Watershed (RCEW) in the Owyhee Mountains of southwest Idaho, has a baseline hydrometeorologic record. Major winter floods of the interior Pacific North-west typically result from strong jet streams transporting warm, moist air northeast from the Pacific across the Coast Range. The December 1964 storm which produced the flood of record for RCEW (109m3/sec) was a prime example. The February 1996 storm system moved east from the Pacific on a course which narrowly missed southwestern Idaho. While streams draining the Blue Moun- tains and western northern Idaho experienced major flooding, RCEW had high within-bank flows; the missing ingredient for RCEW was precipi- tation. Rapid warming produced virtually complete snow-pack melt up to 1600 m elevation in RCEW. The meltwater produced a sharp flow in-crease, peaking on February 8 at the RCEW Outlet Weir. Knowledge of the watershed's physical and hydrologic characteristics and ongoing hydro- logic monitoring concurrent with the nearby storm allows estimating of potential consequences of this and other events. A minor southeast shift of the February 1996 storm track could have contributed over 200mm of warm precipitation to the extant snowpack, which would have resulted in record high streamflows in southwestern Idaho.

Last Modified: 8/2/2014
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