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Idaho: Boise River Watershed

When a significant percentage of a state’s population resides in a watershed with significant flood risk, that watershed will receive focused attention by many agencies with different perspectives. The Boise River watershed, located in southwestern Idaho, is home to over 600,000 residents, more than one-third of Idaho’s population. Despite upstream federal storage projects and community participation in the National Flood Insurance Program, the watershed has more than $10 billion of infrastructure at risk for flood damage. The watershed is one of the highest flood risk priorities for USACE Walla Walla District.

To reduce this risk, many agencies are currently involved in studies to better prepare for and respond to flood events, or to mitigate risk. For example, USACE is conducting a General Investigation with the Idaho Water Resource Board for flood risk reduction and water supply. FEMA completed the Risk MAP Discovery process and is developing updated Boise River flood insurance rate maps (FIRMs). Ada City-County Emergency Management has recently completed an updated County Hazard Mitigation Plan and is interested in developing inundation maps to plan emergency flood response activities. The National Weather Service (NWS) is developing inundation map libraries to post on its Advanced Hydrologic Prediction Service webpage. All of these activities rely on floodplain models and maps.

The Idaho Silver Jackets capitalized on the team’s ability to network, coordinate and cooperate to successfully leverage data, analyses and other resources to develop one version of the “truth” about flood risk in the Boise River watershed. The Silver Jackets team provided the forum to coordinate analyses and resources of several agencies to develop a series of Boise River inundation maps. The hydraulic model used to create the inundation maps was initially developed during a USACE GI study. Local governments contributed additional funds, technical assistance and GIS support to partner with USACE to use the model to map additional flows under the Planning Assistance to States Program. USGS furnished updated river gage data and analyses. NWS provided technical review and funded placement of the inundation map library on its webpage. The Idaho Silver Jackets team developed and executed a watershed flood risk outreach strategy.

The end result was a consistent picture of flood risk and a hydraulic model used by many in the watershed for several purposes at a cost savings to agencies and the taxpayers. Availability of the flood inundation maps on the NWS webpage provides community leaders and residents with a better understanding of flood hazards and assists with planning, mitigation and flood response. Boise residents can go online to evaluate flooding scenarios and determine if flood insurance is necessary. Visit the NWS webpage to access the interactive flood maps posted at: http://water.weather.gov/ahps2/inundation/inundation_google.php?g_datatype=depth&wfo=boi&gage=bigi1 

Google Earth fly-through of 0.2-percent chance flood event (500-year chance) on the Boise River in Idaho. Boise State University’s famous blue turf football stadium is on the left.