What causes flooding in the Boise area?
Flooding in the city of Boise typically occurs in one of the following ways: flash flooding from a heavy rainfall event in the Boise foothills, heavy snowmelt (when combined with rain) in the foothills, or flooding of the Boise River.
Severe thunderstorms in the foothills can result in flash flooding. Runoff can be significant and very little time can pass between when the thunderstorm starts and flood flows begin. Flows can rise very rapidly and carry heavy debris, but typically last a short period of time. The peak season for thunderstorms are June - September.
High runoff can occur from the foothills from rapid snowmelt, especially when warm rain falls on existing snow pack. In these situations, flow rates may not rise rapidly but may last for several days or weeks.
The city of Boise operates and maintains a series of flood control structures on each of the four main foothills gulches (Stuart Gulch, Crane Gulch, Hulls Gulch and Cottonwood Creek). Most of these structures were either installed or upgraded following a major wildfire in the fall of 1996. While these facilities provide vastly improved flood protection, these will not completely protect property in an extreme flooding event.
Boise River Flooding
River flooding is substantially different. There are three upstream dams on the Boise River drainage managed by the Bureau of Reclamation and the U.S. Corps of Engineers that provide for both irrigation water storage and flood response. The need for water storage and space reserves for flood response is a balancing exercise based upon snow pack levels, historic runoff records of snow pack, and reservoir level conditions.
Late spring is the critical time of year when storage is maximized and reserves for flood response are at a minimum. Flooding generally comes from controlled reservoir releases that are known in advance and are fairly long in duration. Attempts are made to keep flows below 6500 cubic feet per second (cfs) in the spring. However, this flow rate is generally exceeded about once every five years. For the Boise River, 7,000 cfs is considered to be at the lower level flood stage; 16,600 cfs is considered a 100-year flood event. For anticipated flows in the Boise River, visit the National Weather Service website.
Flood Plain Management
The city of Boise has a Floodplain Ordinance for the Boise River and foothills gulches. The ordinance applies to properties located in a floodplain. Federal Emergency Management Agency (FEMA) insurance rate mapping applies to development within identified floodplains and areas of shallow flooding.
Questions regarding floodplain areas within Boise should be directed to the Planning and Development Services Department at 384-3830.
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