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Updating Boise River Floodplain Boundaries


Conducted in cooperation with
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Prior to this study, the last flood insurance study was conducted in 1978. Since then, the channel geometry and flood-plain features of the Boise River have changed due to land development and the natural movement of the river. As a result of these physical changes, the current Flood Insurance Rate Maps needed to be updated. Areas that would be inundated by floods along the Boise River are of interest to adjacent cities, the county, and the Federal Emergency Management Agency (FEMA). These parties need information on areas prone to flooding in order to preclude the uneconomic, hazardous, or unnecessary use of the floodplain, with regard to the location of homes, buildings, structures, roads, and other facilities.

USGS staff surveying Boise River and its banks.
USGS staff surveying Boise River and its banks, photos by USGS.

How the USGS Helps

Between October 1997 and December 1998, we collected stream channel cross-section data and documented elevation reference marks established for horizontal and vertical control. The total river length studied was approximately 32.5 miles, which included 26 miles of the Boise River main channel and an additional 6.5 mile reach around the south side of Eagle Island (South Channel of Eagle Island).

Key Results

  • Identified existing elevation reference marks and, where necessary, established new ones
  • Developed a 2-foot contour map of the river channel and flood-plain area
  • Determined water-surface elevations for the 10-, 50-, 100-, and 500-year floods using the surface-water computer model using the Hydrologic Engineering Centers River Analysis System (HEC-RAS) model. No analysis was done to determine the flood discharge values; the U.S. Army Corps of Engineers (1978) values were used.
  • Delineated the calculated floodplain and floodway extents on a 2-foot contour map
  • Prepared and submitted final maps and reports to FEMA


Hortness, J.E., and Werner, D.C., 1999, Stream channel cross sections for a reach of the Boise River in Ada County, Idaho: U.S. Geological Survey Open File Report 99-211.