FEMA Floodplain Maps Effective December 18, 2009
***Interactive Map Tool at bottom of page***
New Gresham and Sandy River Watershed preliminary maps summer 2016
Principal Flood Problems
Columbia River flooding in Multnomah County usually occurs in spring during the Columbia River Basin snowmelt freshet. However, intense winter rainstorms are the primary cause of flooding on the remaining Multnomah County streams, and they occasionally cause Columbia River flooding. Additionally, several localized ponding areas in the county are subject to flooding from seepage through levees during prolonged high Columbia River stages. Intense winter-storm runoff is the primary cause of flooding in the large ponding area along Johnson Creek.
The annual Columbia River snowmelt freshet occurs in May or June and has caused flooding in Portland during high-runoff years. The Willamette River floods during the spring freshet primarily as a result of backwater from the Columbia River. The June 1894, 1948, and 1956 floods, with recurrence intervals of 80, 48, and 18 years, respectively, were snowmelt freshets. Those flood events had peak discharges at The Dalles, Oregon, the nearest gaging station with a reliable discharge measurement of 1,240,000, 1,010,000 and 823,000 cubic feet per second (cfs), respectively; default Read more...
Changes since last mapping
As part of the countywide update, floodplain boundaries within the City of Fairview for portions of the Columbia River, Fairview Lake, Blue Lake, and Fairview Creek were revised based on topographic maps at a scale of 1:1,200 with a contour interval of 2 feet. Additionally, portions of the Columbia River, Sandy River, Willamette River, and Multnomah Channel were revised based on 2 foot contour interval topographic data developed from LiDAR data created by the Puget Sound LiDAR Consortium. The data can be used at a horizontal scale of 1:12,000 (1inch=1,000 feet) or smaller.
The floodway width at cross section F on the Sandy River was revised as part of the countywide update where new topography resulted in a mapped 1-percent annual chance floodplain that was narrower than the effective floodway. The floodway was mapped as coincident with the 1-percent annual chance floodplain.
In accordance with FEMA Guidelines and Specifications for Flood Hazard Mapping Partners, floodways which abut certified levees are to be mapped to the landward toe of the levee. Several locations where the effective mapping did not reflect this were revised and the floodway boundary was moved to the landward toe as determined by new topographic data (see the FIS for Columbia River at cross sections C, D, and Q and for the Multnomah Channel at cross sections D and E).
The floodplain boundaries for the remaining streams were digitized from the effective FIRM and Floodway panels. Aerial photography was used to adjust floodplain and floodway boundaries where appropriate.
A "KML" is a file that can be downloaded and opened directly in Google Earth, ArcGIS Explorer and other viewing software available on the Internet. The KML below is for the Special Flood Hazard Area (SFHA), also known as the "100-year" flood.
- Download KML (updated 2016) - Multnomah Co SFHA
- Flood Insurance Study (FIS) is the publication developed in conjunction with the Flood Insurance Rate Map (FIRM) and discusses the engineering methods used to develop the FIRMs. The study also contains flood profiles for studied flooding sources and can be used to determine Base Flood Elevations for some areas.
- Download County Flood Insurance Study - default Multnomah Co FIS (3.92 MB)
West Consultants has been contracted to revise the FIRMs for the Gresham area; DOGAMI has been contracted to remap much of the Sandy River basin (Risk MAP project: Lower Columbia-Sandy Watershed). Preliminary maps for both projects available at FEMA Map Service Center: https://msc.fema.gov/portal/advanceSearch
Snapshots around the County
None at this time
Note - For general information only - DO NOT use for regulatory purposes, especially for non-modernized areas: see "description" for source information and disclaimer
(hint: once in Viewer select "Basemap" in upper left corner for imagery background)