Countywide FEMA Floodplain Maps Effective December 19, 2006
***See Interactive Mapping Tool at bottom of page***
Principal Flood Problems
Flood season in the Willamette River Basin extends from October through April, with most major floods occurring in December and January. Floods in the areas are normally general in extent rather than limited to a few streams. They are caused primarily by heavy rainfall, sometimes augmented by snowmelt at a time when the soil is saturated by prior rainfall or is frozen. The Willamette River remains above bankfull stage for up to 10 days following a major flood, while smaller tributaries recede in 4 or 5 days. Floodwaters may remain standing in the floodplain for several days after streams recede.
Damage from flooding in Polk County is mainly to farmlands, largely in terms of flooded crops and pastures and fields and bank erosion. However, there is some flooding nearly every year; this results in damage to roads and bridges necessitating increased maintenance and repair patrols during flooding.
The largest flood on record on the Willamette River occurred in 1861 with an estimated peak flow of 500,000 cubic feet per second (cfs) at Salem. Many large floods have occurred since then. The December 1964 flood, unregulated, would have been the largest Willamette River flood since 1861. However, operation of seven upstream flood control storage dams reduced its estimated potential 472,000 cfs flow to 309,000 cfs and its flood level by 7 ft. at Salem. default Read more...
Changes since last mapping
- 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: default Polk Co SFHA KML (339.55 kB)
- A 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 Polk Co FIS (7.31 MB)
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Snapshots around the County
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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)