FEMA Floodplain Maps Effective March 2, 2010
***See Interactive Map Tool at bottom of page***
Principal Flood Problems
Flooding in Yamhill County is caused by intense rainfall from large winter storms moving inland from the Pacific Ocean. This often results in simultaneous flooding on multiple streams such as that which occurred in December 1964. Damaging floods may occur any time between late October and the end of April. Major floods usually occur in December, January, and February. Major floods generally rise from low water to crest in 24 hours or less.
Five large floods with discharges greater than 200,000 cubic feet per second (cfs) were recorded during 25 years of record (1948 to 1973) at the Willamette River upstream gage at Wilsonville, Oregon, approximately 6 miles downstream of the Yamhill County boundary. The largest of these floods occurred in December 1964 with a discharge of 335,000 cfs and a recurrence interval of 120 years. Two other floods of comparable magnitude to the 1964 flood occurred in 1861 and 1850. Although no stream gage data are available for these floods, it is believed they each had an approximate recurrence interval of 100 years.
The largest recorded flood on Yamhill, North and South Yamhill Rivers, and Willamina Creek occurred in December 1964. The peak discharge at the U.S. Geological Survey (USGS) gage on South Yamhill River near Whiteson was 47,000 cfs, with an approximate recurrence interval of 60 years. There are no discharge records available for Yamhill Creek, therefore, the magnitude of the December 1964 flood is unknown. Other major Yamhill River basin floods occurred in January 1972 and, most recently in January 1974. These floods had peak discharges of 37,600 cfs and 37,000 cfs at Whiteson with approximate recurrence intervals of 17 and 15 years, respectively.
There are no stream-gage records available for Chehalem Creek or Hess Creek; however, based on records on North Yamhill River at Pike, the nearest gauged small stream, the December 1964 flood is estimated to have had approximately a 50-year return period on Chehalem and Hess Creeks. Chehalem Creek rises rapidly after rainfall, and, in extreme instances, the creek may rise 1 foot per hour.
Records of past floods on the remaining study reaches in Yamhill County are not well documented. Flood damage has been minor in unincorporated areas due to the sparse development of their floodplains.
The floodplain boundaries for most flooding sources were digitized from the effective FIRM and Floodway panels. Aerial photography and USGS topographic maps were used to adjust floodplain and floodway boundaries where appropriate. Floodplain boundaries of a number of streams in and around the City of McMinnville were revised using topographic mapping with a contour interval of two feet. These reaches include three miles of Baker Creek, four miles of Cozine Creek, and one mile of North Fork Cozine Creek. For North Yamhill River three miles were redelineated. From its mouth to a point 0.2 miles above the Willamette and Pacific Railroad, the right overbank was redelineated; from this point to a point 0.2 miles upstream of State Highway 99W, both overbank areas were redelineated; and from this point to a point 1.3 miles upstream, the right overbank was redelineated. For South Yamhill River ten miles were redelineated. From its mouth to a point 0.5 miles upstream, the left overbank was redelineated; from this point to a point 1.0 mile upstream of SE Three Mile Lane, both overbank areas were redelineated; and from this point to a point 0.8 miles upstream of the McMinnville Municipal Airport runway, the right overbank was redelineated.
- 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.
- 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 KML: Yamhill_Co_SFHA (2.64 MB)
- Download County Flood Insurance Study: default Yamhill Co FIS (3.16 MB)
Snapshots around the County
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Yamhill River North of McMinnville - 2007 (Oregonian)
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)