Countywide FEMA Floodplain Maps Effective September 3, 2010
Principal Flood Problems
Flooding in Umatilla County usually lasts from March through June, although flooding may occur during the winter. Flooding is typically caused by a combination of rainfall and snowmelt. When snowmelt augments rainfall runoff, the floods have both high peaks and large volumes; however, damaging stages seldom last longer than one or two days. default Read more...
Countywide Update - what changed?
As part of the countywide update for Umatilla County, the hydrologic and hydraulic analyses of portions of the Umatilla and Walla Walla Rivers, Meacham Creek, and Squaw Creek were reviewed and updated as necessary.
In 1998, the USACE, Portland District, conducted a hydrologic analysis of flooding sources within the Umatilla Indian Reservation as part of a flood evaluation study. This study included approximately 26 miles of the Umatilla River upstream of Pendleton, Meacham Creek, and Squaw Creek. This analysis was compared to a more recent hydrologic analysis conducted by the Oregon Water Resources Department (OWRD) in 2006 to determine if the addition of newer data was sufficient to significantly change the USACE 1998 peak discharge for the 1-percent-annual-chance flood.
Umatilla River discharge from the USACE study was determined using streamflow data from the Pendleton gage (gage No. 14020850), the West Reservation boundary gage (gage No. 14021000), and the gage near Gibbon (gage No. 1402000). Records from these gages were statistically analyzed utilizing the standard log-Pearson Type III distribution. The USACE analysis agreed well with the more recent hydrologic analysis conducted by the OWRD and was therefore used.
Meacham Creek discharge was determined at the Gibbon gage (gage No. 14020300). Both the
USACE and OWRD studies included a flood frequency analysis for the gage. Peak discharge for the 1-percent-annual-chance flood from the USACE study was significantly higher than the discharge from the OWRD study. Because the OWRD study used a longer period of record as well as computed probabilities to estimate the peak discharges, the newer discharges were used in the hydraulic analysis.
No gaging station exists on Squaw Creek. The 1998 USACE study conducted a direct correlation analysis to establish synthetic flow records for Squaw Creek by transferring observed annual peak discharges for the Gibbon gage (gage No. 14020300). An updated hydrologic analysis was performed using the current regional flood frequency regression equations for the North-Central Region of Eastern Oregon. Because the USACE study used data transferred from a significantly larger drainage basin, discharges developed from the OWRD regional regression equations were used.
Cross sections for the Umatilla River through the Umatilla Indian Reservation, Meacham Creek, and Squaw Creek were developed from the USACE hydraulic model. Additional cross sections were determined using topographic mapping with a contour interval of two feet and aerial photography. Water-surface elevations for these streams were computed using the USACE HEC-RAS computer program.
In the City of Milton-Freewater, shallow flooding was determined along the Walla Walla River left overbank due to decertification of the levee protecting the city. This flooding has a calculated average depth of one foot. The approximate without-levee analysis for the Walla Walla River was conducted in August 2007 using HEC-RAS.
Approximately 29 miles of the Umatilla River floodplain boundaries were revised using topographic mapping with a contour interval of two feet. The portion of the Umatilla River floodplain redelineation begins 4.4 miles above its mouth and continues 29 miles to a point above the Union Pacific Railroad.
- 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 Nov. 2016): Umatilla SFHA
- 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:
None planned at this time.
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)