16-018 Snake River Basin snowpack, inflows, post-wildfire conditions create potential flood risks; Corps encourages seasonal flood awareness

Published April 14, 2016
Dworshak Spillway

Dworshak Spillway

SNAKE RIVER BASIN – An April 1 near-average snowpack and current seasonal runoff forecasts throughout the Snake River Basin prompted regional water managers to U.S. Army Corps of Engineers-Walla Walla District rate the 2016 spring flood potential at normal to slightly above normal. 

“Although most areas typically at-risk of seasonal flooding haven’t experienced any major issues yet this year, the amount of snowpack remaining in the mountains still poses the chance that even regulated flows may approach flood stage, especially if significant precipitation or unexpectedly warm temperatures occur,” said Steve Hall, Walla Walla District’s water-management program manager.

The Walla Walla District’s area of operations includes about 107,000 square miles, primarily encompassing the Snake River Basin, in parts of six states -- Washington, Oregon, Idaho, Wyoming, Utah and Nevada.

Regional Weather Conditions - October 2015 through March 2016. Cumulative precipitation amounts within the Walla Walla District varied from 117-174 percent of average for the lower-Snake River area, and from 98-139 percent of average for the middle- and upper-Snake River areas. 

Basin Seasonal Precipitation - October 2015 through March 2016 (basin precipitation data were obtained from the National Weather Service (NWS) River Forecast Center (RFC), Portland, Oregon.)

Columbia River above The Dalles - 131 percent of normal

Snake River above Ice Harbor - 125 percent of normal

Henrys Fork (upper-Snake) River Basin - 139 percent of normal

Middle Snake Tributaries - 124 percent of normal

Malheur-Owyhee-Boise River Basins - 98 percent of normal

Salmon River Basin - 120 percent of normal

Payette River Basin - 136 percent of normal

Grande Ronde River Basin - 117 percent of normal

Clearwater River Basin - 144 percent of normal

Palouse River Basin - 174 percent of normal

Basin Streamflows - October 2015 through March 2016.  Unregulated streamflows for major basins in the Walla Walla District were 92 percent of average for the Snake River near Heise, Idaho; 108 percent of average for the Boise River at Lucky Peak Dam, near Boise, Idaho; 118 percent of average for the North Fork Clearwater River at Dworshak Dam, near Orofino, Idaho; and 92 percent for Lower Granite Lake inflows on the lower-Snake River near Pomeroy, Washington.

Basin Snowpacks. Snowpack measurements reported by the Natural Resource Conservation Service (NRCS) on April 1, 2016, varied between 102-114 percent of average in the lower-Snake River area, 67-115 percent of average in the middle-Snake River area, and 96-110 percent of average in the upper-Snake River area. Individual sub-basin snowpack reports are available on the NRCS website http://www.wcc.nrcs.usda.gov/basin.html.

Runoff Volume Forecasts. The April 1, 2016, forecasts of spring runoff varied between 39-129 percent of average throughout the District. In general, the forecasts for most sub-basins within the Snake River Basin are just below average to just over average. The April through July runoff volume forecast for the Snake River at Lower Granite Dam is 19.5 million acre-feet (AF) or 98 percent of average.

Reservoir Status. As of April 1, 2016, storage for major reservoirs within the Walla Walla District is normal and varies between 28-99 percent full. Most major reservoirs with flood-risk-management responsibility have adequate space available based on the current volume forecasts and ten-day weather forecasts.

The Jackson and Palisades reservoir system (upper-Snake River, Idaho and Wyoming) is currently about 66 percent full. Based on the system’s 99-percent-of-normal water-supply forecast, there is more than two-times the required flood-risk-management space available. Therefore, the current operating objective is to refill the system while meeting irrigation requirements.

The Boise River reservoir system -- Anderson Ranch, Arrowrock and Lucky Peak dams -- is currently about 72 percent full. Flood-risk-management space is currently meeting requirements based on the April 1 water-supply forecast. Current operations call for maintaining adequate space for potential flood flows while refilling and releasing water for irrigation.

Dworshak Reservoir (North Fork Clearwater River, Idaho) is currently transitioning to refill operations while releasing spring augmentation flows per NOAA’s Federal Columbia River System biological opinion to benefit juvenile salmon and steelhead outmigration. Spring snowmelt in the Clearwater sub-basin appears to be occurring earlier than normal.

Mill Creek Dam and Bennington Lake (Walla Walla, Washington) -- The current flood potential for Mill Creek is normal, based on current snowpack and reservoir capacity. Bennington Lake, Mill Creek Dam’s off-stream storage reservoir, was filled earlier in the month to its normal recreation pool level of about 10-percent capacity.

Additional reservoir storage summaries are available on the NRCS website http://www.wcc.nrcs.usda.gov/basin.html.

The Corps coordinates reservoir system operations with other federal, state and local reservoir managers throughout the region to minimize the potential for seasonal flooding throughout the region, while retaining enough water in storage reservoirs for irrigation and other water-supply needs. Doing so requires constant monitoring and data analysis of ever-changing snowmelt, inflows and weather forecasts during seasonal-flooding months -- typically January through early June. Earlier in the season, conditions are more dynamic and flood potential is less-predictable.

“By the first of April, most of what we’re going to see in snowpack accumulation has peaked, making it somewhat easier to predict what affect precipitation and temperatures will have on reservoir inflows. Flood potential within the district is reevaluated daily throughout the month of April and May,” Hall explained. “Those living in areas where wildfires occurred last year should be especially alert to potential flooding conditions.”

Wildfires in Washington, Idaho and Oregon during 2015 resulted in large, exposed, burned areas, highly prone to flash flooding and erosion. Walla Walla District provided technical assistance to assess post-wildfire conditions and associated flood risks in Clearwater and Idaho counties, Idaho because of the elevated threat to the local communities. For this spring, the greatest threat of flooding in these areas would be caused by rain-on-snow events in April and May. The district is monitoring conditions and coordinating with state and local jurisdictions in the event additional requests for assistance or flood support are needed.

Corps emergency management staff communicate with local officials to obtain on-site observations from communities in which flooding frequently occurs.

BE PREPARED ~ Disasters and emergency situations -- like flooding -- can occur anywhere, often with little or no prior warning. Corps officials encourage everyone to keep local emergency management contact information handy, keep an eye on evolving weather and streamflow conditions, and be familiar with emergency action plans for your specific location. Be prepared and stay informed so you’ll be ready to react if an emergency occurs near you.

BE SAFE ~ The Corps of Engineers discourages in-water recreation during this time of year because of cold water temperatures, higher flow velocities, floating debris and brush along the river banks. Please, be safe around rivers and streams! Visitors with children and dogs should be very cautious about allowing them to approach fast-moving water. Heavy rainfall and higher flows can saturate earthen shorelines, creating unstable walking surfaces.

STAY INFORMED ~ Weather forecast information and extreme weather warnings can be found on the National Weather Service (NWS) website http://weather.gov.  NWS Forecast Stations serving areas within the Corps’ Walla Walla District area of operations include:

NWS Spokane <http://www.wrh.noaa.gov/spokane>  - serves northeastern and far-eastern Washington (includes Garfield and Asotin counties) and northern Idaho

NWS Pendleton <http://www.wrh.noaa.gov/pendleton>  - serves south-central Washington (as far east as Columbia County) and northeastern Oregon

NWS Riverton <http://www.crh.noaa.gov/riw/>  - serves western and central Wyoming

NWS Boise <http://www.weather.gov/boise>  - serves southwestern Idaho and southeastern Oregon

NWS Pocatello <http://www.weather.gov/pocatello>  - serves southeastern Idaho

NWS Missoula <http://www.wrh.noaa.gov/mso/>  - serves central Idaho and northwestern Montana 

NWS Elko <http://www.wrh.noaa.gov/lkn/>  - serves most of northern Nevada

NWS Salt Lake City <http://www.wrh.noaa.gov/slc/>  - serves Utah

The Corps works with states, counties and other public entities to provide necessary resources and information.  The Corps does not have authority to provide disaster assistance directly to individuals. 

The first responsibility for protecting life, homes and property from flood damage rests with the individual. Local governments and agencies, such as flood control districts, may share in this responsibility, and together form a community's first line of defense in preventing flood damages.

Occasionally, however, local resources are not able to minimize the effects of flooding. The Corps’ flood assistance program is intended to supplement state and local governments and special-purpose districts when more help is needed.

Walla Walla District is prepared to assist states and municipalities with flood-management support. That assistance could include technical expertise, supplies and materials, equipment or contracts for emergency flood-fighting work. District flood support teams and technical experts are ready to deploy should local emergency managers request Corps assistance.

State and local agencies needing disaster assistance from the Corps should contact the Walla Walla District Emergency Management Office at 509-527-7146, or 509-380-4538.

Individuals and business owners are encouraged to contact local emergency management agencies to ensure they understand how to prepare, respond and recover from a flood.



Release no. 16-018