Water forecasts

Detroit reflection--a historical photo

The Corps plays a major role in a coordinated and complex system to reduce flood risks and provide water for hydropower generation, fish and wildlife enhancement, navigation, recreation and other uses. Portland District's primary water management mission is to save lives and reduce property damage by reducing flood risks with measures both structural (such as dams and levees) and non-structural (such as improving the natural function of floodplains). We also assist communities in developing responses to flood risks and hazards.

As part of this mission, we own and operate 16 dams in Oregon, maintaining these dams with available resources to meet authorized purposes, the foremost of which is public safety. We also assess levees for integrity and viability. No combination of actions can prevent floods and no single agency can manage them.

Forecasts, reality and teacups

It remains impossible to accurately predict where and when flooding will occur because it is not possible to accurately forecast the weather more than a few days ahead. The amount of rain and variations in temperature over just a few days, for example, can strongly influence the timing and extent of runoff.  A combination of weather conditions, including heavy snow and unexpected warm rain, contributed to the historic floods of 1964 and 1996.

The Corps uses both short-term (10-day) and long-term (3-month) runoff forecasts to help determine the amount of space needed in its flood storage reservoirs to reduce flood risks. Year-round, the Corps uses the short-term forecast, produced by the National Weather Service Northwest River Forecast Center, to make the timeliest decisions possible.

The National Weather Service Northwest River Forecast Center is responsible for issuing official flood warnings and forecasts. The public is encouraged to visit their website for information about rising river levels. The Corps uses information on this site to help estimate river flows. Check out: http://www.nwrfc.noaa.gov/.

Managing summer reservoir levels

Lake levels vary between May and early September and also vary from year to year, depending on inflow to reservoirs from rain and snowmelt. The Corps stores and releases water in the reservoirs during this summer conservation season. Some uses, such as recreation, benefit from keeping water in the reservoir while others, such as irrigation and fisheries enhancement, benefit from the timed release of stored water.

Requirements under the Endangered Species Act and/or a Congressional authorizing document specify the release of a minimum amount of water from each dam to maintain downstream river flows for fish and wildlife. These releases also help maintain water quality and provide for purchased irrigation water. With these releases and surface evaporation during the drier summer months, many reservoirs “draw down” throughout the summer.

The Corps refers to long-term seasonal runoff forecasts of rain and snowmelt volume from the National Weather Service and Natural Resources Conservation Service to plan for the conservation season. Beginning in January, these water volume forecasts are available for the months ahead.


From September through December, the Corps gradually drains its reservoirs to specific, predetermined levels. This regains space to store water during the coming flood season. Exactly how much storage space will be needed remains uncertain from week to week, month to month, year to year.  For most of the dams, the Corps must maintain low water levels in the reservoir whenever possible from December 1 through January 31 to keep storage space available during these wet months to reduce flood damages downstream.

A water control diagram provides the guidelines and analysis for the Corps, along with coordinating agencies, to determine storage and releases for individual dams and across a system of dams. These efforts ultimately reduce localized damage within a specific river or tributary basin and in larger drainage areas such as the Willamette Valley.

They also consider the operational mission specific to that time of year, the current individual and system-wide capacity of the reservoirs and weather and stream flow forecasts. They also use real-time data collected from control points (stream gages) located downstream from a dam. These gages measure the depth of water from dam releases in combination with other uncontrolled runoff that creates the total downstream flow. All combined, this information allows reservoir regulators —most importantly in the winter and spring— to make more accurate and timely water management decisions.

The Corps uses information from the National Weather Service Northwest River Forecast Center to help estimate river flows. The U.S Geological Survey maintains the streamflow gages for river basins in Oregon. A graphic-like rule curve shows the maximum elevation in the reservoir for storing water during the year, with the exception of real-time flood operations. Since Big Cliff and Dexter reservoirs are not used for flood water storage, the Corps operates them differently. State and local agencies manage levee systems, floodwalls or other infrastructure that provides additional riverbank protection against high water. Areas with unregulated stream flows (with no dams or control structures upstream) or those not adequately protected by levees face increased risk of flooding when flows are high.

Related Resources

Reservoir teacup diagrams

Visit river basin “teacup” diagrams for real-time reservoir levels. Each project’s smaller “teacup” includes observed inflow, precipitation levels and its specific rule curve. The diagrams also show releases from dams for the past seven and 30 days:

Using the diagrams

Rolling your cursor over a dam symbol brings up a box with the current elevation of the water in the dam's reservoir, how that elevation relates to the Water Control Diagram, and the amount of water flowing into and out of the reservoir.

Clicking on the dam symbol brings up more detailed water elevation and flow information, including how water levels relate to boat ramp elevations.

The Corps of Engineers and other agencies collaborate on gathering and analyzing data regarding current and projected future reservoir and river level information. This information is hosted on the Northwestern Division Water Management site.