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.