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Our flood risk management mission in the Willamette Valley

Portland District's Willamette Valley flood risk management mission is to save lives and reduce property damage.  During the wettest time of the year (mid-November through January), we operate our 13 dam and reservoir projects in the North and South Santiam, McKenzie, and Coast and Middle Fork Willamette river basins to help reduce flooding downstream.  We store incoming water during significant rain events, and then release it as river levels subside.  River levels may remain high for an extended time after rain events as we release stored water.  Whenever possible, we try to keep river levels at or below the “bankfull” levels established by the National Weather Service.

We cannot prevent floods, but we can significantly reduce the level of floods and the damage they cause.  Our dams can regulate about 88 percent of the total basin runoff that reaches the Jasper gauge above Eugene, 42 percent that reaches Salem, and 27 percent that reaches Portland.  Since 1996, they have provided an annual average of almost $1 billion in flood damage reduction. Click here for a map of the Willamette River Basin.

Spillway gates issues

graphic showing structural problemsOur Dam Safety program has found that most of our dam’s spillway gates are aging and may not operate properly when water levels are high and placing significant pressure on the gates.  This may result in a gate failing to open when desired, or becoming stuck in the open position when attempting to close it.  This would limit or perhaps even compromise our ability to control water releases from that dam.

We assessed the gates in 2010, which resulted in the replacement or repair of critical components at Lookout Point, Dexter, Fall Creek and Hills Creek dams.  Those repairs, however, do not completely eliminate the risk of uncontrolled releases from those gates. 

We continue to pursue the long-term rehabilitation of the gates.  All seven of Dexter Dam’s spillway gates have been strengthened over the past two years.  Strengthening of Big Cliff Dam’s gates is underway this fall.  The need for longer-term gate rehabilitation work remains at seven of our other dams.

For more information about the Willamette Valley dams’ gates, see:

Willamette Valley spillway gates: Posters

Willamette Valley spillway gates: repair status

Willamette spillway gates video on YouTube

Water level impacts

gate tracking diagramWe are reducing the risk of uncontrolled releases while continuing to provide flood reduction support through the winter.  We have lowered the pool elevations at which spillway gates need to be opened, to reduce pressure on the gates and the potential for their failure.  This in turn means the dams may need to release water earlier during flood events, leading to higher than usual downriver flows.

Click here for the pool elevations at which we must open spillway gates at each dam.

We intend to operate the projects much the same as in any other flood season.  However, the gate operating restrictions may mean increased water releases from our reservoirs during severe rainstorms or a series of storms.  The public downstream of our dams may see releases up to bankfull more often and for longer periods of time during and following storm events, as we work to lower the reservoir back to pre-storm levels in order to provide storage for future storms.

For current and projected Willamette reservoir and river elevations and flows, visit:

http://www.nwp.usace.army.mil/Missions/Water.aspx

http://www.nwrfc.noaa.gov/rfc/

Be prepared

We urge property owners and those living and working near the rivers to understand their level of risk and take preparedness actions.  For virtually all emergencies, county emergency management offices are the first sources of information, the first responders, and the designated authorities for deciding upon and announcing evacuations:

 Counties can declare emergencies and request support from the State of Oregon.  If the emergency is of sufficient magnitude, the state may request federal assistance, including Corps emergency response.

People should always remain aware of the dynamic nature of water and storms.  Relying on dams isn’t a substitute for smartly managing one’s property and being prepared for an emergency.  Flood preparation and insurance information is posted at FEMA’s FloodSmart website: http://www.floodsmart.gov.