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Friday, May 18, 2012
Covering Dallas, Monmouth, Independence, Falls City and surrounding areas since 1868
February 21, 2012
Flood. It's the dreaded "F-word."
As Oregonians, we are no stranger to flooding. The rivers and creeks that provide our drinking water, our sources of recreation, and the aesthetic beauty that makes Oregon such an attractive place also give us a source of frustration and helplessness.
The topographic nature of the Willamette Valley, having been formed by a mega-flood, serves as a depository for all the precipitation that falls east of the Coast Range and west of the Cascades. The Willamette Valley Basin drains an area just under 12,000 square miles. The network of streams and rivers flow to the Willamette River, which heads north, and empties into the Columbia River. The Columbia then drains to the Pacific Ocean.
Flood management efforts have been focused on restricting flow through the use of dams, and through channelization. By capturing and holding supplies of water, we have the ability to release measured amounts at intervals that limit potential hazardous conditions. By directing the flow of creeks through populated areas in channels, flow rates are increased, leading to water quality issues.
Do either of these options truly have an effect on frequency or intensity of flooding?
Recent history would say `No,' considering the two 100-year flood events that have occurred within the last 20 years. But could those two events have been much more devastating without the controls in place? It's tough to say.
According to FEMA, the Willamette River has actually declined in peak flows over the last 150 years. The Albany river gauge recorded a peak flow of 340,000 cubic feet per second (cfs) in 1861, down to a mere 117,000 cfs in 1996. While the maximum discharge has dropped, the frequency of events has risen.
If the Willamette River has been "tamed" through flood management along its tributaries, then what is to blame for this increase in recent flood events?
Perhaps the larger cause is due to the lack of the Valley's historic prairie wetlands. Less than 1 percent of historic wetland prairies still exist in Oregon. These former wetland areas have been converted into farmland or paved over, reducing their ability to provide natural holding areas for excess water. Not only do wetlands hold excess water, but they release that water over a period of time, allowing for a more controlled dispersion.
As more wetlands are reclaimed, water holding capacity in the Willamette Valley increases. There is no way to maintain the agriculture production levels currently in practice and restore all the former wetlands. Would that be enough to mitigate the increase in flood events we are experiencing? Probably not.
What can be done? There are many projects being implemented and planned to help increase infiltration of precipitation. Rain gardens and catchment basins provide small-scale solutions. Many of these projects add up to drastically increased rates of infiltration rather than an abundance of surface runoff and an overloading of municipal stormwater systems.
Rather than one big solution to this problem, we may be facing a situation where "every little bit counts." Projects like converting your home driveway to pervious pavement or installing a catchment basin in your frontyard adds up in the long run.
In the last two decades, Polk County has experienced several moderate to severe flood events. Resources are available to assist county residents to identify their level of risk during future flood events.
For information regarding replanting after a flood and other mitigation activities, contact Polk Soil & Water Conservation District at 503-623-9680.
For additional resources, and to document flood damage after an event, contact the Community Development Department of Polk County at 503-623-9237.
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Zach Sutton is a resource management technician with the Dallas-based Polk Soil and Water Conservation District. He can be reached at 503-623-9680 or via e-mai to zachary.sutton@polkswcd.com.
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