 The Corps estimated that the higher spillway would maintain the trapping efficiency of the SRS at or above 31 percent—the fraction being trapped without the higher spillway—for 5 to 10 years after completion of the project.The SRS is about upstream from the confluence of the North Fork Toutle River with the Toutle River and from the larger river's mouth on the Cowlitz River.The structure is about north-northeast of Portland, Oregon.