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The river rises when the water comes

My wife and I recently were rafting on the upper Rogue River, and while we were sitting on a gravel bar having an adult beverage we literally could see the river rise so abruptly that we almost had our Tahitis float away. My guess it was something about water coming out from Lost Creek Lake, but how does that work and how long does it take changes at the dam to alter river flows downstream?

— Greyson, email submission

Well, Greyson — can we call you Grey? — what you and your rafting party discovered was just how water releases from Lost Creek Lake can impact what’s happening downstream on the Rogue, and roughly how long a change in releases triggers change downstream.

The U.S. Army Corps of Engineers typically releases water from Lost Creek Lake into the Rogue River in spring and summer based largely on recommendations from the Oregon Department of Fish and Wildlife.

Usually those changes are no greater than 150 cubic feet per second, either up or down, with the maximum rate of decrease or increase at 300 cfs, according to Corps documents.

So it sounds like you were in the midst of a reservoir release change, and how long that change takes to impact flows downstream is a function of exactly where you are.

Back when Lost Creek Lake went online in 1977, the Corps and ODFW surveyed how long it took water-release increases to reach three key U.S. Geological Survey water gauges, as well as the gauge at the city of Grants Pass water-treatment plant.

Based on the Corps data, it takes about four hours for the increased releases from Lost Creek Lake to travel the 18.4 miles to Dodge Bridge, the first water gauge downstream from the reservoir. It takes about two more hours to traverse the 12.6 miles to reach the gauge just below the old Gold Ray Dam site.

Though the dam has been gone a decade now, the stream gauge remains intact.

The next gauge is at Grants Pass, and the little pulse of extra water needs about four hours to travel the 23 miles between those gauges, according to the Corps.

The last Rogue gauge is at Agness, about 27 river miles east of Gold Beach. It takes about 20 hours to cover the 76 miles from Grants Pass to Agness, according to the Corps.

That’s the last gauge on the Rogue, so there is no accurate measure of the water travel time to the estuary near Gold Beach.

So, to travel from Lost Creek Lake to Agness takes about 30 hours.

Send questions to “Since You Asked,” Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; or by email to youasked@rosebudmedia.com. We’re sorry, but the volume of questions received prevents us from answering all of them.