It’s irrigation season and, as usual, the Middle Deschutes is getting killed.  Below are graphs showing flows in the Deschutes just below the Central Oregon Irrigation District and North Unit Irrigation District diversions at the North Canal Dam just upstream from the Mt. Washington bridge in Bend.  As you can see, over the past two weeks fluctuations in the river have been abrupt and dramatic.  This strands fish and other aquatic life and stirs up sediment that chokes spawning beds. (CORRECTION: in the section below on gas bubble disease, I missed a zero. Gas bubble disease is a real problem around 3,000 cfs, not 300 cfs as originally stated and now corrected. Sorry about that. Flows this morning out of Bowman Dam are are 2,020 cfs with inflows at 4,500 cfs.)

As you can see in this next graph, the dramatic changes in flows occur all the way to Lake Billy Chinook.

The picture is even worse on the Crooked River, although for a different reason.  Designed for flood control and to supply water to Ochoco Irrigation District, the Prineville Reservoir must release water in the spring when very high flows are coming in.  This release is happening now, as you can see in the graph below.

The release of 350 cfs from April 18th through the 20^th was a temporary “pulse” flow, intended to encourage salmon and steelhead smolts to head downstream and out to the ocean.  The river then went down to a level requested by OID for delivery to their patrons.  Over the past two days, however, flows ramped up dramatically to the current level of 1,620 cfs!  This is due to high temperatures that are forecasted for the next few days.  Flows into Prineville Reservoir are currently very high as you can see below.

The Ochoco Mountains have very different geology than the eastern Cascades.  The eastern Cascades have young, porous lava and snowmelt seeps down into the rock and eventually into the aquifer before reemerging as springs.  The Ochocos are much older, are far less permeable, and are subject to “flashy” runoff.  In high temperatures the snowpack will melt quickly, be expressed as surface runoff, and may overfill the reservoir.  Accordingly, water must be released now to guard against the potential for requiring excessive, flood stage flows.

A full reservoir is a good thing, but flows in the Crooked River reaching 3,000 cfs have caused gas bubble disease in the past with significant negative impact on fish including mortality.  High flows have the potential to be bad for a fish population that was already decimated by flows over only 10 cfs last fall. Ideally, snow in the Ochocos would slowly melt over many months, constantly refilling the reservoir through the spring and summer as irrigation withdrawals occur.  A full reservoir now but without replenishment may still result in low flows later in the year.