By Greg McMillan
“If you saw a heat wave, would you wave back?” –Steven Wright
The lower Deschutes River has hit a water temperature of 73 degrees each of the past two days at the Moody gauge. The weather forecast is for more hot weather for everyday of the foreseeable forecast. In other words, it’s not likely that the water temperatures at Moody will go down in the next few days.
There are many factors that cause the lower river to warm up. Foremost is how warm the weather is. But length of day, how warm it stays at night, and finally, the temperature as the river leaves the dams at the Pelton-Round Butte Hydropower Complex.
Here are the temperatures at the Reregulation Dam (the last of the three dams in the three dam complex, located 100 miles upstream) and Moody Rapids (located just above the confluence with the Columbia River).
The dam operators have control over the temperature of the water leaving the Pelton Reregulation Dam. That control is created by blending surface and bottom water from the Lake Billy Chinook reservoir at the Selective Water Withdrawal (SWW) Tower at Round Butte Dam, the upper most of the three dams. We recently took temperatures in the forebay of Round Butte Dam. The surface water was 72 degrees Fahrenheit, and the water at the bottom was 50 degrees Farenheit on the day we took measurements.
The dam operators seek to eliminate the thermal presence of the dams by setting a goal for water temperature that would be the same temperature as if the dams weren’t there. They do this using a complicated formula that includes the temperatures of the tributaries as they enter Lake Billy Chinook and air temperature at Redmond Airport.
In the meantime, cooler, cleaner water sits at the bottom of the reservoir. Last year, during the week of July 19, the dam operators conducted an experiment wherein they increased the bottom draw of reservoir water to 45% of the blend created by the SWW Tower to see how long it would last. They’ve told us that the cold water supply ran out in September. September, when the weather is cooler and the nights are longer.
When the temperature of the discharge water at the Reregulation Dam dropped, so did the temperature at Moody about a day and a half later (the length of time it takes for the water to travel 100 miles downstream).
We believe it’s time to do this “experiment” again. The dam operators will tell us that what they are doing is what is required by their operating license. That what they are doing is trying to return the river to its historic conditions. One wonders what the trout and steelhead in the river would have to say about this? Perhaps their answer will be to just die from thermal stress.
But the dam managers were operating under the same license last summer, and they were able to lower the temperature. Their license also calls for “adaptive management.”
And although we (the DRA) are looking into the historic conditions, we can tell you that climate change wasn’t part of the picture prior to dam construction in the 1950s. The data on pre-dam construction conditions are sparse. We do know that there has been a great deal of riparian recovery in the past thirty years. Shifting to more efficient systems has decreased irrigation withdrawals. Sewage treatment that didn’t exist upstream from the dams now is in place. Do we really want to return to those conditions?
If this heat wave continues, we can’t predict what will happen with the river, because none of us who have been on the river, even for as long as most of the DRA board members have, don’t remember an event like this. As a result, we call on the dam operators to discharge cooler water to improve biological conditions in the lower river.