The Low Down on High Temperatures in the Lower Deschutes River
It’s been a rough summer. No one anticipated the fish die offs and warm water temperatures to the degree we experienced in June and July.
Fortunately, we dodged the bullet with a second heat wave. It was of short duration, and then broken by cooler temperatures, cloud cover and some rain.
But it was long enough to see a trend emerge regarding temperature management at the Pelton-Round Butte Dam Complex. The dam operators were already warming up discharge temperatures, and warming the lower river, until we got a break from the heat. They were once again artificially raising the temperature of the lower river in the face of already naturally warming temperatures. Here’s what it looked like (these temperatures are from the Pelton Reregulation Dam tailrace):
During the same time frame, here is what was happening down at the mouth of the Deschutes River:
There are several observations to be made here:
- As the weather warmed, so did the temperature at the Pelton Reregulation Dam tailrace. This is in accordance with Portland General Electric/Confederated Tribes of the Warm Springs Reservation “Without Project Temperature” calculation that is based on a mathematical model. The WPT calculation estimates (or models) what the water temperature would be if the dams were not present. WPT is then achieved by mixing bottom and surface water at the Selective Water Withdrawal Tower at Round Butte Dam, before it is passed downstream to Pelton Dam and the Reregulation Dam. Here is the equation:
Water Temperature Pelton = 2.8+(0.79)(Water Temperature into LBC)+(0.071)(7-day Ave Air Temperature Redmond Airport)
- This PGE model uses the water temperatures of the tributaries (Metolius, Middle Deschutes and Crooked Rivers) entering Lake Billy Chinook, along with air temperature at Redmond airport, to derive the calculated target temperature (WPT) for dam discharge.
- The lower river, running for 100 miles below the dams, has a daily swing of nearly six degrees during the summer. The tributaries above Lake Billy Chinook have a similar swing in daily temperatures, varying by up to six degrees between morning and late afternoon temperatures.
- The discharge temperature at the dam complex only varies about 1 to 1 ½ degrees daily.
- The temperature discharged from the Reregulation Dam tailrace has a major influence on lower Deschutes River water temperatures.
- The temperature management model warms the lower Deschutes River in spring and summer, but cools it in the fall. Based on annual temperature curves, the spring/summer warming is much greater than the cooling in the fall. Therefore, the lower Deschutes is experiencing net warming during a year-long cycle.
- The temperature model for the dams increases temperature in the lower Deschutes when the lower river is already warm (at times too warm) in spring and summer.
It should be noted that the stated goal of temperature management using the Selective Water Withdrawal Tower is to “eliminate the thermal presence” of the Pelton-Round Butte Complex.
These observations made us curious. How can you manage water temperature with the goal that the dams would be thermally invisible, and yet not have the daily variances that mimic a natural river?
In trying to figure this out, we also wondered what tributary water temperature is being used to calculate the Without Project Temperature goal? Is it the average or mean daily temperature? Is it the minimum daily temperature? This would be the most beneficial temperature for the biology of the river during heat waves. Or is it the maximum, warmest temperature of the day?
We turned to the Portland General Electric/Confederated Tribes of Warm Springs Reservation annual water quality report that was recently submitted to the Federal Energy Regulatory Commission for the 2014-operating year to try to answer these questions.
It turns out that the tributary temperature used to calculate the Without Project Temperature, and corresponding blend at the Selective Water Withdrawal Tower target for dam discharge, is based on a 7-day average of the peak (warmest or maximum) temperature of each day in the 7-day calculation. This means that the overnight cooling, and cooler temperatures that prevail throughout most of the day, are ignored. These cooler temperatures are the temperatures that fish and other organisms rely on to recover from the brief daily period of peak temperatures during summer heat waves, and especially in desert rivers. It should also be noted that the maximum temperature is only of short duration daily. But the model acts as if it is the only temperature of the day.
The red line in the graph above indicates the temperatures from the tributaries above Lake Billy Chinook used in calculating Without Project Temperature (WPT).
In other words, the dam operators are warming the river during spring and summer well beyond what would be its normal state without the dams because they are artificially managing discharge temperature based on the maximum temperature of the above reservoir tributaries, not the minimum or average daily temperature of the tributaries.
Charles Huntington, working under contract to PGE, did the seminal work that was done on understanding water temperature behavior in the lower Deschutes. The work was completed and published in April of 1999. It became the foundation of how water temperature at the dams would be managed.
In the published results of that study, the impact of temperature changes to the biology of the lower river was only minimally examined. The most discussion was in regard to changes in the timing of emergence of salmonid alevins from spawning gravel. The report concludes that juvenile emergence would happen sooner in warmer water conditions. No impact to the success rate of emergence is noted, or suggested, as a benefit of warmer temperatures.
The question regarding what the impacts and effects on other biota (life forms) is asked, but never answered in the report. Impacts to resident fish species, aquatic insects, aquatic plant life and algae are not examined.
No mention is made in the report of the role the lower Deschutes plays as a thermal refuge for upstream migrating adult salmonids in the Columbia River when it reaches maximum daytime temperatures during the summer.
No mention is made of what would happen if increasing temperatures, as a result of dam operations, in the lower river during spring and summer, reached the point of causing heat stress in fish. No defined threshold was set as a maximum river temperature that should be reached before overriding the “mathematical model” to interject human judgment and decision-making.
What is discussed is the role dam discharge temperature plays in affecting temperatures downstream from the dams. In varying conditions, dam discharge temperature can affect lower river water temperatures as far down as the mouth. Yet the dam operators are only held responsible for the dam discharge temperature at the Reregulation Dam, not the impacts downstream.
What We Think Should Be Done
It’s time for a reexamination of how water temperature is managed in the lower Deschutes River. This reexamination should be a more holistic approach to the health of the river and its resident species.
Thresholds should be set where protection of aquatic species is the primary goal for temperature management.
The 2005 dam operating license allows for, and even mandates, adaptive management, meaning changes can be made when new information shows that current methods are flawed. This means PGE, the Tribes, and the agencies responsible for protecting the aquatic health of the river can act now to lower the temperature of water released from the dams.
Deschutes River Alliance: Cooler, cleaner H2O for the lower Deschutes River.
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