Warmer River Temperatures in the Lower Deschutes Harm Aquatic Life

SWW Tower operations have intentionally shifted the thermal regime in the lower Deschutes River. USGS temperature data collected in the lower Deschutes River was recently analyzed. This analysis detailed in our 2022 Water Quality Report shows that since selective withdrawal (SWW) Tower operations started in December 2009 the river is now warmer during most of the year. This is due to the increased proportion of warm surface water from Lake Billy Chinook released directly into the lower Deschutes via the Tower. Prior to installation of the SWW tower, only cool, clean water from the bottom of Lake Billy Chinook was released into the lower Deschutes River.

Figure 16a from the 2022 Water Quality Report depicts the difference in lower Deschutes River temperatures at the USGS Madras Gauge just downstream of Lake Billy Chinook between (1) all years post-tower operations 2009-2022 and (2) the period of record for temperature data at the same site pre-tower operations from the 1970s to 2009. The temperature difference is displayed by calendar day, which means positive values depicted in red indicate dates during the year that are now warmer in the lower Deschutes following SWW Tower operations, whereas negative values depicted in blue indicate dates during the year when the river is now cooler. The larger the value, the warmer or colder the river temperatures are following tower operations.

Figure 16a. Graphed values depict difference between (1) the average 7DADM (7-day average daily maximum) temperature calculated from 13 years during tower operations (12/02/09 to 12/31/22) and (2) the average 7DADM of lower during 21 years before Tower operations (10/01/1971 - 09/30/1988, 11/04/2005 - 12/01/2009) at RM 100 just downstream of the Reregulation Dam tailrace.  Data source: USGS (monitoring location 14092500).

As shown by the graph, the data clearly shows that even though the river is mildly cooler (~0.5C) in the late summer through fall, this cooling is relatively small compared to the warming during the rest of the year (over 2.5C warmer in May). One of the main reasons cited by PGE (the dam operators) for the new temperature regime is that they must save cold water to release into the lower Deschutes River in the fall. This graph shows that the cooling in the fall is negligible and far outweighed by the warming occurring the rest of the year.

Water temperature is very important to river ecosystems. PGE has intentionally made the shift in river temperatures depicted in Figure 16a, but no data-driven explanation has been provided and there has been no proven benefit to aquatic life. On the contrary, the disproportionate warming caused by Tower operations has likely been a contributing factor to the numerous unintentional consequences that have been observed in the lower Deschutes following Tower operations, including:

• Increased nuisance algae/diatoms (Figures A and B): in addition to the excess nutrients contained in the surface water being released by the Tower into the lower Deschutes, warmer water temperatures are likely contributing to the widely observed earlier-in-the-year and more dense growth of nuisance algae and diatoms.

Figures A and B: Mats of algae and stalked diatoms are now a common occurrence in the lower Deschutes River. Photo credit: Rick Hafele.

• Declining aquatic insects: in-part due to the shift in algal community to algae that insects are not able to feed on and an overall reduction of suitable habitat, sensitive aquatic insect groups have declined. Insect declines have primarily been from the mayfly, stonefly, caddisfly, and cranefly groups. Alteration of life cycles, including later hatch timing, has also occurred.
• Fish parasites: warmer water temperatures and excessive algal growth has caused a shift in the macroinvertebrate community that has resulted in conditions that favor high concentrations of worms and snails. This is concerning since some of these worm and snail species serve as intermediate hosts of fish parasites that cause disease and mortality. This is the likely explanation for the increase in black spot disease seen in trout and, of particular concern, C. shasta in Chinook salmon.
• Clean Water Act Violations: from 2020-2022, the DRA has observed water temperatures that exceed the steelhead and salmon spawning and incubation period standard of 13 °C from October 15^th- June 15^th during the month of June. This standard is set to protect this sensitive life history stage of salmonids and, by extension, is important to other sensitive aquatic species. In 2022, the 13 °C standard during the designated time period was violated for 8 days in June and 7 days in October. Clean Water Act Violations of pH and Dissolved Oxygen standards have also been continually observed since the DRA started collecting continual water quality data in 2016.
• Degradation of Cold-Water Refugia for Upper Columbia Salmonids: The lower Deschutes River is one of the more important cold-water refugium for Upper Columbia River Basin adult salmon and steelhead as they migrate into the upper reaches of the Columbia. Increasing the water temperature in the lower Deschutes is counterproductive to larger management goals for salmonids in the Columbia River Basin and potentially eliminates or seriously degrades this important cold-water refugium for anadromous fish. Particularly since the Deschutes River is the only significant thermal refuge in the >250km reach of the Columbia River from The Dalles Dam to Lower Monumental Dam.
• Increasing warmwater fish intrusion into the lower Deschutes: Warmer water earlier in the year is likely what encourages smallmouth bass to migrate from the Columbia River, where they are abundant, up the Deschutes, possibly in search of food resources. The capture of smallmouth bass by steelhead anglers in the lower 40 miles of the Deschutes River during the summers of 2016 and 2017 exceeded anything in recent memory and remain seasonally very abundant. In 2017, walleye were also caught in the lower Deschutes River near its mouth for the first time. Subsequent investigations by the Oregon Department of Fish & Wildlife confirmed smallmouth bass presence in numbers never previously observed by them. The reason for the increased presence of nonnative warmwater fish species are not completely clear, but higher water temperatures in the lower Deschutes River through July compared to pre-SWW Tower temperatures (Figure 16a) is one explanation. The impact this will have on the lower Deschutes River is currently unknown, but increased predation of native fish is unavoidable.

Fortunately, there is a simple and immediate solution to the problems listed above. The DRA has long advocated and will continue to advocate for the increase of cool, clean, bottom-water releases from Lake Billy Chinook. Figure C below shows the temperature data collected by the DRA in 2022 (blue line) graphed with the percent bottom draw from the SWW Tower in Lake Billy Chinook (black line).

Figure C. Rolling 7-day average daily maximum (7DADM) temperature data in the lower Deschutes River and the percent bottom draw from the selective withdrawal tower in 2022. Temperature data collected by the DRA; percent bottom draw data retrieved from a public records request DRA submitted to Oregon DEQ.

As the DRA has observed in previous years, the 2022 data shows that almost immediately following three abrupt increases in cool, clean bottom water during hot summer months, river temperatures responded by cooling substantially. This, and similar trends seen with other water quality parameters (principally pH), demonstrate that maximizing cool water releases from the Tower would produce healthier river conditions in the lower Deschutes River. For more information, please read our 2022 Lower Deschutes River Water Quality Report.