What About Nutrients? A Driving Force of the Negative Shifts in the Lower Deschutes

Prolific algae matts that are now a common occurrence in the lower Deschutes River following SWW Tower operations. Photo credit: Rick Hafele

Increased nutrient levels are one of the most significant contributing factors in the negative shifts that have occurred in the lower Deschutes River since the Selective Water Withdrawal (SWW) Tower started operating in late 2009. Before SWW Tower operations, water from Lake Billy Chinook only released cool, clean water from the bottom of the reservoir into the lower river. The water at the bottom of the reservoir is primarily sourced from the Metolius River which is colder and denser water and therefore sinks. By contrast, the surface water of Lake Billy Chinook is primarily sourced from the warmer nutrient-rich water of the Crooked River.

After the Tower started operating, the amount of surface water released into the Lower Deschutes increased from zero to 100% for almost nine months of the year and a blend of surface and bottom water for the remaining time. This increase in nutrient-rich water entering the lower Deschutes has been problematic, primarily due to the increase in overall algal growth and the proliferation of two nuisance diatom species that create a thick slime-like layer on rock surfaces. Studies on the macroinvertebrate and algal communities by PGE (see PGE’s report and independent statistical analysis of the data by Dr. Edwards) and the DRA have shown a decrease in pollution-sensitive macroinvertebrates such as mayflies, stoneflies and some caddisflies and an increase in pollution tolerant species such as worms and snails. Many of these pollution-tolerant species serve as intermediate hosts for fish pathogens, including Ceratonova shasta, which can cause high mortality in Chinook salmon. Thus, the increased algal growth and shift in species composition has caused a ripple effect in the macroinvertebrate community (i.e., aquatic insects) and, by extension, the rest of the ecosystem including fish.

The nutrients that typically limit algae and aquatic plant growth are phosphorus and nitrogen. In the Deschutes Basin, due to geologically naturally high phosphorus levels, nitrogen is the limiting nutrient. Thus, increasing nitrogen levels in the water results in increased algal and aquatic plant growth. The three tributaries entering Lake Billy Chinook are the middle Deschutes, Metolius, and Crooked Rivers. Of the three tributaries, the Crooked River is highest in nitrogen levels followed by the middle Deschutes, primarily due to the relatively high agricultural use in these two basins (think fertilizer and animal waste runoff).

The table below details nutrient levels collected by the DRA science team at the mouths of each tributary to Lake Billy Chinook since 2015. All show the same nutrient pattern as similar studies performed in the basin with the Crooked River contributing the most nitrogen to Lake Billy Chinook, followed by the Deschutes River. These tributaries, which are warmer and sit on the surface of the reservoir for much of the year relative to the cold Metolius River, are now getting disproportionately released directly into the lower Deschutes River through 100% surface draw that occurs 8-9 months out of the year.

Our most recent blog post on pH discussed how the elevated pH levels in the lower Deschutes are a clear indicator that the river has a nutrient problem. According to the original operating license that currently applies, dam operators are required to develop a plan to reduce pH if it is elevated:

“…if pH at the Reregulating Dam is found to exceed that of the weighted average of the inflows, the Joint Applicants will immediately contact ODEQ and the CTWS WCB to develop an approach to reduce pH that is consistent with maintaining compliant temperature and DO values and surface withdrawal volumes necessary to facilitate smolt movement in Lake Billy Chinook.”

From 2002 Water Quality Monitoring and Management Plan, part of PGE’s § 401 Certification Conditions

Despite continually higher pH levels coming out of the project relative to the three tributaries and continual violations of the pH standard in the lower Deschutes following tower operations, operators have yet to come up with a pH management plan, and enforcement agencies (ie: DEQ) have turned a blind eye.

PGE’s “Lower Deschutes River Pilot Stakeholder Working Group” concluded meeting earlier this year. The original purpose of the working group was to brainstorm ways to cool temperatures in the ever-warming lower river. A large volume of water quality data has been collected by PGE and the DRA, and all the data point to a simple solution that would benefit both temperature and the nutrient/pH problems discussed above: release more cool, clean bottom draw from the SWW Tower. The graph below depicts pH levels (blue line, left vertical axis) along with percent bottom draw (black line, right vertical axis). Note the three abrupt declines in pH occurred when more bottom water was released from the SWW Tower during summer months. This pattern has consistently been observed since the DRA started collecting seasonal continuous pH data in 2016.

Daily maximum pH just downstream of PRB complex and % bottom draw from the SWW Tower. Note the three abrupt declines in pH following large increases in bottom draw. Note that the increase in pH in October is most likely due what is called lake turnover, which occurs in nearly all northern latitude lakes and is when vertical mixing occurs throughout the water column in a reservoir. Data sources: pH – DRA, % bottom draw: PGE annual water quality reports.

The DRA attended all of the Stakeholder Working Group meetings and was the only voice among stakeholders directly advocating for this simple, obvious, and immediate relief available to the lower Deschutes. This, despite the necessary operational changes having no negative effects on power production or the fish passage program. This disappointing, but not surprising outcome, makes it clear why all those concerned about the health of the lower Deschutes River need to keep making their voices heard, and the DRA will continue to be that voice until the river gets the medicine it needs.