TO: Mayor and City Council
THROUGH: Krishna Namburi, Interim City Manager
FROM: Brian D. Martin, PE, Public Works Director
SUBJECT:
title
Update on the pending deep drawdown of Detroit Reservoir in the fall of 2026.
Ward(s): All Wards
Councilor(s): All Councilors
Neighborhood(s): All Neighborhoods
Result Area(s): Natural Environment Stewardship; Safe and Healthy Community; Safe, Reliable and Efficient Infrastructure; Strong and Diverse Economy; Welcoming and Livable Community.
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SUMMARY:
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This report supplements and updates information provided to City Council at its July 28, 2025 meeting at which Council declared an emergency related to the deep drawdown of Detroit Reservoir. The first drawdown is anticipated to occur in the fall of 2026 with drawdowns expected every year thereafter. The drawdowns are expected to increase the levels of turbidity in the North Santiam River which is the source of Salem’s drinking water. This staff report summarizes the results of laboratory testing, which indicate that even relatively low levels of turbidity in the North Santiam River will significantly and adversely impact the City’s water treatment plant. This report then highlights information provided to the City at a meeting with senior officials of the US Army Corps of Engineers on August 26, 2025. At this meeting, the Corps assured the City that it fully understands the potential consequences of the drawdown and indicated its intention to closely monitor levels of turbidity and to manage the drawdown operation in a way that is protective of the City’s water treatment plant. Lastly, this report summarizes work in progress to increase resiliency of the City water system.
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ISSUE:
Information only.
RECOMMENDATION:
recommendation
Information only.
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FACTS AND FINDINGS:
Overview
On December 26, 2024, the National Marine Fisheries Service (NMFS) issued a Biological Opinion (BiOp) requiring the U.S. Army Corps of Engineers (Corps) to conduct deep drawdown operations at Detroit Reservoir to improve fish passage for out-migrating spring chinook and winter steelhead.
A deep drawdown at Detroit Reservoir will increase levels of suspended sediments and turbidity in the North Santiam River. Turbidity is a measure of the clarity of the water and is reported in Nephelometric Turbidity Units or NTU. To illustrate: water with turbidity of 10 NTU or less looks clear; water with turbidity of 50 NTU looks cloudy; water with turbidity exceeding 500 NTU looks muddy.
The City of Salem draws water from the North Santiam River at its water treatment plant located on Geren Island, 27 miles downstream of Detroit Dam. Salem’s drinking water treatment system relies on slow sand filtration, which produces high volumes of clean and safe drinking water.
For sustained operation, slow sand filtration requires the input water to have very low levels of turbidity. The water drawn by the City from the North Santiam River into its treatment plant typically has turbidity of 10 NTU or less. At this low level of intake turbidity, a sand filter can typically operate for about three months before the filter must be taken offline for cleaning (scraping 0.25 to 0.5 inches from the top layer of sand) and reconditioning (allowing a biological layer of microorganisms to regrow on the surface of the sand).
When the Corps conducted deep drawdowns at other reservoirs in the Willamette River system (Green Peter, Cougar, Fall Creek dams), sustained turbidity levels of over 50 NTU were reported below the dams, with peaks above 800 NTU. Slow sand filters are not designed to receive and treat water at these elevated levels of turbidity.
It is well understood that intake water above 10 NTU is detrimental to performance of Salem’s slow sand filtration system. However, when NMFS issued the BiOp, the quantitative relationships between intake water turbidity and operational runtimes were unknown.
Scientific Test Results on the Impacts of High Turbidity to Slow Sand Filters
In February 2025, as part of proactive measures anticipating the deep drawdown of Detroit Reservoir, the City contracted with Carollo Engineers, Inc., to conduct bench-scale scientific testing to evaluate the potential impacts of elevated turbidity levels on the performance of Salem’s slow sand filtration system. Extensive tests have been conducted.
Overall, the ability to treat intake water as it penetrates through a sand filter rapidly decreases with higher turbidity. The ability of the intake water to penetrate through to the filter also decreased with each successive run.
Among other key findings of Carollo’s research, for intake water at high turbidity (tests ranged from 50 NTU to 400 NTU):
(1) In contrast to a runtime of three months for pristine intake water with turbidity less than 10 NTU, the runtime of a filter before failure with turbidity of 50 NTU was less than 21 days in most tested conditions.
(2) Some test runs at high turbidity resulted in runtimes of less than 24 hours.
(3) Particles from the high turbidity intake water penetrated as much as 18 inches into the 36-inch thick sand layer, further degrading the performance of the sand filters and reducing subsequent runtimes substantially after filter scraping.
(4) At higher turbidity, the layer that required to be scraped when reconditioning a sand filter increased to one inch or more.
(5) In some test cases, fine sediments from the intake water coagulated at the surface of the sand filter creating a clay-like top layer. The ability of the City to successfully remove this layer to recondition the filter is uncertain.
(6)The life cycle of a slow sand filter would be reduced from 4 years or more to less than 2 months.
The conclusion, after months of science-based testing, is that the City cannot operate its slow sand filtration system at the Geren Island Water Treatment Plant for more than a few weeks, or even a few days, if the intake water drawn from the North Santiam River is 50 NTU or higher. In addition, each slow sand filter used during a high turbidity event would need to be completely re-sanded at a cost of approximately $3 million per filter, after less than 2 months of runtime.
Summary of Recent Conversations with the US Army Corps of Engineers
The City has a long-standing working relationship with the US Army Corps of Engineers dating back decades. Over the past months, both agencies have been sharing information regarding the planned drawdown and the potential implications for the City’s water system. In April, the City provided a tour of Geren Island Water Treatment Plant for the District Commander and his key staff. On August 26, 2025, City staff met with senior Corps officials at its district office in Portland. Among the takeaways from this meeting:
(1) The Corps is aware of the operational issues and potential impacts on the City’s water system of high turbidity in the North Santiam River.
(2) The Corps is proposing to employ protocols during the drawdown intended to reduce the risks to the City’s system, including: stepped drawdowns over multiple years; turbidity triggers to halt a drawdown; and drawdown duration days.
(3) The Corps and the City committed to continuing to work in close collaboration as the timeframe for the 2026 drawdown nears next fall and not jeopardizing the City’s water system.
(4) Among next steps is the release of a Supplemental Environmental Impact Statement (SEIS) for public comment in late October or early November, 2025. The SEIS will incorporate aspects of the drawdown at Detroit Reservoir into the Environmental Impact Statement for the operation of the Willamette Valley system of dams completed by the Corps in April 2025.
Update of Resiliency Measures
Even though the Corps will do their best to employ protocols during the drawdown to reduce the risks to the City’s water system, we can and should expect turbidity levels to exceed 10 NTU’s for at least a portion of the drawdown, thereby requiring us to rely partially or fully on resiliency measures. The City continues to work on up to four new groundwater wells at Geren Island, accelerate re-sanding of one or more slow sand filters, and construction of a second emergency intertie with the City of Keizer’s water system. Among the benefits of Council’s emergency declaration in July, contracting and procurement processes have been streamlined. Recent accomplishments include:
(1) Contracts have been finalized with Carollo Engineering, Inc., and Summit Water Resources, LLC, to design the groundwater expansion system on Geren Island with the 90-percent design package due in September.
[Note: these are both consultants of record]
(2) Holt Services, Inc., has been awarded a contract to construct the groundwater production wells and drilling is underway.
[Note: quotes were requested from 4 firms and 1 quote was received]
(3) Purchase orders have been issued to Emery and Sons Construction and Schneider Water Services to conduct groundwater well pumping tests and perform preparatory site work.
[Note: Emery is a contractor of record and Schneider is a subcontractor to them]
(4) A source for the four pumps needed for the new wells has been identified and is expected to be under contract before the end of September.
[Note: quotes were requested from 3 firms and 2 quotes were received]
Update on Public Outreach
Public outreach is ongoing.
(1) Every neighborhood association has been offered a presentation. To date, staff have visited 10 neighborhood association meetings.
(2) The City’s public website provides information on the drawdown at https://www.cityofsalem.net/community/household/water-utilities/salem-s-drinking-water#detroit-drawdown <https://www.cityofsalem.net/community/household/water-utilities/salem-s-drinking-water>
BACKGROUND:
On December 26, 2024, the National Marine Fisheries Service (NMFS) issued a Biological Opinion (BiOp) requiring the U.S. Army Corps of Engineers (Corps) to conduct deep drawdown operations at Detroit Reservoir to improve downstream fish passage. Drawdown operations at Detroit Dam can cause high levels of turbidity in the North Santiam River, the source of Salem’s drinking water. Elevated turbidity levels present a severe threat to the operation of the slow sand filter system at the City’s water treatment plant, which is the primary method for treating Salem’s drinking water supply. Deep drawdown operations are currently planned to begin in the fall of 2026 and are projected to occur annually thereafter. On July 28, 2025, City Council approved an emergency declaration to procure necessary materials, equipment and services to develop alternative water sources and build resiliency due to the system limitations imposed by the drawdown.
Robert D. Chandler, PhD, PE
Assistant Public Works Director
Attachments:
None.