Oregon City presents findings on harmful algal blooms in Clackamas Cove
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Summary
City water quality coordinator Marcos Kubo told the planning commission the cove’s summer isolation, sediment phosphorus release and invasive vegetation help drive cyanobacteria increases; staff recommended starting with invasive‑species removal and mechanical mixing before pursuing river‑exchange options.
Marcos Kubo, water quality coordinator for the City of Oregon City, told the planning commission the city has detected increased algal concentrations in Clackamas Cove and has preliminary evidence that internal phosphorus release from sediments drives summer cyanobacteria blooms.
Kubo said the cove becomes hydrologically isolated in summer — "it’s essentially cut off and acts like a lake" — when water depth drops below about 12 feet and the gravel bar at the entrance limits river influence. That isolation, combined with temperature stratification, creates low-oxygen conditions at the bottom that can release phosphorus from the top centimeters of sediment, he said. "We know the phosphorus is [the] limiting factor for harmful algal blooms," Kubo said.
The presentation compared water-column profiles from 2024 and 2025, showing a higher near-surface chlorophyll signal in 2025 and spikes in cyanobacteria. Kubo cautioned that chlorophyll alone does not prove toxicity but noted cyanobacteria (harmful algae) were also observed. He summarized the city’s monitoring as showing larger bottom phosphorus concentrations (reported in presentation as increasing to roughly 1.4 at depth versus surface values that rarely exceeded about 0.03), and stressed those values reflect the team’s sampling and not a human contact advisory.
Commissioners asked about the gravel bar and whether dredging could help. Kubo said staff have a roughly 20-year lookback (2005–2024) showing substantial rock aggregation and that large storms have shifted deposits and even damaged data loggers. He described dredging as a likely temporary solution: "Any kind of dredging is a temporary solution," he said.
On management options, Kubo described two paths: treating the cove as a lake with active management (removing invasive aquatic plants such as Eurasian milfoil, mechanical mixing, bubblers or aerators, and, as a last resort, algaecide), or enhancing exchange with the river — the latter would require substantial permits and work in the river. He urged starting with lower‑risk measures: "I would say starting with the low‑hanging fruit, like removing the invasive species," and then evaluating whether additional measures are needed.
Kubo told commissioners that removal of invasive species could provide a 3–5 year window with fewer invasives if reintroduction is prevented, and that at least three years of monitoring would be needed to identify a trend in algal response because annual conditions (temperature, precipitation) strongly affect stratification and internal loading. He closed by noting the model used to estimate inflow needs has been calibrated and results are expected soon.
The presentation was given as part of the city’s pollutant discharge permit reporting and flagged as relevant to redevelopment considerations for Clackamas Cove; no immediate regulatory action was taken at the meeting.
