Loch-tober Continues
By Dani Backman
Each October, we highlight the science and stories behind lake management across the region. After stopping at Moses Lake last week for WALPA 2025, we travel north to Snohomish. Let’s take a closer look at how science and collaboration are shaping the lake’s future.
Loch-tober 2025: Restoring Blackmans Lake
Blackmans Lake, originally known as Stillaguamish Lake, meaning “People of the River” in Lushootseed, is a treasured community resource in the City of Snohomish. The lake, renamed for the Blackman family who operated a logging camp there from 1875 to 1884, offers scenic beauty, recreation, and wildlife habitat. Two public parks, Hill and Ferguson, provide access for boating, fishing, and lakeside enjoyment.
Like many Northwest lakes, Blackmans faces recurring harmful cyanobacteria (“blue-green algae”) blooms that cloud the water, restrict recreation, and threaten human and animal health. To restore and protect the lake, Herrera partnered with the City of Snohomish to develop and implement a Lake Cyanobacteria Management Plan. One of the plan’s key recommendations, implemented in April 2025, was a high-dose aluminum sulfate (alum) treatment to reduce phosphorus levels and curb toxic algae growth.
What are cyanobacteria and why are they a problem?
Cyanobacteria are naturally occurring microscopic organisms, but under nutrient-rich conditions they can grow rapidly, forming dense surface scum. Some species produce cyanotoxins such as microcystin and anatoxin, which can harm people, pets, and wildlife through contact or ingestion. In Blackmans Lake, blooms typically occur from August through October, occasionally exceeding state health guidelines and triggering public health warnings.
Why Blackmans Lake?
Monitoring between 2022 and 2023 found that phosphorus is the primary nutrient driving algae growth. Most of the phosphorus originates internally, from lake sediments that release phosphorus during summer when oxygen levels drop to zero near the bottom. Warmer temperatures accelerate this process, fueling blooms.
How Alum Treatment Helps
To combat this, Herrera implemented a sediment inactivation treatment using alum. Alum binds with dissolved phosphorus to form aluminum phosphate, a stable compound that locks up phosphorus in the sediment, preventing its release into the water column. The treatment is expected to reduce internal phosphorus loading for several years.
Post-treatment monitoring will track phosphorus, chlorophyll-a, and water clarity to evaluate effectiveness and inform future management actions.
Next Steps
The City and Herrera will continue monitoring lake conditions, evaluate oxygenation system options for long-term control, and engage the community in watershed stewardship to reduce external nutrient inputs. Together, these actions will help ensure Blackmans Lake remains a healthy and vibrant part of Snohomish for years to come.
______________________________________________________________________
Contributing Author: Ruth Thirkill, Ph.D
Early morning preparation for Alum application on Blackmans Lake.
Early morning preparation for Alum application on Blackmans Lake.
Ducks enjoying the Caribbean blue waters of Blackmans Lake after the end of the Alum application.
Sechi disk depth (a measurement of water clarity) prior to treatment (left) versus after the April 2025 treatment (right).
Posted In:
Water