Lake Stevens Restoration History

The following is a history of efforts to improve Lake Stevens water quality. For current information on lake health, recreational opportunities, lake maps and more visit Lake Stevens page. 


Lake Stevens is the largest natural lake in Snohomish County. The lake covers 1013 surface acres and has a maximum depth of 150 feet. Approximately 80% of the lake lies within the city limits of the City of Lake Stevens; only the southeastern shore is located in unincorporated Snohomish County. The watershed, which is the entire land area that drains to the lake, covers 3,485 acres. 

From the 1950s through the 1980s, Lake Stevens experienced declining water quality, with poor water clarity and blooms (thick growth) of algae. Nutrient pollution, particularly phosphorus, was the primary cause of the poor water quality. Phosphorus enters the lake through runoff from the residential and commercial lands in the watershed. The amount of phosphorus coming from the watershed has increased over 50% since the 1980s. In addition, over time, phosphorus has built up in the sediments at the bottom of the lake. The phosphorus flowing from the surrounding watershed and coming from the sediments in the lake are the two primary sources that feed unwanted algae growth.

You Can Help Protect Lake Stevens

Lake Stevens needs your help to stay healthy.  Learn about small changes you can make on your property to protect your lake and become LakeWise. Find more information at For those of you that live in the City of Lake Stevens, you can check out the I Love Lake program offered by the City of Lake Stevens. 

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Actions to Improve Water Quality

Overall, Lake Stevens has good water quality compared to many lakes in Snohomish County. However, because of its large size and depth, the lake should be even clearer and have fewer algae blooms than it does. On-going efforts are needed to keep the lake healthy.

The City of Lake Stevens and Snohomish County have been working to control the watershed sources of phosphorus. Both jurisdictions work with property owners to encourage actions that prevent pollution, such as picking up pet waste, maintaining septic systems, and covering bare soils. Street sweeping and regular maintenance of the drainage system also help to protect the lake.

Since the early 1990s, the City and the County have also worked to control phosphorus coming from the lake sediments, first with an aeration system and more recently with alum treatments.

Aeration System

To help control phosphorus being released from the lake sediments, an aeration system was installed in the center of the lake in 1994. The aeration system provided oxygen to the bottom waters of the lake to offset the chemical reaction that releases phosphorus from the sediments when oxygen levels are low. The aeration system was helpful in controlling phosphorus release and improving the water quality of Lake Stevens for nearly 20 years.

Design of former aeration system


However, in recent years, the aerators were losing effectiveness at maintaining low phosphorus levels in the lake because there was no longer enough iron in the sediments to bind all the phosphorus. Based on this finding, plus the high costs of operating and maintaining the aeration system (approximately $100,000 per year), the City and the County decided to retire the aeration system in 2012.

Alum Treatments

As an alternative to relying on the aerators to control phosphorus pollution from the lake sediments, the City and County decided to conduct small, annual aluminum sulfate (alum) treatments.

Alum is a safe and widely-used chemical for removing impurities, such as phosphorus, from drinking water supplies and from lakes. The aluminum in alum strips phosphorus out of the lake water and sinks to the lake sediments where it permanently binds phosphorus in the sediments, making it unavailable for algae growth.

The long-term plan developed by the City and the County calls for annual alum treatments over the next ten or more years. The alum treatments will help remove phosphorus that washes into the lake from the surrounding watershed each year as well as inactivate a portion of the phosphorus in the lake sediments. In the long run, the repeated alum treatments can be just as effective, or more effective, than operation of the aeration system.

Alum Treatment at Lake Stevens

Lake Stevens2

The dose of alum being applied to Lake Stevens each year is very small, 50 to 100 times smaller than the doses used in recent years at Lake Ketchum and Green Lake. The low alum doses create a light floc that sinks slowly to the lake bottom, removing phosphorus and a small portion of the algae and other plankton in the lake.

The small annual alum treatments began in June 2013. There were also alum treatments in the spring in 2014 and 2015. In 2016, the alum treatments were moved to occur each fall. The City and County are working closely with the Washington State Department of Fish and Wildlife to ensure that the alum treatments do not impact fish and other life in the lake. The next Alum treatment will be in October 2019.

Water Quality Monitoring Results

Snohomish County Surface Water Management (SWM) monitors the water quality of Lake Stevens every month. The monitoring data show that lake conditions have improved since the beginning of the alum treatments.

Phosphorus (key nutrient for algae)

Phosphorus concentrations in the upper waters were slowly increasing during the last years of aeration, as shown in the first graph. However, phosphorus concentrations during the years with alum treatments (2013-2017) have declined by 44% compared to the last six years of aeration (2007-2012).


Some other lakes have also seen lower phosphorus during the last three years, possibly because of warm, dry summers and the state-wide ban on phosphorus in lawn fertilizers, but the other lakes had average decreases of only 25%. So, the improvements at Lake Stevens have been more significant.

Phosphorus levels in the bottom waters are higher than in the upper waters. Since 1999, phosphorus in the bottom waters at 40 meters (130 feet) deep was steadily increasing (see next figure). This reflects the declining effectiveness of the aeration system, release of phosphorus from the lake sediments, as well as increasing pollution from the surrounding watershed. However, phosphorus concentrations in the bottom waters have dropped 31% during the 2013-2017 alum treatments compared to the last six years of aeration. In contrast, most other lakes in Snohomish County have seen increases in phosphorus in the bottom waters in recent years.


Chlorophyll a (Algae)

Algae are tiny plant-like organisms that are essential for a healthy lake. Fish and other lake life depend on algae as the basis for their food supply. However, excessive growths of algae, called algae blooms, can form unsightly scums and sometimes release toxins. Chlorophyll a is a measure of the amount of algae in a lake.

The algae levels in Lake Stevens are relatively low. However, similar to the changes in phosphorus levels, chlorophyll a concentrations during the years of alum treatments (2013-2017) have declined compared to the last six years of aeration (see graph).


Lake Stevens does still experience occasional nuisance algae blooms, especially in the late winter and early spring, even with the alum treatments. However, the aeration system and later the alum treatments are helping to keep phosphorus levels low and control most of the nuisance algae growth.

Water Quality Outlook

Overall, Lake Stevens is in good condition. However, the lake is at risk of future water quality declines unless phosphorus pollution is controlled.


The small, annual alum treatments are helping control the phosphorus that feeds nuisance algae. The alum treatments remove phosphorus from the water column and over time inactivate phosphorus in the lake sediments.

However, the alum treatments cannot completely offset the impacts of pollution coming from properties in the watershed. Any future increases in phosphorus pollution flowing into the lake may lead to nuisance algae growth. Residents around the lake can make a difference by making simple changes around their properties that reduce phosphorus pollution. These actions include picking up pet waste, avoiding fertilizers or using phosphorus-free fertilizers, diverting water from roofs and driveways into stable vegetated areas rather than piping it to the lake, covering bare soils, and planting more native shrubs and trees near the water.

More ideas about protecting the health of Lake Stevens can be found at or at

Aquatic Plants

Aquatic plants are also important in a lake ecosystem. Plants provide food and shelter for fish and other aquatic animals, stabilize the shoreline and bottom sediments, and in some cases increase water clarity by out-competing algae for nutrients.

Lake Stevens supports moderate levels of native aquatic plants. However, Lake Stevens has had a serious problem with Eurasian watermilfoil, an invasive, non-native plant. The milfoil can choke out native plants and form dense stands that are a nuisance to humans and wildlife.

In the late 2000s, Eurasian watermilfoil grew into a major problem. The thick mats of milfoil were so dense that they interfered with swimming and boating in many portions of the lake.


The City of Lake Stevens, conducts spot  herbicide treatment  to kill milfoil plants in the densest areas. Residents and recreational users of the lake should be sure to check their boats and trailers and remove any Eurasian watermilfoil fragments to prevent the spread of this plant within Lake Stevens and to other lakes.

Project Funding

The efforts to protect and improve the water quality of Lake Stevens are manageed by the City of Lake Stevens and funded by both the City and Snohomish County. Because most of the lake lies within the city limits, the City is currently paying for 89% of the water quality work and the County pays 11%.