Download our “Welcome to the Lake” booklet for more information on Cottage Lake!
Cottage Lake is located approximately four miles east of Woodinville. The lake covers 63 acres and the majority of the shoreline is surrounded by private homes. Cottage Lake Park is at the north end of the lake and provides public access with a large dock and a small beach.
Fishing: Fishing season runs from the last Saturday in April to October 31st.
Lake Water Quality Issues:
Cottage Lake contains high phosphorus concentrations and is therefore on Washington State’s 303(d) list of lakes not meeting state water quality standards. High phosphorus levels lead to increased algae blooms that impact water quality and the long-term usability of a lake. These algae blooms, when severe, are potentially toxic to humans. Past studies show that phosphorus is coming primarily from incoming streams (Daniels Creek and Cottage Creek). Other suspected sources include harmful lawn-care products and ill-maintained septic systems.
Algae Blooms and Cyanobacteria (blue-green algae):
Due to the high phosphorus levels in the lake, Cottage Lake is home to a lot of algae (microscopic single-celled plants). During hot, sunny weather, the algae multiplies quickly and this is referred to as an “algae bloom”(algae forms large dense patches visible on the surface of the water). Most species of algae are harmless and the blooms are just a nuisance. Cyanobacteria (commonly referred to as “blue-green algae”, but really a bacteria) is another organism living in the lake that multiplies during warm, sunny weather – again, cyanobacteria is generally harmless, but some can produce toxins which can be harmful to animals and humans. In particular, Microcystis (a type of cyanobacteria) has produced toxin blooms in other lakes in our area in the past (including Lake Sammamish). Another type of cyanobacteria has also been linked to swimmer’s itch.
Cottage Lake Algae Watch (also see our new Toxic Algae page):
- Aug 18th, 2019 – samples again “below the recreational guidance value” and therefore safe for water access
- Aug 5th, 2019 – two weeks of good samples so the lake has been re-opened
- July 21st, 2019 – lake closed and warning signs up (see https://www.nwtoxicalgae.org/ for latest status)
- August 30th, 2018 – lake closed and warning signs up (lasted until early October)
- Sept 2017: King County tested the sample we gave them and it contained high levels of microcystins, which is a common toxin produced by blue-green algae. The lake was closed (no swimming, keep pets out) for several weeks (until the samples were below the health limit). This was the third year in a row where we had a toxic algae bloom at the end of a hot summer.
Other Problems (updated!):
In addition to the high phosphorus levels and algae, Cottage Lake is also home to at least three identified noxious weeds: Purple Loosestrife, Fragrant Water Lilies, and Eurasian Milfoil. Purple loosestrife can clog streams and crowd out beneficial native plants and is very difficult to remove once it is established. Fragrant water lilies grow in dense patches and crowd out native plants. The can even create stagnant areas with low oxygen levels underneath the leaves which impacts fish and other wildlife. Eurasian Milfoil was recently found (Fall 2007) and can also have a very serious impact on the ecosystem and recreation. See the new milfoil page for more information.
To learn more about Washington State’s 303(d) list and phosphorus go to:
To read the Washington State Dept. of Ecology “TMDL” or “Total Maximum Daily Load” Analysis report
for Cottage Lake go to:
For more information on “TMDL” analysis please see: