Colby Lake

Colby Lake

Lake Description

Colby Lake is a ~70-acre, shallow lake located in Woodbury. It is part of a multi-lake system, receiving water from Wilmes Lake to its north and contributing water downstream to the Bailey Lake wetland.

Colby Lake receives contributions from Wilmes south basin and its 2,924 acre drainage area, of which 1,075 acres are impervious.

Colby Lake is surrounded by City of Woodbury parks and trails, and 2.3 miles of shoreline.

Lake Details

Water Quality in Colby Lake

Colby Lake is a shallow lake with a maximum depth of 3.4 meters. The lake’s watershed is 10.6 square miles, 6.5 of which come from Wilmes Lake. The remaining 4.3 square miles of the drainage area contribute water directly into Colby Lake either through direct runoff or through stormwater infrastructure. Collectively, these lakes make up the majority of SWWD’s Northern Watershed (NWS) which is dominated by recent suburban development.

Average Secchi depth in Armstrong Lake is 0.9 meters, which is just below the Minnesota State Standard of 1.0 meters. A Secchi disk is a black and white circular disk used to measure water transparency or turbidity by assessing the depth the disk is visually present under the surface of the water. The more turbid or “murky” the water, the shallower the Secchi depth will be, the more transparent or “clear” the water, the deeper the Secchi depth will be.

Interpreting the Data

Water quality data is collected as part of a regular monitoring schedule. Once data is collected, it is interpreted. It can be used to understand the general quality of the water being monitored, potential pollutants that may need to be addressed, or to understand the overall quality of the aquatic and surrounding environment. SWWD regularly monitors for Total Phosphorus (TP), Chlorophyll a, turbidity or transparency, and chloride.

Monitoring Data

Colby Lake Vegetation & Management

Another way to learn more about the health and quality of a waterbody is by monitoring and managing aquatic vegetation. 

Aquatic plants are a natural part of most lake communities and play a key role in the aquatic food chain, converting basic chemical nutrients in the water and soils into plant matter, which becomes food for all other life forms.

Aquatic plants also help to regulate the chemical makeup of water, which improves water clarity (transparency/turbidity). Water clarity plays an important role in the production of algal blooms, either promoting or controlling algal blooms by too little or not enough sun exposure, limiting the natural process of photosynthesis. 

Colby Lake’s aquatic plant species richness is average (9), but it does not meet the standard for a lake of its size (11).
When an invasive species takes over a waterbody, either preventing the growth of other beneficial native species and creating a monoculture, or preventing navigation or recreation on the lake, aquatic invasive species (AIS) management may be required.

Learn more about how SWWD conducts AIS management

Aquatic Health Under the Magnifying Glass

Aquatic plants are an important source of habitat for aquatic animals like fish, reptiles and amphibians, and macroinvertebrates. The AMCI (Aquatic Macrophyte Community Index) is a way to measure the presence of native, nonnative, and invasive aquatic macrophytes, and further understand the overall health of the waterbody. During a survey, the number of macroinvertebrate species like snails, crayfish, Water Boatman, Dragonfly larve, and beetles is counted to create the AMCI score and rate species richness. The highest AMCI score at Colby Lake was recorded in 2015 at 47 species. This score has seen a slight decline since then, however scored at 44 species during the 2024 survey, indicating moderate biological integrity.

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