Understanding Lake Water Quality Grades

The Minnehaha Creek Watershed District’s water quality lake grading method was developed in 1989. The lake grade shows how one lake measures up compared to other area lakes and indicates the perceived condition of the open water. Three water quality measurements are combined to create each lake grade. However, there are also other factors that contribute to lake condition, including precipitation. aquatic plants, fisheries, harmful chemicals, and over-use.

Understanding Lake Water Quality Grades

The grades consider what is average or normal for lakes in a given area. What is a “C” in one part of the country might not be a “C” elsewhere. However, within the seven-county metro area, grades can be compared.

Lake GradeRelative RankingDescription
A90% and upCrystal clear, beautiful. These lakes are exceptional and can be enjoyed by recreation users without question or hesitation.
B70 - 90%Generally good water quality but algae may limit swimming, particularly toward the end of the summer.
C30 - 70% Average quality. Swimming, boating, and fishing may be undesirable relatively early in the season. Algae blooms occasionally.
D10 - 30%Severe algae problems. People are generally not interested in recreation on these lakes.
FLowest 10% Not enjoyable. The lake would have severe limitations to recreational use.

The Three Water Quality Measurements

  • TP is the abbreviation of total phosphorus. Total phosphorus usually is a limited food source (nutrient) for algae and plants. An increase in total phosphorus relates closely to increased algae, frequency of algal blooms, and an increased quantity of blue-green algae.        
  • CHL-A is the abbreviation of chlorophyll-a. Chlorophyll-a is the green pigment in algae and plants and is essential to photosynthesis. A measure of chlorophyll-a in the water estimates the algal abundance.        
  • SD is the abbreviation of Secchi disk depth or water clarity. The lower the Secchi disk is visible, the clearer the water appears.        

Improving Low Water Quality Lake Grades  

Raising a lake’s grade requires extra work. It may mean changes in drainage patterns, filtration, or detention of storm water. And not all lakes have the same potential (e.g. shallow lakes can have only so much Secchi disk visibility). But everyone plays a role in helping a lake live up to its full potential. For example, your home and yard may contribute to the poor quality of a neighborhood lake. Preventing pet  waste, leaves, home and auto chemicals, and house gutters runoff  from entering storm drains can help keep your lake or stream clean. Storm drains are a direct, untreated route to the nearest lake or stream.

For more information on what you can do click here.