Number Twenty-one Lake

Water clarity at Number Twenty-one Lake

Clear water. Across summers from 2018 to 2023, Number Twenty-one Lake averaged 3.16 m (10 ft) of visibility down into the water, so you can typically see several feet down in midsummer. This lake sits close to the line between clarity classes, so treat the label as approximate and the depth reading as the real number.

Nobody has taken a water sample here that we can find. Michigan's environmental agency measures clarity on lakes like this one from satellite images instead, which is why this page reports how clear the water is and no phosphorus or algae readings.

Clear is not the same as clean. A lake can be easy to see through and still carry high nutrient levels, and a naturally tea-stained lake can be perfectly healthy. The nutrient and algae readings on other lakes come from phosphorus and chlorophyll-a measured in a laboratory, and clarity alone cannot stand in for them.

The same satellite images put summer algae here at roughly 4.2 micrograms per litre. Estimating algae from orbit is far less exact than measuring clarity, so that figure is worth knowing and is not part of any rating.