The Lakes Where Water Quality Is Slipping

Lakes with the steepest declines

A declining lake is one where the long-term direction of the data is worsening — not a lake that had one bad summer. The lakes below combine a low current grade with a documented downward trend across several years ofSecchi depth, phosphorus, and chlorophyll-a readings.

The "years of data" column matters: a decline backed by a decade of sampling is a far stronger signal than one drawn from three or four seasons. Open any lake to see the year-by-year clarity and nutrient charts behind its trend.

How we detect a trend

A trend is a direction, not a snapshot. We look at a lake's repeated measurements over multiple years and ask whether the available tracks are moving the wrong way. Specifically, the overall trend is a majority vote of three series when they exist: Secchi depth (clarity), total phosphorus (the nutrient that feeds algae), and chlorophyll-a (the algae itself). That is different from the letter grade, which scores phosphorus, nitrogen, chlorophyll-a, and turbidity against EPA ecoregion classes and does not score Secchi. A lake can lose feet of clarity and still hold its letter until nutrients and algae catch up — which is why the trend table is worth reading next to the grade. Lakes without enough repeated sampling are left untrended. The full approach is on the methodology page and in Lake Water Quality Grades Explained.

What drives a lake downhill

Declines are a nutrient story. Phosphorus is the master variable in most freshwater lakes: add more of it and algae grows, clarity drops, and the grade falls. The common sources are consistent across states:

• Watershed development — new rooftops, roads, and lawns replace forest and wetland, sending more stormwater and nutrients straight to the water.
• Agricultural runoff — fertilizer and manure phosphorus washing off row crops and pasture, the dominant driver in agricultural basins.
• Failing septic and aging wastewater — nutrient leakage from shoreline homes and undersized treatment upstream.
• Invasive species — infestations that disrupt the food web and, in some lakes, release sediment-bound phosphorus back into the water.
• Warmer, longer summers — higher water temperatures extend the growing season for cyanobacteria (blue-green algae) and intensify blooms.

These forces rarely act alone. A lake ringed by new development in an agricultural county, warming year over year, is exposed to all of them at once — which is why declines cluster geographically rather than scattering at random.

The other side of the ledger

Decline is not destiny. For every lake slipping, others are recovering because their watersheds are sending less phosphorus downstream. 3,162 lakes in our data show improving trends, and the clearest success stories now hold A or B grades while still trending upward. If you want the hopeful version of this data, read the lakes that made the biggest comeback and browse the full improving-lakes ranking. To see the complete list of lakes headed the wrong way, open the declining-lakes ranking.