Are Midwest lakes getting cleaner or more polluted?

Every covered state has lakes trending in each direction. Of lakes with sufficient monitoring data, the largest improvements show up in clarity gains and phosphorus reductions where shoreline and watershed management have improved. The largest declines correlate with shoreline development, agricultural runoff, and climate-related warming. Browse the leaderboards below for specific lakes.

How are these trends calculated?

Each lake's metric (clarity, phosphorus, chlorophyll-a) is the median of all EPA Water Quality Portal samples in that year. We fit a linear regression across the annual medians and rank lakes by the relative magnitude of change. The current sample cache spans roughly 2020–2025, which is short for climate-scale claims — treat these rankings as directional signal, not definitive verdict.

What causes a lake to improve or decline?

Common drivers: shoreline development, agricultural runoff, invasive species (zebra mussels often increase clarity by filtering the water), road salt, climate warming, watershed land-use change, and changes in upstream wastewater treatment. A single lake's trend usually reflects several of these together.

Lake Climate & Long-Term Trends

Which lakes across our 12 covered states are getting cleaner — and which are getting worse. Trends calculated from EPA Water Quality Portal samples, year by year. Every lake page links to a full chart with sources.

Most improving

Largest relative improvement per year on any single metric.

C
Railroad Lake
Scott County, Iowa · Chlorophyll-a
↑ Improving
-51.81 µg/L/yr
C
Blue Grass Lake
Scott County, Iowa · Chlorophyll-a
↑ Improving
-51.81 µg/L/yr
C
Lake of the Hills
Scott County, Iowa · Chlorophyll-a
↑ Improving
-51.81 µg/L/yr
B
Lambach Lake
Scott County, Iowa · Chlorophyll-a
↑ Improving
-51.81 µg/L/yr
B
Spectacle Lake
Chippewa County, Michigan · Water Clarity
↑ Improving
+1.36 m/yr
D
Dry Lake
Ramsey County, North Dakota · Chlorophyll-a
↑ Improving
-39.71 µg/L/yr
B
Lac Lake
Carlton County, Minnesota · Water Clarity
↑ Improving
+1.94 m/yr
C
Chisago Lake
Chisago County, Minnesota · Chlorophyll-a
↑ Improving
-6.96 µg/L/yr
C
Lake Tschida
Grant County, North Dakota · Chlorophyll-a
↑ Improving
-7.36 µg/L/yr
C
Lake Tschida
Grant County, North Dakota · Chlorophyll-a
↑ Improving
-7.36 µg/L/yr
B
Kroon Lake
Chisago County, Minnesota · Chlorophyll-a
↑ Improving
-11.57 µg/L/yr
C
Deadfish Lake
Carlton County, Minnesota · Water Clarity
↑ Improving
+1.96 m/yr

Most declining

Largest relative decline per year on any single metric.

D
Thayer Lake
Union County, Iowa · Chlorophyll-a
↓ Declining
+34.40 µg/L/yr
B
Indian Lake
Kalamazoo County, Michigan · Phosphorus
↓ Declining
+43.70 µg/L/yr
B
Boot Lake
Kalamazoo County, Michigan · Phosphorus
↓ Declining
+43.70 µg/L/yr
B
Ingerson Lake
Allegan County, Michigan · Phosphorus
↓ Declining
+41.90 µg/L/yr
F
Blue Lake
Monona County, Iowa · Chlorophyll-a
↓ Declining
+48.51 µg/L/yr
C
Mariposa Lake
Jasper County, Iowa · Chlorophyll-a
↓ Declining
+8.37 µg/L/yr
F
Silver Lake Max Depth
Delaware County, Iowa · Chlorophyll-a
↓ Declining
+21.39 µg/L/yr
B
Mattson Lake
Chisago County, Minnesota · Chlorophyll-a
↓ Declining
+8.02 µg/L/yr
C
Green Castle Lake
Marshall County, Iowa · Chlorophyll-a
↓ Declining
+8.22 µg/L/yr
B
Finger Lakes
Boone County, Missouri · Chlorophyll-a
↓ Declining
+5.90 µg/L/yr
F
Rocky Fork Lake
Boone County, Missouri · Chlorophyll-a
↓ Declining
+5.90 µg/L/yr
D
Clear Lake
Rock County, Wisconsin · Water Clarity
↓ Declining
-1.85 m/yr

Methodology

Data: EPA Water Quality Portal — secchi depth, total phosphorus, and chlorophyll-a samples.
Annual values: median of all samples for that lake in that year.
Trend fit: linear regression across annual medians; classified improving, declining, or stable when change/year is below 2% of the long-run mean.
Eligibility: lakes with at least four years of data on the metric being ranked.
Limitation: the current sample window is short (~2020–2025). Treat trends as direction, not certainty. We will revisit this page as the sample window widens.

Looking for a single lake? Open any lake page from Minnesota, Wisconsin, Illinois, Michigan, Iowa, Ohio, Pennsylvania, New York, Missouri, Indiana, North Dakota, South Dakota and follow the Trends link.

All improving lakes →All declining lakes →Cleaner lake finder →Grading methodology →