Lake Climate & Long-Term Trends
Which lakes across US lakes with nutrient and algae readings 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.
- —Herkimer County, New York · Phosphorus↑ Improving-12.05 µg/L/yr
- —Scott County, Iowa · Chlorophyll-a↑ Improving-51.81 µg/L/yr
- —Scott County, Iowa · Chlorophyll-a↑ Improving-51.81 µg/L/yr
- —Scott County, Iowa · Chlorophyll-a↑ Improving-51.81 µg/L/yr
- —Scott County, Iowa · Chlorophyll-a↑ Improving-51.81 µg/L/yr
- —Pinellas County, Florida · Phosphorus↑ Improving-28.55 µg/L/yr
- —Chisago County, Minnesota · Chlorophyll-a↑ Improving-8.81 µg/L/yr
- —Windham County, Vermont · Chlorophyll-a↑ Improving-1.89 µg/L/yr
- —Polk County, Wisconsin · Chlorophyll-a↑ Improving-12.08 µg/L/yr
- —Carlton County, Minnesota · Water Clarity↑ Improving+1.96 m/yr
- —Chittenden County, Vermont · Chlorophyll-a↑ Improving-4.14 µg/L/yr
- —Oneida County, Wisconsin · Water Clarity↑ Improving+1.52 m/yr
Most declining
Largest relative decline per year on any single metric.
- —Johnson County, Iowa · Phosphorus↓ Declining+92.60 µg/L/yr
- —Jasper County, Iowa · Phosphorus↓ Declining+51.35 µg/L/yr
- —Carlton County, Minnesota · Phosphorus↓ Declining+75.10 µg/L/yr
- —Carlton County, Minnesota · Phosphorus↓ Declining+74.60 µg/L/yr
- —Carlton County, Minnesota · Phosphorus↓ Declining+74.00 µg/L/yr
- —St. Louis County, Minnesota · Phosphorus↓ Declining+74.30 µg/L/yr
- —Scott County, Iowa · Phosphorus↓ Declining+43.20 µg/L/yr
- —Adair County, Iowa · Phosphorus↓ Declining+37.20 µg/L/yr
- —St. Louis County, Minnesota · Phosphorus↓ Declining+73.05 µg/L/yr
- —Carlton County, Minnesota · Phosphorus↓ Declining+72.65 µg/L/yr
- —Guthrie County, Iowa · Phosphorus↓ Declining+69.90 µg/L/yr
- —Jones County, Iowa · Phosphorus↓ Declining+74.25 µg/L/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, Florida, Colorado, North Carolina, Vermont, Texas, Oregon, Arkansas, Oklahoma, Kansas, Nebraska, Nevada, Arizona, Idaho, Montana, New Mexico, Tennessee, Alabama, Virginia, Kentucky, Utah, Washington, Wyoming, Georgia, Massachusetts, Ontario, Québec, Nova Scotia, British Columbia, Newfoundland and Labrador and follow the Trends link.