New Hampshire Lake Water Quality

301 of the 650 New Hampshire lakes in this index carry a recent clarity, phosphorus or algae reading, spread over 10 counties. Those readings come from the New Hampshire Department of Environmental Services, alongside federal programs and volunteer monitoring networks. The rest are listed for their size, depth and location, with no recent nutrient or algae sampling behind them. The measured lakes spread across the range: low phosphorus and algae readings in the deeper basins, higher readings in the agricultural south of the state.

Opechee Bay-Deep Spot Lake is the largest lake in this index at about 46,720 acres, one of 26 here that clear 500 acres. The rest run smaller, and small and shallow is where nutrient loading shows up first, because there is less water to dilute it and the bottom sediment stays within reach of the sun. Most measured New Hampshire lakes sit in the mesotrophic band: moderate productivity, decent clarity, capable of supporting both warm-water and cool-water fisheries. This is the band where well-managed working lakes tend to land. Carroll has the lowest nutrient and algae readings on average. With only 10 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.