New Hampshire Lake Water Quality

New Hampshire has 315 graded lakes in our dataset, sampled across 10 counties, drawing on monitoring by the New Hampshire Department of Environmental Services plus federal and volunteer measurements. Only about 15% of New Hampshire's lakes pull an A grade — the smaller, agricultural-watershed lakes that dominate the dataset tend to carry persistent nutrient loads. 7% of lakes fall in the D or F brackets.

Mesotrophic is the dominant trophic class — a workable middle ground between the pristine oligotrophic deep lakes and the algal-pressured eutrophic shallow ones. Carroll ranks highest on average county grade (B). Rockingham sits lowest. With only 10 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.