46 stream surveys at the chapter's two sites, read against 83,297 volunteer samples nationwide. Each result has its own page with the evidence behind it.
Site 1 finds fewer pollution-sensitive animals than its setting predicts, and the gap appears to be widening (p = 0.055, about 11 percentage points a decade).
Great Seneca CreekSteadyScores held near 16.5 before 2013 and 16.6 since; the overall score shows no trend.
Great Seneca CreekWatchBetween 2005–2009 and 2020–2024, damselflies and common net-spinning caddisflies became rarer while dobsonflies and aquatic worms became regulars.
Water chemistryWatchChloride in the creek climbs from about 85 mg/L in summer to winter peaks of 159, still below the EPA's 230.
Neighbouring streamsGood news11 of 31 long-record sites within 30 km show improving scores, the opposite of Site 1's direction.
National pictureContext19,960 sites across the lower 48 states have usable surveys; most are in IA, NJ, VA.
National pictureContextNationally, sensitive animals fall from 40% of groups found in near-natural watersheds to 22% where a quarter or more is paved.
The Rockville Chapter's two monitoring sites on Great Seneca Creek in Germantown, Maryland, surveyed since 2005: how stream health has moved, which animals are changing and how the creek compares with what its setting predicts.
How the chapter's creek compares with each larger watershed it sits inside. To zoom through every watershed in the country, use Explore.
Chloride from road salt and nitrate from fertilizer and sewage, measured with Salt Watch and Nitrate Watch strips at the chapter's creek and lake since 2024.
Every Clean Water Hub site within 30 km of the chapter with a long enough record, most of them Fairfax County streams across the Potomac: are they changing the same way?
What thousands of volunteer surveys across the country show, and the expected-score model trained on them that the chapter's sites are judged against.
Where the numbers come from, how they were cleaned, and the problems found in the volunteer data along the way.
