Great Seneca Creek Record IWLA Rockville Chapter · Save Our Streams
Izaak Walton League · Rockville Chapter · Save Our Streams

Great Seneca Creek Record

46 stream surveys at two sites between 2005 and 2026, read against 83,297 volunteer surveys nationwide. The creek's overall score has held steady, but it is losing pollution-sensitive animals faster than its setting explains.

Data as of 2026-10-27

not to scaleRiffle: fast, shallow water over stoneswhere Save Our Streams samplesPool: slower and deeperKick-seine netstir the stones upstream; animals wash inMost animals counted cling toand under the stones
Illustration. Animals in the lens are drawn far larger than true.
46chapter surveys
2005–2026years on record
83,297usable surveys nationwide
20,049sites with data

Key findings

Great Seneca CreekConcern

Compared with what its setting predicts

Site 1 finds fewer pollution-sensitive animals than its setting predicts, and the gap is widening (p = 0.029, about 11 percentage points a decade).

 Great Seneca CreekSteady

The score has held steady

Scores held near 16.5 before 2013 and 16.6 since; the overall score shows no trend.

 Great Seneca CreekWatch

The mix of animals is shifting

Between 2005–2009 and 2020–2024, damselflies and common net-spinning caddisflies became rarer while dobsonflies and aquatic worms became regulars.

 Water chemistryWatch

Road salt reaches the creek every winter

Chloride in the creek climbs from about 85 mg/L in summer to winter peaks of 159, still below the EPA's 230.

 Neighbouring streamsGood news

Streams across the Potomac are mostly improving

11 of 31 long-record sites within 30 km show improving scores, the opposite of Site 1's direction.

 National pictureContext

Where volunteers monitor

19,960 sites across the lower 48 states have usable surveys; most are in IA, NJ, VA.

 National pictureContext

Paved watersheds lose sensitive animals

Nationally, sensitive animals fall from 40% of groups found in near-natural watersheds to 22% where a quarter or more is paved.

Explore

Great Seneca Creek

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.

Water chemistry

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.

Neighbouring streams

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?

National picture

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.

Data and methods

Where the numbers come from, how they were cleaned, and the problems found in the volunteer data along the way.

Next

  1. Tags drawn from volunteers' notes (storms, construction, spills) to look for local causes of the loss of sensitive animals.
  2. Scanned paper data sheets for the years with no Hub record, and plain-English questions over the tables.
  3. A scheduled refresh, with an alert when a new survey falls outside its expected range.