Stoneflies return to Rock Creek as waters clear

July 11, 2026Piedmont Uplands

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(Plecoptera) are emerging along Rock Creek and its tributary Cabin John this July — a suite of fourteen records over the past three months marking one of the clearest ecological signals of improving water health. These mayfly-like insects are relentless indicators; their nymphs cannot tolerate even trace levels of organic pollution, and they vanish entirely from waters under stress.

The distribution tells a hopeful story. Maryland's mainstem of Rock Creek has now achieved full attainment for all designated uses, a milestone reached in 2024 after years of stormwater and sewage management work. The DC sections, by contrast, remain on the impaired list — yet even there, stonefly nymphs are establishing themselves in cleaner pockets of the channel. A stonefly on the wing in midsummer is a stream announcing, quietly but unmistakably, that it is coming back.

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