This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Perennial by stout rootstocks; culms stout or slender, smooth, obtusely triangular or nearly terete, stiff, leafy, 2°-6° tall. Leaves elongated, 2"-3' wide, rough-margined, the upper often overtopping the culm, those of the involucre 3-6, their bases often brown or black, the longer much exceeding the terminal, compound umbel; spikelets ovoid-oblong, obtuse, 1 1/2" - 5" long, in capitate clusters of 3-15 at the ends of the raylets, or some or all of them stalked; scales ovate or lanceolate, acute or subacute; bristles 6, entangled, smooth, much longer than the achene, much exserted beyond the scales and brown or reddish at maturity; stamens 3; style 3-cleft; achene 3-angled, oblong, slender-beaked, nearly white.
Scirpus atrocinctus Fernald, characterized by black bases of the involucral leaves, is of northern range and may be specifically distinct.