Perianth bristles absent. 2n = 38.

Fruiting summer to fall. Shores with relatively large water-level fluctuations; 20-300 m; Ind., Mass., Mich., Wis.

Schoenoplectus purshianus var. williamsii is known only from the type locality in Massachusetts, several counties in Michigan, one county in Indiana, and two counties in Wisconsin. In southwestern Michigan it is reportedly frequent on sandy shores of lakes with considerable water-level fluctuation spring to fall; the typical variety is infrequent in those habitats and often grows on shores with relatively uniform water levels (A. E. Schuyler 1972).