This is widely scattered across north-central regions of North America in varied kinds of lakes, streams, rivers and estuaries, especially calcareous. However, it has become rare to locally extinct within the central Mississippi River watershed, as well as several peripheral parts of its range. In Ky., it is known only from the Bluegrass region, but it may be overlooked further west (K). As well as a few recent colls. from streams or ponds in FAYE (JC) and MADI (EKY, NY), there is a 1830s collecton of R. Peter (MICH, NY) from "Kentucky River", probably in MADI or FAYE. E. nuttallii differs from canadensis in its leaves, which are mostly 1-1.7 mm wide (versus 1.8-2.5 mm), with L/W ca. 5-10 (versus 2-5), and more spaced at growing tips (versus densely overlapping); 2n = 48 (versus usually 24). Its rarely collected flowers are smaller and they separate in bud from mother plants soon when mature (versus produced on a long flexuous stalk). In Europe, both canadensis and especially nuttallii have spread to become locally weedy in eutrophic waters; hybridization remains virtuallly unknown (Cr, FNA 22). Differences in habitat are not clear but nuttallii tends to be more vigorous, and it may do relatively well in more fast-moving water, with average depth of about 0.2-0.5 cm. See also Rybicki et al. (2013).