This aquatic plant is widespread in streams across cool-temperate regions of North America, but rare to absent in southeastern states. Nomenclature here follows W rather than FNA 3, which combined longirostris with the Eurasian aquatilis and the circumboreal trichophyllus. In Ky. longirostris has been found only in a few streams of the central Bluegrass region (especially Lees Br. of WOOD), plus a recent discovery by P. Haragan in Floyd's Fk. of JEFF (EKY). Even Short (1828-9, with coll. at PH) did not know it well, noting only one site, under the name "R. fluviatilis (River crow-foot). A small aquatic plant found in shallow-running waters. Found in Elk-horn on the George-town road: rare." Although rare here at the southern edge of its range, the species is moderately tolerant of eutrophic turbid waters, so long as there is regular flow; see also Albert & Minc (2004) and Croft & Chow-Fraser (2007). It appears to be increasing in streams of BOUR and HARR where cattle have been excluded in recent years and riparian vegetation allowed to regrow (M. Brock & M. Hines, pers. comm.).