The St. John River is the longest stretch of un-dammed river east of the Mississippi. The St. John runs north and east for the first 140 miles of its length, and its drainage is found in Quebec, Maine, and New Brunswick. There are no dams on the Upper St. John River above Grand Falls, New Brunswick.
Since the headwaters are to the south, they thaw first in the spring, sending surging water north and east to meet still frozen river. The water dams up behind the ice, causing the ice to pile up onto itself, ultimately being forced downstream, scouring the river banks. It is this violent action by the ice that creates the unique habitat for the federally endangered Furbish Lousewort plant along the upper stretches of river.