There are two rivers which are part of the setting in A Separate Peace by John Knowles. Both rivers rather converge at Devon School, but they are distinctly different in most ways. The Devon River is the familiar, less brackish, and more used river; The Nagaumsett is not necessarily the opposite, but it is certainly a more ominous body of water.
Three characteristics which distinguish the Nagaumsett River are that it is never used by the Devon students; it is brackish (salty from the nearby ocean) rather than pure; and it is dark (full of silt and seaweed). All of this is found in the following quote from the text:
[The boys] never used this lower river, the Nagaumsett. It was ugly, saline, fringed with marsh, mud and seaweed.
The Nagaumsett is symbolic of the undercurrent of war for the boys at Devon. Ironically, the worst thing that happens at Devon during the course of this novel (Finny's fall) happens at the Devon River, not the Nagaumsett.