oxbow lakes. The image above nicely shows the process; the river (presumably glacial meltwater, judging by the color) enters at lower right, curls off the frame at the top, and exits lower left. The transported solids carve away at the outer walls of the curves (and get deposited on the insides of the curves). In another few decades, that narrow isthmus will be breached, the water will flow directly across, and the curve at the top will transform from river segment to lake, and later eventually to marsh and then bog and then field and finally woods.
For an impressive depiction of the complexity of oxbows in a floodplain, see Radical Cartography.
Photo credit Cameron Davidson.