A river can cut itself off from its own loop and leave a lake stranded on land

Rivers on flat ground curve into loops called meanders, which grow tighter over time as the outside of each bend erodes and the inside deposits sediment. Eventually the river breaks straight through the narrow neck of a loop, abandoning the old curve as a crescent-shaped oxbow lake, slowly cut off from the new, shorter channel.

— Fluvial geomorphology scholarship, Meander — standard river-channel process