A meandering river looks, from above, like it's tracing a fixed, permanent pattern of loops across a floodplain. It isn't. Meanders are constantly migrating, slowly reshaping themselves through the same two processes acting in opposite directions on opposite sides of every curve — and given enough time, that migration can eventually cut a bend off entirely, leaving behind a crescent-shaped lake with no river running through it at all.
Erosion on the outside, deposition on the inside
Wherever a river bends, water flows faster along the outer edge of the curve and slower along the inner edge, a direct consequence of the physics of moving water going around a corner. That faster outer flow has more energy to erode the riverbank, cutting it back over time, while the slower inner flow deposits sediment it's no longer carrying fast enough to keep suspended, building up a gently sloped bar on the inside of the bend. The combined effect, repeated over years and decades, is that the outer bank retreats and the inner bank advances — the meander doesn't just sit there, it migrates sideways and downstream, and its curve grows steadily more exaggerated with each cycle of flooding and channel adjustment.
When the neck finally gives way
As a meander's loop grows more pronounced, the neck of land separating one part of the curve from another, further along the same loop, grows correspondingly narrower. Eventually, often during a flood when the river is carrying unusually high flow and cutting with unusual force, the river breaches that narrow neck directly, carving a new, shorter channel straight across it. Water then preferentially follows that new, shorter path, and the old, longer loop is abandoned, its ends silting up and sealing off from the main channel. What's left behind is an oxbow lake — a crescent of standing water tracing the exact shape of a bend the river no longer flows through, slowly filling with sediment and vegetation over subsequent decades until it eventually disappears back into the surrounding floodplain.
What we're still unsure about
The basic mechanics of meander migration and oxbow lake formation are well understood and observed directly in rivers around the world — this isn't a matter of scientific dispute. What remains genuinely harder to predict precisely is the timing and exact location of any specific future cutoff event: while geomorphologists understand the general conditions that make a neck cutoff more likely (a sufficiently narrow neck, a sufficiently large flood), forecasting exactly when and where a particular meander will breach involves enough local variability in sediment, vegetation, and flow conditions that it remains more a matter of probabilistic assessment than confident, specific prediction.
This sits inside River Systems & Fluvial Processes, one of seven topics in Physical Geography, one of five domains in Geography, one of seventeen subjects the app can quiz you on.