Seen from the air, a lowland river loops back and forth across a broad, flat floodplain. Those loops, meanders, are not fixed; they grow and move. Flowing water and sediment transport explain the whole landscape.
Why it works
Water flowing around a bend keeps its momentum and is flung toward the outside of the curve, where it runs deepest and fastest. Fast water erodes the outer bank into a steep cut bank; on the inside, slow water drops sand in a gentle point bar. So each bend eats outward and builds inward, and the meanders swing sideways across the valley. When two bends nearly meet, a flood can cut through the narrow neck; the abandoned loop becomes an oxbow lake. Floods spill over the banks, slow down and drop silt, building the floodplain. And a river can only erode down to its base level: sea level overall, or a lake or reservoir locally.
Key idea
In a meander, the fastest water is on the outside of the bend, eroding a cut bank; the inside builds a point bar. Rivers cut down toward base level; raise base level and they deposit, lower it and they cut down.
Worked examples
Worked example: How winding?
A river's channel is 36 km long between two towns 20 km apart along the valley. What is its sinuosity?
- Sinuosity = channel length ÷ valley length $= 36 \div 20 = 1.8$.
- Above about 1.5, a river is called meandering.
Answer: 1.8: meandering.
Worked example: A dam
A dam is built across a river. What happens upstream, and why?
- The reservoir is a new, higher local base level.
- Where the river enters it, the water slows and drops its sediment, slowly filling the reservoir.
Answer: The river deposits sediment and fills the reservoir.
Common mistake
A common guess is that the water on the inside of a bend is fastest, because that path is shorter. It is on the outside. Check the banks: the outside bank is the steep, eroding one, and the inside is the sandy bar where you can wade.