Rivers & erosion

A river looks peaceful, yet over long periods it shapes whole landscapes. It gnaws at rock, carries sand and gravel along and drops them again somewhere else. Wearing rock away and carrying it off is called erosion. Water does most of that work, but wind and ice play their part too. Deep valleys, wide loops and broad river mouths are all the work of rivers.

Wearing away and laying down

How much a river moves depends on its speed. Fast water has a lot of force and even drags stones along. When the river slows down, the water loses this force and lets the carried particles sink to the bottom. In its upper course in the mountains it flows fast and mainly wears away, in flat land it becomes slow and lays material down.

Meanders: the loops of the river

In flat land a river rarely flows dead straight but in loops, the meanders. In a bend the water flows fastest at the outer bank and wears that bank away. At the inner bank it is slow and lays down sand. In this way the loop wanders further over time and becomes more strongly curved.

From the valley to the delta

In its upper course the fast river cuts a narrow, steep valley into the rock, often shaped like a V. The closer it comes to the sea, the slower and wider it gets. At the mouth the water is so calm that almost all the carried material settles. It piles up into new land called a delta: seen from above it often looks like a triangle, just like the Greek letter delta.