Science Puzzle
Why Don’t Rivers Flow in Straight Lines?
Look at almost any river crossing flat land and it winds back and forth in great loops called meanders. Now and then a loop gets cut off and left behind as a curved lake.
Water runs downhill, and the shortest way down is a straight line. Why don't rivers flow in straight lines?
The Answer
Because any small bend makes itself bigger. On a bend the river wears away its outer bank and builds up its inner bank, so the bend grows, and a straight river can't stay straight for long.
Round a bend, the water on the outside has further to go and flows faster. Fast water carries more sand and gravel and scours the outer bank away, leaving a steep cliff. On the inside the water is slower, so it drops what it was carrying and builds a gently sloping beach. One bank moves outwards and the other follows, and the loop swings wider.
The water also spirals as it goes round. Water at the surface is flung towards the outer bank and dives, while water along the bottom is pushed back towards the inner bank, carrying sediment with it. In 1926 Albert Einstein wrote a paper explaining this and pointed out it's the same reason tea leaves gather in the middle of a stirred cup.
Even a river that starts dead straight has small irregularities: a fallen tree, a patch of softer soil. Each one nudges the flow to one side, and the feedback does the rest. Eventually two loops grow so close that the river breaks through the narrow neck in a flood, takes the short cut and leaves the old loop behind as an oxbow lake.
The principle: Feedback in erosion. On a bend the faster water on the outside wears the bank away while slower water on the inside drops sediment, so any small bend keeps on growing.