Re: Curve of a river.

As a river curves, the outside of the curve is:
A. eroded by the river
B. built up as the river deposits its sediment
C. neither eroded nor built up
D. the slowest part of the river

Water moving along the inside of the curve moves more slowly. When it slows down it no longer has as much energy to carry sediment, so some of its sediment load is dropped or "deposited" on the inside of the bend. This type of deposit is called a "point bar".

If the river is flowing over sediment that is easily eroded, over time the river channel will migrate in the direction of the outside of the curve. Eventually the curves or "meanders" become so big that when the river floods it finds a shorter path between them and cuts a new channel. The cut-off meanders become what are called "ox-bow lakes". If you look at a map of the lower Mississippi River Valley you will see its meanders and ox-bow lakes. Even rivers flowing over hard bedrock erode the outsides of their bends, but how far the bends can migrate is usually limited by the hardness of the rock and the topography of the valley.

This still leaves the question of why the water moves faster around the outside of the bend. This will take a little physics to explain. Water flowing downhill under the force of gravity will always follow the path of least resistance. If you look closely at a stream, you can see that there is part of the channel where the water flows faster than in the other parts. This fastest-flowing part of the stream is called the "thalweg". One of the laws of physics is about "inertia": a mass in motion will remain in motion unless acted on by a force. In this case the mass in motion is the flowing water. The water in the thalweg flows fastest because that part of the channel has the least resistance to its flow. When the water reaches a bend in the river, because of its inertia the water tries to keep flowing in a straight line but is met by an obstacle (the river bank) that forces it to change direction. The greater speed of the water in the thalweg gives it more "kinetic energy". Since it is harder to stop something that is moving fast than something that is moving slow, its greater energy carries it up against the obstacle, and the thalweg hugs the outside of the curve. If you have ever watched the bobsled competition in the winter olympics, the path of the sled as it travels down its run is similar to what happens to the water in the thalweg of a stream.

I hope this helps to answer your question.

---- Sean Sherlock, Geologist