Describe how the speed of flowing water changes where a river bend?

It's all about fluid dynamics. The water in a river acts like a single substance. It has the tendency to stay together. One part of the fluid is not easily pulled apart from the rest. So when the water comes to a curve, the whole "bunch" of water wants to stay together and travel downhill at the same time. However, the water at the outside of the curve has to travel faster to keep up with the water on the inside of the curve because it must cover more distance in the same amount of time.

The same fluid principles also apply to air and account for the lift that occurs when air travels over an airplane wing.