In a flowing river, the water does not move at the same speed throughout the entire depth or cross-section of the river. The speed of the water can vary at different levels, and this phenomenon is known as velocity distribution or velocity profile.
Typically, in most natural rivers, the water near the surface tends to move faster compared to the water near the riverbed or bottom. This variation in velocity is due to factors such as friction with the riverbed and banks, turbulence, and the presence of obstacles like rocks or vegetation. These factors create drag or resistance, which slows down the flow of water near the riverbed.
The velocity distribution in a river is often described using a concept called "shear flow." Shear flow refers to the gradual change in velocity from the fastest-moving water at the surface to the slowest-moving water near the riverbed. The exact shape of the velocity profile depends on various factors, including the river's slope, depth, channel shape, and the roughness of the riverbed.
It's worth noting that in some engineered or controlled channels, such as canals or flumes, efforts may be made to create a more uniform velocity profile by carefully designing the channel shape and controlling the flow conditions. However, in natural rivers, variations in velocity are commonly observed.