The Speed Of A River

The gradient of the river influences the speed but other factors need to be considered. Water flowing down a steep slope or gradient has higher velocity than one flowing down a gentle gradient. A river that plunges down a steep slope as a waterfall is much faster than in a river that travels down a gentle slope.

Other Factors:

Roughness Of The Channel

Material such as rocks in the channel can influence the speed. Whether rocks on the river bed are smooth or rough or uneven. Rocks that protrude out from the bank can slow the pace of the water as friction slows it down as it passes the obstacles.

The Shape Of The Channel

The shape of the channel or its cross section affects the extent to which water is in contact with its channel. This is known as the wetted perimeter. The greater the wetted perimeter, the greater the friction between the water and the banks and the bed of the channel, and the slower the flow of river.

Wetted Perimeter

Wetted perimeter is calculated by adding the length and the breadth of the channel in contact with the water

Example 1:

A river 2 meters deep and 3 meters wide will have a volume of 6 sq meters (2mx3m) and a wetted perimeter of 7 metes (2m+3m+2m). The 7 meters will be represent the friction slowing the river down

Example 2:

A river 4 meters deep and 6 meters wide will have a 24 sq meters volume and a wetted perimeter of 14 meters.

But the shape of the river is a major influence! A river with the same volume of water as Example 2 but with a different shape will have a different friction value.

Example 3:

The river is 1 meter deep but 24 meters wide.

This means that the wetted perimeter is 26 meters almost double that of Example 2 which means that the river will be slower as a larger part of the river energy is used to overcome friction.

The gradient of the river channel is only one factor to influence the speed of the river.