What typically causes a decrease in water velocity in a channel?

A decrease in water velocity in a channel can be attributed to several factors that impede the flow of water. Increasing channel roughness refers to the presence of irregularities along the channel bed and banks, such as rocks, vegetation, and sediment buildup, which create friction and turbulence. This friction opposes the flow, thus reducing the velocity of the water.

Decreasing channel slope is another important factor; a shallower slope means that water has to travel a longer distance to fall the same elevation, resulting in a slower flow. In essence, the gravitational pull acting on the water is less pronounced at lower slopes, leading to reduced water velocity.

When both increasing roughness and decreasing slope occur simultaneously, the effects compound, presenting a more significant impact on the flow velocity. The channel becomes not only rougher but also less steep, contributing to an overall decrease in water velocity. Therefore, the combination of both factors leads to a more pronounced reduction in water velocity compared to either factor alone, making the choice that includes both increasing roughness and decreasing slope the most comprehensive answer to the question.