If a stream has a steep slope or gradient, it will have a faster velocity, which means it will be able to carry larger materials in suspension. At flood stage, rivers flow much faster and do more erosion because the added water increases the stream's velocity.

While people often assume that small, steep mountain streams flow faster than large, wide rivers, the opposite is frequently true in terms of mean velocity. Science shows that as a river progresses downstream and its volume (discharge) increases, its channel typically becomes smoother and deeper, which reduces friction against the riverbed and banks. This allows the water to move more efficiently. While a mountain stream may look "fast" because of turbulent white water and rapids caused by rocks, the actual average speed is often slowed down by that same roughness. Large, deep rivers like the Amazon or the Mississippi often have higher average velocities because the majority of the water is moving through a deep, unobstructed "thalweg" far from the friction of the edges. However, the speed of any specific body of flowing water is ultimately determined by three primary factors: the gradient (slope), the volume of water, and the roughness of the channel. Therefore, while a major river is often faster on average, a narrow stream during a flash flood can temporarily reach much higher speeds than a calm river.