Which factor affects the velocity of river flow the most?

The slope of the river is a crucial factor influencing the velocity of river flow. When the slope of the riverbed is steep, gravitational forces propelling the water downstream are stronger, leading to an increase in velocity. Steep gradients allow water to flow more quickly, as there is less resistance and it can move rapidly over the incline.

Additionally, while factors like river width, surface roughness, and water volume also impact flow rates, their effects are often secondary compared to slope. A wider river might suggest slower flows due to spreading water, surface roughness introduces turbulence which can slow down velocity, and while a greater volume of water can enable faster flow under certain conditions, the maximum velocity is still primarily determined by how steeply the riverbed descends. Therefore, the slope stands out as the most significant factor affecting the velocity of river flow.