What is a primary factor influencing the capacity of a river to transport sediment?

The capacity of a river to transport sediment is primarily influenced by water velocity. This is because the speed of the water flow determines how much sediment can be picked up, carried, and deposited. Faster-moving water has greater energy, allowing it to lift heavier particles and transport larger quantities of sediment downstream. When the velocity is increased, all particle sizes — from silt to gravel — become more easily mobilized. Conversely, if the water is moving slowly, it may not have enough energy to transport sediment effectively, leading to deposition instead.

While other factors mentioned can have some influence on sediment transport — such as water temperature affecting viscosity or chemical composition influencing the interaction of sediment with water — they do not have as direct and significant an impact as water velocity does. Bedrock type might influence the amount and type of sediments available in the river, but it does not determine the river's transport capacity in the same manner as the velocity of the water. Overall, water velocity stands out as the key factor in the dynamics of sediment transport within river systems.