Final answer:
The driving force behind a river’s flow is gravity. While sediment load, velocity, and riverbed shape affect flow characteristics, gravity is the primary force that causes water to move downhill towards the ocean. The correct answer is option c) gravity
Step-by-step explanation:
The driving force behind a river's flow is gravity. Gravity pulls river water downhill, propelling it toward the ocean. As rivers or streams flow away from their source, they can widen and their currents slow down due to the gradient decrease. This process, combined with an increase in volume from tributary contributions, results in more sedimentation.
Sediments like gravel, sand, silt, and clay are deposited by moving water, and these can vary based on the velocity of the water. However, it's important to note that sediment load, velocity, and the shape of the riverbed all play a role in how a river flows, but they are not the initial driving force.
Additionally, flow rate and velocity are related. For instance, the higher the velocity of the water, the greater the flow rate. Nonetheless, the flow rate also depends on the river's size and shape. The equation Q = AU, where Q stands for flow rate, A for the cross-sectional area, and U for the velocity, helps to determine the volume flow rate of a river.