Which of the following is a cause of river erosion?

The flow of water and hydraulic forces is a primary cause of river erosion because the movement of water exerts pressure on the riverbed and banks. As water flows, it carries sediment and other materials away, which gradually wears down the landscape through hydraulic action. This occurs when fast-moving water creates turbulence that can dislodge and lift particles from the riverbed. The greater the velocity of the water, the more powerful the erosion process becomes, allowing rivers to carve out valleys and shape the land over time.

In contrast, the other factors mentioned do not directly contribute to the erosion process in the same way. For instance, the presence of dams typically slows down river flow, which can lead to sediment buildup rather than erosion. Plant growth along the banks can actually stabilize soil and reduce erosion by anchoring the banks, while the water's temperature, though important for various ecological aspects, does not play a significant role in the physical process of erosion itself.