In a fluvial environment, deposition of the bed load is dependent on which factors?

• A. The color of the grains
• B. The velocity of the water and size of the grains
• C. The depth of water
• D. The temperature

Answer: (B)

Bed-load movement in rivers is controlled by the energy of the flowing water and the size of the particles. The flow’s shear stress, which grows with velocity, must exceed a threshold that depends on grain size to entrain and move particles along the bed. Larger grains require higher velocities to be mobilized, so when the flow slows below what is needed for those grains, they deposit. This makes the combination of water velocity and grain size the primary factors controlling bed-load deposition.

Colors of grains and temperature don’t affect the mechanics of transport. Depth of water can influence the overall hydraulic conditions, but it’s the velocity and grain size that most directly determine whether bed-load stays in motion or settles.