In a hydrologic context, the deposition of bed-load in rivers is primarily governed by which factors?

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

Answer: (A)

The main idea is that whether bed-load grains are moved or deposited depends on the balance between the flow’s energy and how easily a grain can be mobilized. Flow velocity sets the transport capacity and the shear stress acting on the bed. Grain size determines the threshold needed to entrain a particle—the bigger the grain, the higher the flow energy required to lift or roll it along the bottom. When velocity (and the resulting shear stress) falls below that threshold, bed-load grains can no longer be kept in motion and they settle, forming deposits on the streambed. Larger grains need stronger flow to stay in motion, so they tend to deposit first as flow slows, while smaller grains can continue moving or be carried differently, but the crucial control for bed-load deposition is the combination of flow velocity and grain size. The color or temperature of the water don’t directly govern whether bed-load is deposited. Channel depth and width affect flow patterns, but they aren’t the primary factors controlling bed-load deposition. Mineralogy influences density and other properties, but it isn’t the main driver of whether bed-load grains deposit under typical conditions.