As a stream moves from high mountains to lower areas toward sea level, it tends to erode more along its banks than downcutting into its bed.

As a stream moves from high mountains to lower areas toward sea level, it tends to erode more along its banks than downcutting into its bed.

• A. It downcuts into its bed more
• B. It erodes the banks more than the bed
• C. It stops eroding
• D. It widens only through deposition

Answer: (B)

As streams descend from steep mountains into flatter regions, the energy of the flow shifts from mainly carving downward to widening the channel. Near sea level the base level (sea level) limits how deep the bed can be cut, so vertical incision slows. The still-robust flow tends to erode the sides of the channel through lateral processes like bank undercutting and meander migration. This causes the valley to widen as the outer banks erode and the meanders shift, even while the bed remains relatively lower in erosion. So the river erodes the banks more than it downcuts the bed as it approaches sea level.

As streams descend from steep mountains into flatter regions, the energy of the flow shifts from mainly carving downward to widening the channel. Near sea level the base level (sea level) limits how deep the bed can be cut, so vertical incision slows. The still-robust flow tends to erode the sides of the channel through lateral processes like bank undercutting and meander migration. This causes the valley to widen as the outer banks erode and the meanders shift, even while the bed remains relatively lower in erosion. So the river erodes the banks more than it downcuts the bed as it approaches sea level.