IB Geography A.1.4 the Formation of Typical River Landforms Notes

Waterfalls Retreat Upstream Through Differential Erosion

1. A waterfall can form where resistant rock overlies softer rock across the channel.
2. Hydraulic action and abrasion erode the softer rock more quickly and deepen a plunge pool.
3. Undercutting leaves an unsupported overhang of resistant rock that eventually collapses.
4. Repeated collapse makes the waterfall retreat upstream and leaves a steep-sided gorge.
5. Niagara Falls shows this sequence where resistant dolostone overlies weaker shale.

Floodplains and Levees Grow Through Repeated Flood Deposition

1. A floodplain is the wide, flat valley floor beside a river and develops through meander migration and overbank deposition.
2. Floodwater loses velocity when it leaves the channel because it becomes shallower and meets greater surface friction.
3. Coarser sediment is deposited first near the channel, while finer alluvium travels farther across the floodplain.
4. Repeated floods build raised natural levees beside the channel and layers of fine sediment across the wider floodplain.

1. Artificial levees may reduce flooding locally but can transfer risk downstream if they speed or confine floodwater.
2. The levee cross-section isolates how repeated overbank floods sort sediment beside the channel.

Meanders Migrate as Erosion and Deposition Act on Opposite Banks

1. Faster flow follows the outside of a bend, where hydraulic action and abrasion create a river cliff.
2. Slower, shallower flow on the inside bend deposits sediment and creates a slip-off slope.
3. Helicoidal flow transfers sediment from the outer bank of one bend toward the inner bank downstream.
4. Continued outer-bank erosion and inner-bank deposition make the bend migrate across the floodplain.

Deltas Form When Deposition Exceeds Sediment Removal

1. A delta develops where a river enters a lake or sea, loses velocity and deposits more sediment than waves, tides and currents remove.
2. Deposition blocks the original channel and divides flow among distributaries.
3. Arcuate deltas have a broad curved front where wave action redistributes sediment, as in the Nile Delta.
4. Bird's-foot deltas extend narrow distributaries seaward where river deposition dominates, as in the Mississippi Delta.
5. Cuspate deltas form a pointed shape where wave approach concentrates deposition around the river mouth, as at the Tiber.

1. Inland deltas such as the Okavango form where a river spreads across a low-gradient interior basin and loses water through evaporation and infiltration.