river landforms

Answer: A waterfall forms where a river flows over a band of hard rock overlying softer rock (1 mark). The soft rock below erodes faster through hydraulic action and abrasion, creating an overhang of hard rock (1 mark). The overhang eventually collapses under gravity into the plunge pool below (1 mark). The fallen debris is swirled around by the waterfall, deepening the plunge pool through abrasion, and the waterfall retreats upstream leaving a gorge (1 mark).

Answer: A waterfall forms where a river flows over a band of hard rock overlying softer rock (1 mark). The soft rock below erodes faster through hydraulic action and abrasion, creating an overhang of hard rock (1 mark). The overhang eventually collapses under gravity into the plunge pool below (1 mark). The fallen debris is swirled around by the waterfall, deepening the plunge pool through abrasion, and the waterfall retreats upstream leaving a gorge (1 mark).

Answer: Meanders develop pronounced bends with erosion on the outside and deposition on the inside of each curve (1 mark). Over time, the neck of the meander narrows as erosion continues on both outside bends (1 mark). During a flood, the river breaks through the narrow neck, taking the shortest route (1 mark). Deposition then seals off the cut-off meander from the main channel, creating an ox-bow lake (1 mark).

Answer: Upper course landforms are created mainly by erosion - interlocking spurs form where the river cannot erode laterally around hard rock projections, and waterfalls form where hard rock overlies soft rock (1 mark). Gorges form behind retreating waterfalls (1 mark). Lower course landforms are created mainly by deposition - levees form from repeated flood deposition, and flood plains are wide, flat areas of accumulated alluvium (1 mark). Estuaries form where the river meets the sea (1 mark). The middle course shows a mix of both - meanders have erosion on the outside (river cliff) and deposition on the inside (slip-off slope) (1 mark). Ox-bow lakes form from meander cut-offs (1 mark). Upper course landforms are steep and rugged while lower course landforms are flat and wide, reflecting the changing energy of the river from source to mouth.

Answer: Deposition is extremely important in creating river landforms, particularly in the lower course where the river's energy is lowest. Levees are formed entirely by deposition - when a river floods, coarse sediment is deposited near the channel, building up raised banks over successive flood events (1 mark). The Mississippi River's natural levees are several metres high, demonstrating the effectiveness of this process (1 mark). Flood plains are also created by deposition - each flood adds a thin layer of alluvium, building up deep, fertile soils over thousands of years (1 mark). The Nile flood plain has been fertilised by annual deposits for millennia, supporting agriculture (1 mark). However, erosion also creates important landforms - waterfalls like High Force on the River Tees are formed by differential erosion, and meanders involve erosion on the outside bend (1 mark). Interlocking spurs and V-shaped valleys in the upper course are entirely erosional (1 mark). Even depositional landforms depend on erosion upstream to supply the sediment (1 mark). In the middle course, both processes are equally important - meanders show erosion (river cliff) and deposition (slip-off slope) working together (1 mark). In conclusion, deposition creates the most extensive landforms (flood plains can be many kilometres wide), but it cannot operate without erosion supplying the sediment, making both processes essential (1 mark).