River landforms

Question 1

Straight river channels

Answer 1

• rare occurence
• thalweg moves from side to side
• These channels generally have a central ridge of deposited material, due to the water flow pattern

Question 2

Meandering river channels

Answer 2

• Most rivers meander to some extent
• Upland streams meander less, most pronounced found on floodplains where the soft nature of the river banks facilitates lateral erosion
• Meanders v. common because spiraling is normal behaviour of normal flowing fluids on the surface of a rotating planet

Question 3

Braided river channels

Answer 3

• Formed by steep channel gradient, a large proportion is coarse material
• easily erodible bank material and high variable discharge
• Common in periglacial and semi-arid areas.
• During high discharge, rivers transport large amounts of sediment
• braiding occurs when sediment exceeds river capacity, = deposition.
  Also when river gradient becomes shallower (lower course).
• Begins with mid-channel bar that grows downstream. coarse bed load deposited first followed by finer material. Upstream end becomes stabilised with vegetation. Island localises and narrows to form more permanent eyots. Vegetation traps further sediment and increases height of eyot so only briefly covered at periods of very high discharge
  E.g. Waitaki river

Question 4

Waterfalls

Answer 4

Band of resistant rock over band of less resistant rock. Differential erosion results in less resistant rock undercutting more resistant rock creating an overhang. Plunge pool created by sediment colliding with bank. As process continues and waterfall retreats, a gorge will be formed

Question 5

River Cliff

Answer 5

• water erodes lower section of the river bank, causing upper section of river bank to become unsupported and therefore unstable
• eventually, unsupported upper part of bank collapses, causing steep cliff to form
• occur when water outside the meander’s bend erodes a steep valley wall. occurs on wide flood plain, when meander shifts from side to side to reach valley wall

Question 6

floodplain

Answer 6

• large flat area of land near a river, forms due to erosion and deposition
• over time, meanders erode a v-shaped valley into wider, flat valley
• when river floods, sediment transported by river leaves channel and spreads out onto river bank
• water loses energy when no longer flowing through channel, meaning sediment is deposited onto flat plain
• over time, builds up making it higher. usually v fertile soil

Question 7

levees

Answer 7

• natural embankments along river that build up due to repeated flooding
• sediment being transported rises onto floodplain in a flood
• heavy coarse material deposited first as requires most energy
• finer light sediment needs less energy, so transported and deposited further from channel.
• over time, builds up and becomes more well-formed. means river banks can be higher with no flooding (natural flood defence)

Question 8

delta

Answer 8

• form at mouth of river, where river carries out large amount of sediment
• when river meets body of water, large difference in velocity, so river mass deposits sediment
• forms series of sediment islands with smaller streams and rivers flowing through them