Fluvial depositional landforms

Floodplain
1. Area of moderately flat land formed from debris deposited when the river is in flood
  1. The width of floodplains is roughly proportional to river discharge.
  2. Convex floodplains
    1. The low-gradient floodplains of most large rivers are broad and have slightly convex (product of sedimentation) cross sections, the land sloping away from the riverbank to the valley sides, this because the channel banks and levees grow faster than the flood basins and may stand 1–15 m higher
  3. Flat floodplains
    1. Small floodplains are flat or gently concave in cross-section, natural levees are small or absent and the alluvial flats rise gently to the valley sides
      1. Formed by lateral accretion (sedimentation on the inside of meander bends) or alluviation in braided streams
Alluvial fan
1. Is a cone-shaped body that forms where a stream flowing out of mountains debouches on to a plain
  1. The steepness of the fan slope depends on the size of the stream and the coarseness of the load, with the steepest alluvial fans being associated with small streams and coarse loads.
    1. Alluvial fans are dynamic landforms so int and external factors control their evolution (Nicholas et al 2009)
River terraces
1. A terrace is a roughly flat area that is limited by sloping surfaces. They are the remains of old valley floors left after river downcutting.
  1. structural benches
    1. Resistant beds in horizontally lying strata may produce flat areas on valley sides
  2. Types
    1. Paired terraces
      1. form where the vertical downcutting by the river is faster than the lateral migration of the river channel
        Terraces of the same elevation on opposite sides of either a stream or river. The river downcuts evenly on both sides
    2. Unpaired terraces
      1. form where the channel shifts laterally faster than it cuts down
        occur when either a stream or river encounters material on one side that resists erosion, leaving a single terrace with no corresponding terrace on the resistant side
    3. Bedrock (S) terraces and alluvial terraces (A)
      1. Strath (S) terraces start in valleys where a river cuts down through bedrock to produce a V shaped valley, the floor of which then widens by lateral erosion
        Rock-floored terraces are pointers to prolonged downcutting, often resulting from tectonic uplift
      2. Accumulation (A) terraces are relicts of alluvial valley floors. They often form a staircase
2. Formation and survival
  1. Processes that promote river terrace formation
    1. Crustal movement (tectonic and isostatic)
    2. Eustatic sea level changes
      1. Static sea levels favour lateral erosion and valley widening.
    3. Climatic changes
      1. Climatic changes affect stream discharge and the grain size and volume of the transported load
    4. Stream capture
      1. A part of a stream captures a part of another stream. This is a one-off process and creates just one terrace level.
    5. Terraces tend to survive in parts of a valley that escape erosion (ex. slip-off slopes {inside bank of a meander} and spurs {ridges})
Water
1. Flowing water is a considerable geomorphic agent in most environments, and a dominant one in fluvial environments. Flowing water carves many erosional landforms, including rills and gullies, bedrock channels, and alluvial channels
River profiles, drawn from source to mouth, are normally concave, although they often possess knickpoints marked by steeper gradients
The discharge or flow (Q) in the large rivers goes from 5.000 to 200.000 m3/s

Next up

Fluvial depositional landforms

Description

Resource summary

Media attachments

Similar

	Created by alejandra escobar almost 6 years ago