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Unit 43


Unit Overview

This unit examines aggradational landforms created by fluvial processes. The main sections are as follows:

  • Alluvial fans
  • Streams to the sea
  • Deltas

Streams not only degrade landscapes - they also can build landscape features. An alluvial fan is one such feature; an ephemeral stream that emerges from highland areas creates it.

Associated with alluvial fans and bajadas (i.e., a coalescence of alluvial fans) are arroyos, which cut into and erode the depositional landforms. As streams move closer to the sea, depositional landforms become more common. Floodplains are dominant depositional landforms, and they are associated with meanders, oxbow lakes, meander scars, retreating bluffs, and levees. When a stream is rejuvenated, it resumes down-cutting, thereby forming terraces. The terraces are remnants of the older floodplain existing above newer bluffs. As a stream deposits its material into the sea, a delta is formed. The delta represents the final aggradational landform along the course of a stream.

Teaching Objectives

  • To identify the types of landforms built by streams
  • To examine the formation and development of the stream floodplain
  • To investigate the evolution of river deltas

Glossary of Key Terms

Alluvial fan A fan-shaped deposit consisting of alluvial material located where a mountain stream emerges onto a plain; primarily a desert landform.
Arroyo Gullies cut into alluvial fans due to increased streamflow.
Artificial levee An artificially constructed levee built to reinforce a natural levee, most often along the lowest course of a river.
Bajada A coalesced assemblage of alluvial fans that lines a highland front (also known as an alluvial apron); primarily a desert feature.
Braided stream One carrying a high sediment load that subdivides into many intertwined channels that reunite some distance downstream; as these channels shift position, stretches of deposited alluvium become divided between them, giving the "braided" appearance.
Delta The often major sedimentary deposit surrounding and extending beyond the mouth of a river where it empties into the sea or a lake; frequently assumes a triangular configuration, hence its naming after the Greek letter of that shape.
Distributaries The several channels a river subdivides into when it reaches its delta; caused by the clogging of the river mouth by deposition of fine-grained sediment as the stream reaches base level and water velocity declines markedly.
Entrenched meander Meanders of a rejuvenated stream incised into hard bedrock from overlying floodplain topography; caused by the uplifting of the land surface above the stream's base level.
Ephemeral stream An intermittently flowing stream in an arid environment; precipitation (and subsequent stream flow) is periodic, and when the rains end the stream soon runs dry again.
Floodplain The flat, low-lying ground adjacent to a stream channel built by successive floods as sediment is deposited as alluvium.
Meander The smooth, rounded curves or bends of rivers that can become quite pronounced as floodplain development proceeds; also characteristic of many ocean currents, which, after the passage of storms, can produce such extreme loops that many detach and form localized eddies.
Midstream bar A midchannel sandbar that is deposited where sediment-clogged water of a stream significantly slows in velocity.
Natural levee A river-lining ridge of alluvium deposited when a stream overflows its banks during a periodic flood; when the river contracts after the flood, it stays within these self-generated "dikes" or levees.
Oxbow lake A lake formed when two adjacent meanders link up and one of the bends in the channel, shaped like a bow, is cut off.
Terrace The higher-lying remnant of an old floodplain that stands above the bluffs lining the newer floodplain of a rejuvenated river; when two terraces lie at the same elevation, they are called paired terraces.

Unit Outline

  • Alluvial fans
    • An ephemeral stream (one that flows intermittently) deposit alluvial material where it emerges onto a plain
      • alluvial fans are best developed in arid areas
    • Formation of alluvial fans includes two stages
      • formation of a midstream bar
      • formation of a braided stream
    • An alluvial apron (bajada) forms when alluvial fans coalesce
      • over time, can weather into desert pavement
  • Streams to the sea (Fig. 43.4)
    • Major rivers eventually drain into the sea, forming deltas
    • As a river begins to deposit material, it erodes laterally and forms meanders (bends)
    • The floodplain (Fig. 43.5)
      • the flat, low-lying area on either side of a river's channel, created as meanders erode laterally and downstream
      • oxbow lakes form when a meander is cut off (Fig. 43.6)
      • bluffs are created by meandering streams (Fig. 43.5)
      • a meander scar often remains on the landscape where a meander once existed
      • natural levees are broad ridges of deposited material along both sides of a flyer
        • artificial levees can also be constructed
    • Terraces (Fig. 43.10)
      • when a stream is rejuvenated, it cuts downward, and the remains of the old higher floodplain level are visible as terraces
        • terraces are sometimes paired on either side of a river at the same elevation
        • terraces can be constructed of rock or alluvial material
        • if land is uplifted, whole meander belts produce entrenched meanders
  • Deltas (Fig. 43.12)
    • Large fan-shaped areas of alluvial deposits at the mouths of rivers
    • Distributaries develop where the mouth of the river becomes clogged with sediment, carrying water in several directions
    • Factors influencing delta formation
      • stream volume
      • configuration of offshore continental shelf
      • strength of currents and waves
    • Deltaic plain is the flat land portion of a delta
    • Birdfoot delta can develop if distributaries are kept open by dredging
    • The delta profile (Fig. 43.14)
      • finest sediment is deposited in bottomset beds where water is quiet
      • top set beds lie under deltaic plain, constantly added to
      • foreset beds are built from leading edge of topset beds
        • Mississippi River

Review Questions

  1. Draw a general diagram of a delta profile, using Fig. 43.13 and Fig. 43.14 as references.
  2. What are terraces? Paired terraces?
  3. List the three major factors that influence delta formation.