Unit 41


Unit Overview

This unit examines the various processes responsible for stream dynamics. The main sections are as follows:

  • Erosion and the hydrologic cycle
  • Streams and basins
  • The stream as a system
  • Stream functions and valley characteristics
  • Factors in stream erosion
  • The concept of the graded river system

Streams are incredibly important erosional agents. Fluvial erosion begins when the precipitation rate exceeds the infiltration rate, thereby initiating runoff. Erosion eventually leads to rills, which form the initial component of a drainage network, which is part of a drainage basin. Stream erosion occurs through hydraulic action, abrasion, and corrosion, and the resultant load is transported by traction, saltation, suspension, and solution.

Velocity is a critical factor in a stream's ability not only to erode but also to transport material. Under ideal conditions, the highest velocity prevails in the middle of the stream channel where the stream is deepest. Erosion can also occur outside the stream channel; this occurs during flood events. Floodplains, which are depositional landforms, result from flooding. Many streams epitomize steady-state systems, because energy and matter move in a particular way and work is evenly distributed throughout the stream. Streams ultimately become graded, for a longitudinal profile is established that allows the stream's load to be transported with neither degradation nor aggradation at any part of the profile.

Unit Objectives

  • To discuss the processes associated with the erosion of hillslopes
  • To outline the factors influencing the erosional activity of streams and to discuss the mechanisms of stream erosion and sediment transport
  • To characterize the river as a system and to identify the processes associated with this system

Glossary of Key Terms

Abrasion The erosive action of boulders, pebbles, and smaller grains of sediment as they are carried along a river valley; these fragments dislodge other particles along the stream bed and banks, thereby enhancing the deepening and widening process.
Absolute base level The elevational level lying a few meters below sea level.
Alluvium Sediment laid down by a stream on its valley floor; deposition occurs when the stream's velocity decreases and the valley fills with a veneer of unconsolidated material. Since soil particles washed from slopes in the drainage basin form a large part of these deposits, alluvial soils are usually fertile and productive.
Base level The elevational level below which a stream cannot erode its base.
Corrosion The process of stream erosion whereby certain rocks and minerals are dissolved by water; can also affect coastal bedrock that is susceptible to such chemical action.
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.
Denudation The combined processes of weathering, mass movement, and erosion that over time strip a slope of its soil cover unless the local rate of soil formation is greater.
Drainage basin The region occupied by a complete stream system formed by the trunk river and all its tributaries (also known as a watershed).
Flood An episode of abnormally high stream discharge; water overflows from the stream channel and temporarily covers its floodplain (which continues to build from the alluvium that is deposited by the floodwaters).
Floodplain The flat, low-lying ground adjacent to a stream channel built by successive floods as sediment is deposited as alluvium.
Fluvial processes Flow processes involving running water; the Latin word for river is fluvius.
Graded river The slope of a river channel as measured by the difference in elevation between two points along the stream course.
Headward erosion The upslope extension, over time, of the 'head' or source of a river valley, which lengthens the entire stream network.
Hydraulic action The erosional work of running water in a stream or in the form of waves along a coast. In a river, rock material is dislodged and dragged away from the valley floor and sides; where waves strike a shoreline, the speed and weight of the water, especially when air is compressed into rock cracks by the power of the waves, can fracture and erode coastal rocks quite rapidly.
Infiltration capacity The rate at which a soil is able to absorb water percolating downward from the surface.
Local base level The base level for a river that flows into a lake at whatever altitude it may lie.
Saltation The transportation process that entails the downstream bouncing of sand- and gravel-sized fragments along the bed of a moving stream.
Sheet erosion The erosion produced by sheet flow as it moves fine-grained surface materials.
Solution The transportation process by whereby rock material dissolved by corrosion is carried within a moving stream.
Stream capacity The maximum load of sediment that a stream can carry within a given discharge.
Suspension The transportation process whereby very fine clay- and silt- sized sediment is carried within a moving stream.
Temporary base level Base level of a stream formed of especially hard, resistant rock, limits further upstream channel incision for a time.
Traction Transportation process that involves the sliding or rolling of particles along a riverbed.

Unit Outline
  • Erosion and the hydrologic cycle
    • Infiltration capacity of a soil is the rate at which it is able to absorb surface water
    • Runoff results when the infiltration capacity is exceeded
    • Splash erosion occurs when large raindrops dislodge soil
      • splash erosion on a slope can result in loss of soil cover, or denudation, in time
      • vegetation cover lessens the effects of splash erosion
  • Streams and basins
    • Sheet flow is rain that is not absorbed by soil
      • sheet flow (sheet wash) results in sheet erosion
      • continued runoff can form rills, brooks, creeks, and streams
    • A river system has several parts
      • trunk river
      • tributary streams
    • A river system as a whole forms a drainage basin (watershed)
      • drainage basins are separated by divides, or topographic barriers
      • sediment yield is the total amount of sediment leaving a basin, but some remains as deposited alluvium
    • Conditions that affect erosion rates in drainage basins
      • precipitation
      • vegetation
      • relief
      • underlying lithography
      • human impact
  • The river as a system
    • Energy and the work in a river system (Fig. 41.4)
      • water generates kinetic energy as it flows downstream, powered by Sun
      • potential energy is transformed into kinetic energy
      • a river is a steady-state system, water and energy are input and output equally
        • tendency for the least work to be done
        • tendency for work to be uniformly distributed
        • graded river profile is a compromise between these two principles
    • A graded river system
      • a river is considered graded if its load is transported with no degradation or aggradation to its profile
    • Factors that influence a river's tendency to be graded
      • independent factors (not under river's control)
        • discharge
        • sediment load
        • ultimate base level
      • semidependent factors (somewhat under river's control)
        • channel width
        • channel depth
        • bed roughness
        • grain size of sediment
        • velocity
        • meander/braid tendency
      • dependent factor (under river's control)
        • slope
  • Stream functions and valley characteristics
    • Rivers are not static, they are constantly changing
      • organic model describes a river's life cycle with stages of youth, maturity, and old age
    • A river's course lengthens over time in several ways
      • headward erosion
      • building of deposits in delta
      • valley lengthening
    • Valley deepening occurs when water in V -shaped cross-section rushes downslope
    • Erosion by rivers
      • hydraulic action is the physical removal of rock and debris by the water itself
      • abrasion is the further removal of materials by rock and sediment that have already been dislodged, and are being carried downstream
      • corrosion is the process by which certain types of rock are dissolved by water
        • limestone
        • sandstone
    • Transportation by rivers (Fig. 41.8)
      • traction is sliding or rolling of particles in the river bed by hydraulic action
      • saltation is traction and suspension, particles bounce along the river bed
      • suspension carries very fine sediment, does not contact the river bed
      • solution carries dissolved particles, does not contact the river bed
    • Deposition by rivers
      • as distance increases from source, particle size decreases, as does erosional power
  • Factors in stream erosion
    • Stream power
      • stream capacity is the maximum load that a stream can carry with a given discharge
      • stream competence refers to the effectiveness of erosion
        • the faster the stream, the more effective it is at eroding materials
    • Stream floods
      • a flood occurs when a river overflows its channel
      • floods occur infrequently, effects are very limited over long time periods
      • floodplains, the low-lying areas adjacent to the channel, are built up by the alluvial deposits of floodwaters
    • Base levels
      • the level below which a stream cannot erode its bed
      • absolute base level lies no more than a few meters below sea level
      • the lake level is the local base level of a river
      • temporary base level is reached when stream erosion reaches a particularly resistant or hard layer of rock, limiting upstream channel incision for a time
  • The concept of the graded river system
    • Not all rivers are graded, and not all graded rivers have a smooth profile

Review Questions

  1. Define the terms flood and floodplain.
  2. Diagram the river as an open system, using Fig. 41.4 as a guide.
  3. What are the four major processes at work when a stream is transporting its load?