Chapter 4: Fluvial Processes in Riparian Areas

Channel Adjustment

  1. Channels constantly adjust in width, depth, profile and planform patterns in response to variations in discharge and sediment.
  2. A dynamic equilibrium is reached when the long term balance between water and sediment is such that the channel neither aggrades nor degrades. This can occur when climate remains relatively stable and no disturbances occur (fire, stream regulation etc.).
  3. Dynamic stability can be expressed as the product of sediment load and sediment size in proportion to (but not equal) to the product of discharge and channel slope.

    Figure 4.14. Relationship among stream discharge (Q), channel slope (S), sediment discharge (Q s), and channel sediment size (D 50 that is the median grain size of the channel sediment.

    • Changes to any one of the factors can affect each or all of the remaining three factors.
      • If the sediment load is increased, the channel will aggrade as the transport capacity is exceeded and sediment is deposited.
      • If the discharge increases, the channel will degrade through scour as sediment is picked up to satisfy the transport capacity.
      • During the period of major adjustment the channel can be considered unstable.
      • Changes that can unbalance the dynamic equilibrium are bank stabilization, channelization, river regulation, road construction, sand and gravel mining, and vegetation removal in response to land use change.
    • In southeastern Arizona, many valleys experienced entrenchment during the late 1800's through the early 1900’s because of disturbances of their dynamic equilibrium. Several causes have been suggested for the regional entrenchment, including climate, fire, intensive grazing.