If the world were geometrically simple, all channels would be perfectly straight with frictionless sides, and all water would move through them at the same speed. We have already seen that this is not the case. In the real world, friction slows the water interacting with the walls of the channel.

Moreover, in reality, channels are not straight. Thus:


  • Terminology: Floodplains: All of this meandering happens as the stream "hoses" across a flat floodplain. How does this surface come into being?
  • Making a floodplain: When ground level is above the longitudinal profile, the stream will erode. Longitudinal Profile - How does the stream "know" when to stop cutting downward and start meandering?
    Complications of base level:

    Stream types:


  • Stream deposition: What happens during flood? Floodplain is inunated when the river overflows its banks. Think about the velocity of the water as it moves over the floodplain, the ultimate broad shallow channel. Thus, whereas you get only sand and larger deposited in channel, you get mud in floodplain. Furthermore, most of it piles up next to the channel, forming a levee. Thus, the channel it heightened.

  • Point bar sequence: Each package of sediment deposited at a point bar forms a dipping drape of sediment over the earlier point bar. Coarser clasts accumulate at the bottom, while finer ones are at the top. A boring through a point bar therefore shows a fining-upward sequence. The uppermost portions show features characteristic of shallower water, such as ripples. Because it is deposited by a moving flow, nothing finer than sand occurs. This typical sequence makes channel deposits easy to identify in ancient rocks.

    With this info we can now identify fluvial environments in the rock record: Layers of mud that are linear in map view, interspersed with ribbons of sand showing fining-upward sequence.
    Two associated environments:

    Deltas: The interface between the stream and marine environments. Transitional depositional environment.

    Alluvium and Alluvial fans: On land in arid regions, mountains can channel rain water into intermitent streams which may empty onto a valley floor. Flow tends to be flashy and intermittent, at times resembling mudflows more than streams. These streams carry much sediment but lose competence as soon as they are not confined by mountains.

    Result: Alluvial fan: A conical deposit characterized by large clasts radiating from a point source.


    Now, something cool. Check out this alluvial fan link.


    Key concepts and vocabulary: