Big concept I: The Hydrologic Cycle: Earth's waters are stored in reservoirs, but each reservoir receives water and donates water to other reservoirs. The result is a hydrologic cycle: the process by which water circulates between the reservoirs.

What powers water movement: Two things, gravity and change of physical state.

  • Big concept II: In the modern natural world, reservoirs are at equilibrium. I.e. for each reservoir, the amount of water removed is balanced by an amount coming in.

  • Water for human use:

  • Humans can temporarily (time scale of 100s to 1000s of years) upset the equilibrium of reservoirs.

  • Big concept III: Hydrology (local condition of hydrologic cycle) is influenced by climate, runoff, and geologic conditions:

  • Humidity and rainfall: Basic facts: Hydrology and runoff:

    Today we are concerned with the runoff section. How does it occur and what do associated geological features look like?

    Stream properties:

  • Discharge. Total amount of water moving past a particular point per unit time. Discharge = velocity * Cross sectional area. Thus, if a stream has cross sectional area of 100 m2 and velocity of 2m sec-1, then

    Discharge = 100m2 * 2m/sec

    = 200m3/sec

    Of course, this varies over time.

    Erosion: The gradient of the stream determines the amount of energy to be released by the flowing water. High gradient ==> high energy. A portion of this energy is transferred to adjacent rock material as erosion. Streams erode by three mechanisms:

    Transport: Once rock material is eroded, it can be transported. Streams transport material as three different "loads":

    Note, there is a relationship between a stream's velocity (energy) and the size clast that it can transport. Clay sized particles require very little to keep them in suspension, whereas house sized boulders require much.

    We use two terms to describe a stream's ability to transport sediment:

    These are not the same things. Minihaha Stream at Glen Echo Park stream had the competence to carry automobile-sized clasts, but probably little overall capacity because it wasn't that big.
    Sink my car in the mouth of the Mississippi, and it will not move, thus, for all its capacity, the Mississippi has little competence at its mouth.
    Note!! All the stream descriptors we are using can change along the stream's length. The Potomac River at Great Falls is a very different stream than the Potomac River at Fort Washington, south of DC.


    Key concepts and vocabulary: