Running Water and Random Numbers
idea by A. Colleen Yows
To model the development of drainage basins by using random rolls of a number cube.
Water, a dominant geologic agent on Earth, makes significant changes to our planet's surface. Channels and valleys are evidence of the erosional effects of running water. Point bars, levees, terraces, alluvial fans, and deltas are deposits associated with running water.
|This exercise focuses on the development of channels and drainage basins (the total area contributing water to the stream channel). Is there an orderliness to channel development? Or is there a randomness to the channel patterns? In truth, the answer is yes to both questions. Channel patterns are related to and sometimes controlled by underlying rock type and structure, especially in the presence of faults, joints, domes, and basins.|
On the otherhand, where bedrock is relatively uniform and flat, a beautifully random, tree-like (dentritic) channel pattern develops. The tracing to the left shows a dentritic channel network from the country of Yemen bordered by the Red and Arabian Seas. Channel intersections point downstream, so the flow of water is down to the bottom of the page.
In nature, streams continuously change shape and orienation in response to the amount of water and sediment being carried in the channel. Random patterns develop when discharge (the volume of water moving downstream per unit time) and load (mateial being transported by the stream) are variable e.g., in braided streams.
A roll of a number cube provides an excellent simulation of the random development of stream channels.
What about water on the other planets? Liquid water does not exist on the surfaces of the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, or Pluto. But data from Mars and many of the outer planet satellites lead scientists to conclude that water ice does indeed exist on these bodies today. Past climatic conditions on Mars were probably different enough from current conditions to allow flowing liquid water on the surface. How do we know? The evidence comes from photographs taken by orbiting spacecraft and lander craft which depict channels and valley networks on the surface of Mars.
Review and prepare materials listed on the student sheet.
Review the key words so the students can label features they see on their diagrams.
Each pair of students begins with one sheet of graph paper. If there is enough time, encourage them to tape another sheet of paper onto the bottom to continue the exercise.
Remind students to choose the dot (water source) before tossing the number cube to ensure randomness.