Chapter 10 Answers

Review Question Answers:

1. Approximately 1% of the Earth’s water is liquid fresh water.
2. Approximately 30% of the Earth’s fresh water is groundwater.
3. A trellis drainage pattern typically forms on sedimentary rock that has been tilted and eroded
4. Many of the streams in the southwestern part of Vancouver Island flow to the ocean as waterfalls because the land has been uplifted relative to sea level over the past several thousand years.
5. The fastest water flow on a straight stretch of a stream will be in the middle of the stream near the surface.
6. 1 millimeter sand grains will be eroded if the velocity if over 20 centimeters per second and will be kept in suspension as long as the velocity is over 10 centimeters per second.
7. If the flow velocity is 1 centimeter per second, particles less than 0.1 millimeters (fine sand or finer) can be transported, while those larger than 0.1 millimeters cannot. At this velocity no particles can be eroded.
8. A braided stream can develop where there is more sediment available than can be carried in the amount of water present at the rate at which that water is flowing. This may happen where the gradient drops suddenly, or where there is a dramatic increase in the amount of sediment available (e.g., following an explosive volcanic eruption).
9. If a meander is cut off it reduces the length of a stream so it increases the gradient.
10. The average gradient of the Fraser River between Hope and the Pacific Ocean is 0.28 meters per kilometer (or 28 centimeters per kilometer).
11. In coastal regions of B.C. the highest levels of precipitation are in the winter, and large parts of most drainage basins are not frozen solid. As a result stream discharges tend to be greatest in the winter.
12. In most parts of Canada winter precipitation is locked up in snow until the melt season begins, and depending on the year and the location that happens in late spring or early summer. If the thaw is delayed because of a cold spring, and then happens very quickly, flooding is likely. Some regions also receive heavy rainfall during this period of the year.
    

    Exercise Answers

    Exercise 10.1 How long does water stay in the atmosphere?

    The volume of the oceans is 1,338,000,000 km³ and the flux rate is approximately the same (1,580 km³/day).

    What is the average residence time of a water molecule in the ocean?

    1,338,000,000/1,580 = 846,835 days average residence time for water in the ocean (or 2320 years)

    Exercise 10.2 The effect of a dam on base level

    How does the formation of a reservoir affect the stream where it enters the reservoir, and what happens to the sediment it was carrying?

    The velocity of the streams slows to zero and most of the sediment is deposited quickly.

    The water leaving the dam has no sediment in it. How does this affect the stream below the dam?

    With nothing to deposit, the water below the dam can only erode, so there will be enhanced erosion below the dam.

    Exercise 10.3 Understanding the Hjulström-Sundborg Diagram

    

    1. A fine sand grain (0.1 millimeters) is resting on the bottom of a stream bed.
       1. What stream velocity will it take to get that sand grain into suspension? Roughly 20 centimeters per second.
       2. Once the particle is in suspension, the velocity starts to drop. At what velocity will it finally come back to rest on the stream bed? Roughly 1 centimeters per second.
    2. A stream is flowing at 10 centimeters per second (which means it takes 10 seconds to go 1 meter, and that’s pretty slow).
       1. What size of particles can be eroded at 10 centimeters per second? No particles, of any size, will be eroded at 10 centimeters per second, although particles smaller than 1 millimetre that are already in suspension will stay in suspension.
       2. What is the largest particle that, once already in suspension, will remain in suspension at 10 centimeters per second? A 1 millimetre diameter particle should remain in suspension at 10 centimeters per second.

    Exercise 10.4 Determining stream gradients

    

    The length of the creek between 1,600 meters and 1,300 meters elevation is 2.4 kilometres, so the gradient is 300 divided by 2.4 = 125 meters per kilometer.

    1. Use the scale bar to estimate the distance between 1,300 meters and 600 meters and then calculate that gradient. 5.2 kilometres, with a gradient of 700 divided by 5.2 = 134 meters per kilometer.
    2. Estimate the gradient between 600 meters and 400 meters. 3.6 kilometres, with a gradient of 200 divided by 3.6 = 56 meters per kilometer.
    3. Estimate the gradient between 400 meters on Priest Creek and the point where Mission Creek enters Okanagan Lake. 4 kilometres, with a gradient of 60 divided by 4.0 = 15 meters per kilometer.

    Exercise 10.5 Flood probability on the Bow River

    1. Calculate the recurrence interval for the second largest flood (1932, 1,520 m³/s). Ri = 96/2 = 48 years
    2. What is the probability that a flood of 1,520 m³/s will happen next year? 1/48 = 0.02 or 2%
    3. Examine the 100-year trend for floods on the Bow River. If you ignore the major floods (the labelled ones), what is the general trend of peak discharges over that time? In general the peak discharges are getting lower (from an average of around 400 m³/s in 1915 to an average of about 300 m³/s in 2015)

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       “Physical Geology – 2nd Edition” by Steven Earle is licensed under CC BY 4.0 Adaptation: Renumbering, Remixing

       https://openpress.usask.ca/physicalgeology/