Answers to Chapter 14 Review Questions

1. Approximately 1% of the Earth’s water is liquid fresh water.

2. Approxmately 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 mm sand grains will be eroded if the velocity if over 20 cm/s and will be kept in suspension as long as the velocity is over 10 cm/s.

7. If the flow velocity is 1 cm/s particles less than 0.1 mm (fine sand or finer) can be transported, while those larger than 0.1 mm 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 m/km (or 28 cm/km).

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.

13. Ri = (n+1)/r (where n is the length of the record) and r is the rank of the flood in question. In the Ashnola River case Ri = (65+1)/2 = 33. The probability of such a flood next year is 1/Ri, or 1/33 which is 0.03 or 3%.