Chapter 13 Streams and Floods
The topics covered in this chapter can be summarized as follows:
|13.1 The Hydrological Cycle||Water is stored in the oceans, glacial ice, the ground, lakes, rivers, and the atmosphere. Its movement is powered by the sun and gravity.|
|13.2 Drainage Basins||All of the precipitation that falls within a drainage basin flows into the stream that drains that area. Stream drainage patterns are determined by the type of rock within the basin. Over geological time, streams change the landscape that they flow within, and eventually they become graded, meaning their profile is a smooth curve. A stream can lose that gradation if there is renewed uplift or if their base level changes for some reason.|
|13.3 Stream Erosion and Deposition||Erosion and deposition of particles within streams is primarily determined by the velocity of the water. Erosion and deposition of different-sized particles can happen at the same time. Some particles are moved along the bottom of a river while some are suspended in the water. It takes a greater velocity of water to erode a particle from a stream bed than it does to keep it in suspension. Ions are also transported in solution. When a stream rises and then occupies its flood plain, the velocity slows and natural levées form along the edges of the channel.|
|13.4 Stream Types||Youthful streams in steep areas erode rapidly, and they tend to have steep, rocky, and relatively straight channels. Where sediment-rich streams empty into areas with lower gradients, braided streams can form. In areas with even lower gradients, and where silt and sand are the dominant sediments, meanders are common. Deltas form where streams flow into standing water.|
|13.5 Flooding||Most streams in Canada have their highest discharge rates in spring and early summer, although many of B.C.’s coastal streams are highest in the winter. Floods happen when a stream rises high enough to spill over its banks and spread across its flood plain. Some of the more significant floods in Canada include the Fraser River flood of 1948, the Saguenay River flood of 1996, the Red River flood of 1997, and the Alberta floods of 2013. We can estimate the probability of a specific flood level based on the record of past floods, and we can take steps to minimize the impacts of flooding.|
Questions for Review
- What is the proportion of liquid (not frozen) freshwater on Earth expressed as a percentage of all water on Earth?
- What percentage of that fresh water is groundwater?
- What type of rock, and what processes, can lead to the formation of a trellis drainage pattern?
- Why do many of the streams in the southwestern part of Vancouver Island flow to the ocean as waterfalls?
- Where would you expect to find the fastest water flow on a straight stretch of a stream?
- Sand grains can be moved by traction and saltation. What minimum stream velocities might be required to move 1 millimetre sand grains?
- If the flow velocity of a stream is 1 cm per second, what sizes of particles can be eroded, what sizes can be transported if they are already in suspension, and what sizes of particles cannot be moved at all?
- Under what circumstances might a braided stream develop?
- How would the gradient of a stream be affected if a meander is cut off?
- The elevation of the Fraser River at Hope is 41 metres. From there it flows approximately 147 kilometres to the sea. What is the average gradient of the river (metres per kilometre) over that distance?
- How do B.C.’s coastal streams differ from most of the rest of the streams in Canada in terms of their annual flow patterns? Why?
- Why do most serious floods in Canada happen in late May, June, or early July?
- There is a 65-year record of peak annual discharges on the Ashnola River near Princeton, B.C. During this time, the second highest discharge was 175 m3/s. Based on this information, what is the recurrence interval (Ri) for that discharge level, and what is the probability that there will be a similar peak discharge next year?
Answers to Review Questions can be found in Appendix 2.