|In this lesson, you will create a Mind Map to help you organize the information from this Unit in a comprehensive overview. As well, certain river topics needing further explanation will be presented.|
Recall that a delta is a level area of alluvial deposits at the mouth of a river where it enters the ocean or a lake. Deltas are often popular areas for farming due to the presence of fertile alluvial soils. Deltas are associated with the old stage of a river. There are four main types of deltas that you should be familiar with.
A) Bird's Foot or Digitate Delta
A bird's foot delta forms where sediment is deposited in relatively calm offshore waters. An example of a bird's foot delta is the Mississippi river delta.
B) Arcuate Delta
An arcuate delta forms when a river meets the sea in a place where the waves, currents, and tides are strong. It is often bow shaped and has a number of distributaries flowing across it. An example is the Nile delta of Egypt.
C) Estaurine Delta
When the mouth of a river enters the sea and is inundated by the sea in a mix with freshwater and very little delta, it is called an estuary. An example of a estuarine delta is the Seine river delta in France or the Mackenzie river delta in Canada.
D) Cuspate Delta
A cuspate delta is formed when a river drops sediment onto a straight shoreline with strong waves. Waves force the sediment to spread outwards in both directions from the river's mouth making a pointed tooth shape with curved sides. An example is the Tiber delta in Italy.
of the Fraser River Delta
A) 10 000 years ago
B) 5 000 years ago
What type of delta does the Fraser river appear to have at this point in time?
The Fraser river delta continues
to grow at a rate as much as five metres per year in places.
At this pace, in approximately
Recall that if a relatively flat landscape is uplifted, this will allow a river flowing on that landscape to start eroding vertically instead of laterally. Hence, the river begins to act more like its youthful stage and is said to be rejuvenated.
What can cause a landscape to uplift?
One way to uplift a surface is through isostatic rebound. This occurs after a surface sheds a tremendous weight such as the mass of ice sheets during the last ice age. The depressed land surface begins to slowly rise up over the course of thousands of years. This creates landforms such as raised beaches that used to be at sea level. Another potential cause for uplift is the movement of tectonic plates in a certain fashion.
Can a river be rejuvenated without uplift of the surface?
Yes. A drop in the base level of the river due to a drop in sea level or lake level would also cause a river to downcut.
What landforms may result from river rejuvantion?
Since the river is reverting back to its youthful stage, landforms such as waterfalls, rapids, and potholes may result. Rapids occur where the stream current is moving with a greater velocity than usual and where the water surface is broken by obstructions, but without a sufficient break in slope to form a waterfall. Rapids commonly result from a sudden steepening of the stream gradient or the presence of a retsricted channel. A pothole is a deep, round hole formed in the rock of a river bed by gravel whirling in water.
Another landform that may result from river rejuvenation is a river terrace.
eg Matheson River Area, Melville Peninsula, Nunavut.
"This is an aerial view of the Matheson River area, looking inland towards the Prince Albert Hills in the western portion of Melville Peninsula. Former river terraces form low benches above the modern river, and outwash plains with braided channel patterns inscribed on their surfaces form higher sandy expanses in front of the bedrock hills. The photo also shows a raised beach along the snow bank in the center of the picture. The river has cut through older deposits as the land emerged from the sea" (Geological Survey of Canada).
The Effects of Clearcutting
Clearcutting is a logging technique in which all trees are removed from an area, typically eight hectares (i.e. twenty acres) or larger.
In the previous lesson you looked at the effect of hydro dams on a river's variables. Predict how cleacutting would alter the discharge, sediment load, erosional transport, channel width and depth, velocity, channel roughness, and gradient of a river. Afterwards, check the answer key.
River Basin Management
A river basin is a term used to designate the area drained by a river and its tributaries.
In turn, these tributary streams have their own drainage basins. In mountainous areas, the separation between these basins occurs along the peaks. These are known as divides or watersheds (see Figure 13.2 on page 258 of Planet Earth: A Physical Geography).
On a large scale, the Rocky Mountains is referred to as "The Great Divide" as it sheds water either to the west or east as evidenced on the map below.
For more information on drainage basins in Canada, go to The Atlas of Canada.
Focussing on British Columbia, the map below shows the drainage basin of the Fraser river in green. The area outlined in red is the Fraser Headwaters Study Area. The headwaters are the source and upper reaches of a river. The Fraser river starts on Mt. Robson in the Rockies. The Fraser river is approximately 1400 km in length.
Whether one is talking about a drainage basin on a large or small scale, management of that basin is critical. Improper management can lead to inceased runoff and erosion resulting in flooding, siltation, and other problems.
Examples of Improper Management
- Poor farming
- Plough with the contours of
The map below indicates the drainage area of the lower portion of the Fraser river.
A) Carefully read the information presented above.
B) Design a mnemonic device to remember the
ways to better manage a river basin.
Press the button on the Action menu to proceed.