Page 29 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Climate Systems and Climate Change xxv Visualizing Processes and landscapes
. NEW! Geosystems in Action present highly-visual presentations of core physical processes and critical chapter con- cepts. These features include links to mobile-ready media and MasteringGeography, as well as GeoQuizzes and integrated active learning tasks that ask students to analyze, explain, infer, or predict based on the information presented.
Stream channels meander, or curve
from side to side in a snakelike pattern, where a stream’s
gradient is low and it flows through fine sediments. Meanders form
because the portion of the stream with maximum velocity shifts from one side of the stream to the other as the stream bends, thus affecting erosion and deposition along the
stream’s banks (GIA 15.1). Through these “scour-and-fill” processes, a meandering stream moves position laterally across its valley and creates a distinctive landscape (GIA 15.2).
PROFILE OF A MEANDERING STREAM
15.1a The cross sections show how the location of maximum flow velocity shifts from the centre along a straight stretch of the stream channel to the outside bend of a meander. The oblique view shows how the stream erodes, or “scours,” an undercut bank, or cutbank, on the outside of a bend, while depositing a point bar on the inside of the bend.
[Vladimir Melnikov/Shutterstock.]
Maximum velocity
Pool (deep)
Undercut bank erosion:
Areas of maximum stream velocity (darker blue) have more power to erode, so they undercut the stream’s banks on the outside of a bend.
Areas of maximum velocity
FORMATION OF AN OXBOW LAKE
Neck
Itkillik River in Alaska [USGS.]
The neck narrows even more due to undercutting of its banks.
Point bar deposition:
On a bend’s inner side, stream velocity decreases, leading to deposition of sediment and forming a point bar.
ACTIVE EROSION ALONG 15.1b A MEANDER
Notice how this stream in Iowa has eroded a steep cutbank on the outside of a bend.
Explain: Explain the relationship between stream velocity, erosion, and deposition in the formation of a meander.
Step 1:
A narrow neck is formed where a lengthening meander loops back on itself.
Neck
Stream meander
The stream erodes through the neck, forming a cutoff.
Cutoff
Step 2:
Point bar
Step 4:
Undercut bank
Step 3:
An oxbow lake forms as sediment fills the area between the new stream channel and its old meander.
Cutbank
Oxbow lake
[USDA NRCS.]
15.2a Over time, stream meanders migrate laterally across a stream valley, eroding the outside of bends and filling the insides of bends. Narrow areas between meanders are necks. When
discharge increases, the stream may scour through the neck, forming a cutoff, as seen in the photograph.
Direction of flow
Cutoff
15.2b The diagrams below show the steps often involved in forming an oxbow lake; this photo corresponds to Step 3, the formation of a cutoff. As stream channels shift, these processes leave characteristic landforms on a floodplain.
STREAM MEANDERING PROCESSES
Stream valley landscape:
A neck has recently been eroded, forming a cutoff and straightening the stream channel. The bypassed portion of the stream may become a meander scar or an oxbow lake.
Follow up: In your own words, describe the sequence of steps in the process that forms an oxbow lake.
Animation
Stream Processes, Floodplains, Oxbow Lake Formation
470
Visit the Study Area in MasteringGeographyTM to explore meander and oxbow lake formation.
Visualize: Study a geosciences animation Assess: Demonstrate understanding of meander and of meander and oxbow lake formation. oxbow lake formation (if assigned by instructor).
An unparalleled visual program includes a variety of illustrations, maps, photographs, and composites, providing authoritative examples and applications of physical geography and Earth systems science.
GEOquiz
1. Explain: Explain the processes that cause a gentle bend 2. Summarize: Summarize the process by which a stream, over along a stream to become a deeply looping meander. time, could produce the landscape in the GIA 15.2a photograph.
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Figure 17.11 The geomorphic handiwork of alpine glaciers. As the glaciers retreat, the new landscape is unveiled. Inset photos are surface and aerial views from Norway. [Photos by Bobbé Christopherson; waterfall by Robert Christopherson.]
Arête
Horn Col
Paternoster lakes
Glacial erratics
Hanging valley
Hanging waterfall
Cirques
U-shaped valley
U-shaped glacial trough
Postglacial
Tarn
U-shaped valley, aerial
▲Figure 13.19 Seismic activity in or near Canada, 1627 to 2012. Damaging earthquakes (M > 5) are strongly associated
with the Western Cordillera and the valleys of the Ottawa and St. Lawrence Rivers. [Used by permission of the Minister of Public Animation Works and Government Services Canada; Natural Resources Canada, Geological Survey of Canada.] Elastic Rebound
Hanging waterfall
Tarn
U-shaped valley
GEOSgYeSoTsEyMstSemINs iAnCaTcItOioNnM1EA5NDMERESA:NSDTREERAINMGCSHTARENANMESL DEVELOPMENT
geosystems in action 15 MEANDERING STREAMS