Page 501 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 15 River Systems 465
Effects of Dams on Sediment Transport As discussed in the chapter-opening Geosystems Now, dams disrupt natural river discharge and sediment regimes, usually with detrimental effects on river systems. For example, Glen Canyon Dam on the Colorado River near the Utah– Arizona border controls discharge and blocks sediment from flowing into the Grand Canyon downstream. Con- sequently, over the years, the river’s sediment supply was cut off, starving the river’s beaches for sand, dis- rupting fisheries, and depleting backwater channels of nutrients.
In 1996, 2004, 2008, and 2012, the Grand Canyon was artificially flooded with dam-controlled releases in an unprecedented series of scouring–redistribution– deposition experiments for sediment movement. The first test lasted for 7 days. The later tests were of shorter duration and were timed to coincide with floods in tributaries that supplied fresh sediment to the system.
The results were mixed and the benefits turned out to be limited—the flood releases disrupted ecosystems and eroded some sediment deposits even while building up others. With such a limited sediment supply, not enough sediment is present in the system to build beaches and improve habitat even when high discharge occurs (more information is at www.gcmrc.gov/).
Recent dam removals have allowed scientists to study post-dam sediment redistribution. Focus Study 15.1 dis- cusses dam deconstruction and other stream restoration practices. Along with the Glines Canyon Dam in Wash- ington State (Figure 15.1.1), two dam removals in Canada are pictured, a smaller dam in Ontario (Figure 15.1.2) and a larger one in British Columbia (Figure 15.1.3).
Channel Patterns
A number of factors, including the sediment load, affect the channel pattern. Multiple-thread channels, either braided or anabranching (defined below), tend to occur in areas with abundant sediment or in the lowest reaches of large river systems. Single-thread channels are either straight or meandering. Straight channels tend to occur in headwater areas where gradient is high. In lower- gradient areas with finer sediments, meandering is more common; this is the classic river pattern in which a single channel curves from side to side in a valley or canyon.
Multiple-Thread Channels With excess sediment, a stream might become a maze of interconnected chan- nels that form a braided stream pattern (Figure 15.15). Braiding often occurs when reduced discharge lowers a stream’s transporting ability, such as after flooding, or when a landslide occurs upstream, or when sediment load increases in channels that have weak banks of sand or gravel. Braided rivers commonly occur in glacial en- vironments, where coarse sediment is abundant and slopes are steep, as in New Zealand, British Columbia, Yukon Territory, Alaska, Nepal, and Tibet. This pattern
▲Figure 15.15 A braided stream. The braided Waiho River chan- nel in the glaciated landscape of the west coast of new Zealand’s South Island. [David Wall/Alamy.]
also occurs in wide, shallow channels with variable discharge, such as in the U.S. Southwest.
In large river systems, an anabranching channel pat- tern sometimes occurs, in which multiple large chan- nels are present across a vast floodplain (Figure 15.16).
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▲Figure 15.16 An anabranching channel pattern. The anabranching channel of the Paraná River in South America. [nASA.]
Active side channel Paraná River
Past channels