Page 499 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 15 River Systems 463
▲Figure 15.11 A turbulent stream. The high-gradient, turbulent Maligne River flows through a bedrock canyon in the Canadian Rockies. [Ashley Cooper/Corbis.]
point and carry larger amounts of sediment. As this de- bris moves along, it mechanically erodes the streambed further through the process of abrasion, with rock and sediment grinding and carving the streambed like liquid sandpaper.
Fluvial erosion by hydraulic action and abrasion causes streams to erode downward (deepen), erode lat- erally (widen), and erode in an upstream direction (lengthen). The process whereby streams deepen their channel is known as channel incision (an example on the San Juan River is discussed ahead). The process of lateral erosion is discussed in the next section with meandering river channels.
The process whereby streams lengthen their chan- nels upstream is called headward erosion. This type of erosion occurs when the flow entering a main chan- nel has enough power to downcut, such as occurs at the break in slope where a gully enters a deep valley (Figure 15.12). Headward erosion can also occur from groundwater sapping, in which groundwater seeps out of the ground at the head of the channel and weakens the channel’s upstream endpoint. Headward erosion can eventually cause an eroding part
of one stream channel to break
through a drainage divide and
capture the headwaters of another
stream in an adjacent valley—an
event known as stream piracy.
tributaries of an arid-region river in Baja California Sur, Mexico. [Rg B Ventures LLC dba SuperStock/Alamy.]
vegetation, and human activity in a drainage basin. Dis- charge is also closely linked to sediment transport— increased discharge moves a greater amount of sediment, often causing streams to change from clear to murky brown after a heavy or prolonged rainfall. Sediment is moved as dissolved load, suspended load, or bed load by four primary processes: solution, suspension, saltation, and traction (Figure 15.13).
The dissolved load of a stream is the material that travels in solution, especially the dissolved chemical compounds derived from minerals such as limestone or dolomite or from soluble salts. The main process contrib- uting material in solution is chemical weathering. Along the San Juan and Little Colorado Rivers, which flow into the Colorado River near the Utah–Arizona border, the salt content of the dissolved load is so high that human use of the water is limited.
The suspended load consists of fine-grained clastic particles (bits and pieces of rock). They are held aloft in the stream until the stream velocity slows nearly to zero, at which point even the finest particles are de- posited. Turbulence in the water, with random upward
Dissolved load
Sediment Load When stream en- ergy is high and a supply of sedi- ment is present, streamflow propels sand, pebbles, gravel, and boulders downstream in the process known as sediment transport. The mate- rial carried by a stream is its sedi- ment load, and the sediment supply relates to topographic relief, the nature of rock and soil through which the stream flows, climate,
Stream current
Suspended load
Bed load
Rolling grain
Traction
Saltation
Streambed
▲Figure 15.13 Fluvial transport. Fluvial transportation of eroded materials as dissolved load, suspended load, and bed load. Bed load includes grains transported by traction (rolling or dragging of materials along the streambed) and saltation.
Headward erosion
▲Figure 15.12 Headward erosion. Headward erosion in several