Page 500 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

464 part III The Earth–Atmosphere Interface
motion, is an important mechanical factor in holding a load of sediment in suspension.
Bed load refers to coarser materials that are moved by traction, which is the rolling or dragging of materials along the streambed, or by saltation, a term referring to the way particles may bounce along in short hops and jumps (from the Latin saltim, which means “by leaps or jumps”). Particles transported by saltation are too large to remain in suspension but are not confined to the sliding and rolling motion of trac- tion (see Figure 15.13). Stream velocity affects these processes, particularly the stream’s ability to retain particles in suspension. With increased kinetic en- ergy in a stream, parts of the bed load are rafted up- ward and become suspended load.
During a flood (a high flow that overtops the chan- nel banks), a river may carry an enormous sediment
load, as larger material is picked up and carried by the enhanced flow. The competence of a stream is its ability to move particles of a specific size and is a function of stream velocity and the energy avail- able to move materials. The capacity of a stream is the total possible sediment load that it can transport and is a function of discharge; thus, a large river has higher capacity than a small stream. As flood flows build, stream energy increases and the competence of the stream becomes high enough that sediment trans- port occurs. As a result, the channel erodes, a process known as degradation. With the return of flows to normal, stream energy is reduced, and the sediment transport slows or stops. If the load exceeds a stream’s capacity, sediment accumulates in the bed, and the stream channel builds up through deposition; this is the process of aggradation.
Sediment Transport during a Flood We saw in the previous section that discharge can change quickly in response to precipitation events in a watershed. Greater discharge increases flow ve- locity and therefore the competence of the river to transport sediment as the flood progresses. As a result, the river’s ability to scour materials from its bed is enhanced.
As an example, Figure 15.14 shows changes in the San Juan River channel in Utah that oc- curred during a flood. The channel was deep- est on October 14, when floodwaters were highest (blue line plotted in Figure 15.14a). During this time, the channel bed eroded. The flood and scouring process moved a depth of about 3 m of sediment from the depicted cross section. By October 26, with the discharge re- turning to normal, the energy of the river was reduced, and the bed again filled as sediment redeposited. This type of channel adjustment
Right bank
9/15
10/14
10/26 9/15
Left 1687 m3.s-1 bank
512 m3.s-1 186 m3.s-1
9/9 — 18 m3.s-1
9/9 10/26
10/14
6
3 1.5 0 –1.5
(a)
Scouring occurs as discharge increases
September 15 186 m3 · s–1
is ongoing, as the system continu- ously works toward equilibrium, maintaining a balance between discharge, sediment load, and channel form.
◀Figure 15.14 How a flood affects a stream channel. (a) Channel cross sections showing the effects of a flood on the San Juan River near Bluff, Utah. (b) Details of channel profiles during four stages of the flood. [Adapted from “The Hydraulic geometry of Stream Channels and Some Physiographic Implications,” by L. Leopold and
T. Maddock, USgS Professional Paper 252, p. 32, 1941.]
Original channel profile
September 9 18 m3 · s–1
October 14 1687 m3 · s–1
Deposition occurs as discharge decreases
October 26 512 m3 · s–1
(b) Channel profiles by date
Gauge height (m)