Page 496 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

460 part III The Earth–Atmosphere Interface
potential to kinetic energy determines the ability of the stream to do geomorphic work and depends in part on the volume of water involved.
A stream’s volume of flow per unit of time is its discharge and is calculated by multiplying three vari- ables measured at a given cross section of the channel. It is summarized in the simple expression
Q = wdv
where Q = discharge, w = channel width, d = channel depth, and v = stream velocity.
Discharge is expressed in cubic metres per second (m3 ∙ s−1). According to this equation, as Q increases, one or more of the other variables—channel width, channel depth, stream velocity—must also increase. How these variables interact depends on the climate and geology of the fluvial system.
Changes in Discharge with Distance Downstream In most river basins in humid regions, discharge increases in a downstream direction. The Mackenzie River is typi- cal, beginning as many small streams that merge suc- cessively with tributaries to form a large-volume river ending in the Beaufort Sea. However, if a stream origi- nates in a humid region and subsequently flows through an arid region, this relationship may change. High po- tential evapotranspiration rates in arid regions can cause discharge to decrease with distance downstream, a pro- cess that is often exacerbated by water removal for irriga- tion (Figure 15.7). This type of stream is an exotic stream.
The Nile River, one of Earth’s longest rivers, drains much of northeastern Africa. But as it flows through the deserts of Sudan and Egypt, it loses water, instead of gaining it, because of evaporation and withdrawal for
agriculture. By the time it empties into the Mediterra- nean Sea, the Nile’s flow has dwindled so much that it ranks only 36th in discharge.
In the United States, discharge decreases on the Colorado River with distance from its source; in fact, the river no longer produces enough natural discharge to reach its mouth in the Gulf of California—only some agricultural runoff remains at its delta. The river is de- pleted by high evapotranspiration and removal of water for agriculture and municipal uses; shifting climatic patterns are adding to the river’s water losses.
As discharge increases in a downstream direc- tion, velocity usually increases. Stream velocity is af- fected by friction between the flow and the roughness of the channel bed and banks. Friction is highest in shallow mountain streams with boulders and other obstacles that add roughness and slow the flow. In streams where contact with the bed and banks is high or where the channel is rough, as in a section of rap- ids, turbulent flow occurs and most of the stream’s en- ergy is expended in turbulent eddies. In wide, lowland rivers, where friction is reduced by less contact of the flow with the bed and banks, the apparent smoothness and quietness of the flow mask the increased veloc- ity (Figure 15.8). The energy of these rivers is enough to move large amounts of sediment, discussed in the next section.
Changes in Discharge over Time Discharge varies throughout the year for most streams, depending on pre- cipitation and temperature. Rivers and streams in arid and semiarid regions may have perennial, ephemeral, or intermittent discharge. Perennial streams flow all year, fed by snowmelt, rainfall, groundwater, or some combination of those sources. Ephemeral streams flow only after precipitation events and are not connected
▲Figure 15.7 Declining discharge with distance downstream. The Virgin River in southwest Utah, a tributary of the Colorado River, is a perennial stream in which discharge decreases in the lower reaches as water is removed for irrigation, is transpired by riparian vegetation (water-loving plants), and is lost to evapotranspiration in this semiarid climate. [Bobbé Christopherson.]
▲Figure 15.8 Increasing stream velocity and discharge downstream. A high-discharge, high-velocity section of the Bow River in Banff national Park, Alberta. [Robert Christopherson.]