Page 319 - Laboratory Manual in Physical Geology

continent. The continental divide in North America River Valley Forms and Processes
is an imaginary line along the crest of the Rocky The form or shape of a river valley varies with these main
Mountains (see red line on small map in FIGURE 11.4 ). factors:

Rainwater that falls east of the line drains eastward into
the Atlantic Ocean, and rainwater that falls west of the ■ Geology —the bedrock geology over which the stream
line drains westward into the Pacific Ocean. Therefore, flows affects the stream’s ability to find or erode its

North America’s continental divide is sometimes called course ( FIGURE 11.2 ).
“The Great Divide.” ■ Gradient —the steepness of a slope—either the
slope of a valley wall or the slope of a stream
Stream Weathering, Transportation, and along a selected length (segment) of its channel
Deposition ( FIGURE 11.6 ). Gradient is generally expressed

Three main processes are at work in every stream. in meters per kilometer or feet per mile. This is
Weathering occurs where the stream physically determined by dividing the vertical rise or fall
erodes and disintegrates Earth materials and where it between two points on the slope (in meters or feet)
chemically decomposes or dissolves Earth materials by the horizontal distance (run) between them
to form sediment and aqueous chemical solutions. (in kilometers or miles). For example, if a stream
Transportation of these weathered materials occurs when descends 20 meters over a distance of 40 km, then
they are dragged, bounced, and carried downstream (as its gradient is 20 m/40 km, or 0.5 m/km. You
suspended grains or chemicals in the water). Deposition can estimate the gradient of a stream by studying
occurs if the velocity of the stream drops (allowing the spacing of contours on a topographic map.
sediments to settle out of the water) or if parts of the Or, you can precisely calculate the exact gradient
stream evaporate (allowing mineral crystals and oxide by measuring how much a stream descends
residues to form). along a measured segment of its course. Learn
The smallest valleys in a drainage basin occur at more about calculating slope (gradient) at this
its highest elevations, called uplands ( FIGURE 11.1 ). In site featuring The Math You Need,

the uplands, a stream’s (tributary’s) point of origin, or When You Need It math tutorials for
head , may be at a spring or at the start of narrow runoff students in introductory geoscience

channels developed during rainstorms. Erosion (wearing courses: http://serc.carleton.edu/
away rock and sediment) is the dominant process here, mathyouneed/slope/index.html
and the stream channels deepen and erode their V-shaped ■ Base level —the lowest level to which a stream
channels uphill through time—a process called headward can theoretically erode. For example, base level is
erosion . Eroded sediment is transported downstream by achieved where a stream enters a lake or ocean. At
the tributaries. that point, the erosional (cutting) power of the
Streams also weather and erode their own valleys stream is zero and depositional (sediment accumula-
along weaknesses in the rocks (fractures, faults), soluble tion) processes occur.
nonresistant layers of rock (salt layers, limestone),
and where there is the least resistance to erosion (see ■ Discharge —the rate of stream flow at a given time
FIGURE 11.2 ). Rocks composed of hard, chemically and location. Discharge is measured in water vol-

resistant minerals are generally more resistant to erosion ume per unit of time, commonly cubic feet per
3
and form ridges or other hilltops. Rocks composed of second (ft /sec).
soft and more easily weathered minerals are generally less ■ Load —the amount of material (mostly alluvium,
resistant to erosion and form valleys. This is commonly but also plants, trash, and dissolved material) that
called differential erosion of rock. is transported by a stream. In the uplands, most
Headward tributary valleys merge into larger stream streams have relatively steep gradients, so the
valleys, and these eventually merge into a larger river streams cut narrow, V-shaped valleys. Near their
valley. Along the way, some new materials are eroded, and heads, tributaries are quick to transport their load
deposits (gravel, sand, mud) may form temporarily, but downstream, where it combines with the loads of
the main processes at work over the years in uplands are other tributaries.
erosion (headward erosion and cutting V-shaped valleys)
and transportation of sediment. Therefore, the load of the tributaries is transferred
The end of a river valley is the mouth of the river, to the larger streams and, eventually, to the main river.
where it enters an outlet waterbody (lake, gulf, ocean) or a The load is eventually deposited at the mouth of the river,
dry basin. At this location, the river water is dispersed into where it enters a lake, ocean, or dry basin.
a wider area, its velocity decreases, and sediment settles out From a stream’s headwaters to its mouth, the
of suspension to form an alluvial deposit such as a delta gradient decreases, discharge generally increases, and
(in water) or an alluvial fan ( FIGURE 11.5 ). If the river water valleys generally widen. Along the way, the stream’s

enters a dry basin, then it will evaporate and precipitate load may exceed the water’s ability to carry it, so the
layers of mineral crystals and oxide residues (in a playa). solid particles accumulate as sedimentary deposits
Stream Processes, Landscapes, Mass Wastage, and Flood Hazards ■ 289