Page 322 - Laboratory Manual in Physical Geology

Rise or fall between two points, channels. These channels may become highly sinuous,
measured vertically

Gradient = or meandering (see FIGURES 11.1B , 11.7 , 11.8 ). Erosion
Distance (run) between the two points,
measured horizontally occurs on the outer edge of meanders, which are
called cutbanks . At the same time, point bar deposits

120 (mostly gravel and sand) accumulate along the inner
A 60 ft edge of meanders. Progressive erosion of cutbanks and
Gradient from A to B = = 40 ft/mi
1.5 mi deposition of point bars makes meanders “migrate”
100 over time.
100
Channels may cut new paths during floods. This
60 ft can cut off the outer edge of a meander, abandoning it to
80 80 become a crescent-shaped oxbow lake (see FIGURE 11.1C ).
When low gradient/high discharge streams become
Elevation in feet (ft) 60 1.5 mi B 60 lines stream patterns. These consist of braided channels with
Topographic
overloaded with sediment, they may form braided
contour
linear, underwater sandbars ( channel bars ) and islands
(see FIGURES 11.1B and 11.1D ).

Some stream valleys have level surfaces that are
40
40 higher than the present floodplain. These are remnants
of older floodplains that have been dissected (cut
40 ft
20 by younger streams) and are called stream terraces .
Sometimes several levels of stream terraces may be
3mi
developed along a stream, resembling steps. The steep
0 20 slopes or cliffs separating the relatively horizontal stream
0
C terraces are escarpments .
1
2 Where a stream enters a lake, ocean, or dry basin,
3 its velocity decreases dramatically. The stream drops
4 0 its sediment load, which accumulates as a triangular
5 or fan-shaped deposit. In a lake or ocean, such a
Distance in miles (mi)
40 ft 6
Gradient from B to C = = 13 ft/mi deposit is called a delta . A similar fan-shaped deposit

3.0 mi
of stream sediment also occurs where a steep-gradient
FIGURE 11.6 Stream gradient (slope). The gradient of a stream abruptly enters a wide, dry plain, creating an
stream is a measure of the steepness of its slope. As above, gradient alluvial fan .
is usually determined by dividing the rise or fall (vertical relief)
between two points on the map by the distance (run) between
them. It is usually expressed as a fraction in feet per mile (as above) ACTIVITY
or meters per kilometer.
A second way to determine and express the gradient of a 11.5 Mass Wastage at Niagara
slope is by measuring its steepness in degrees relative to horizontal.
Thirdly, gradient can be expressed as a percentage (also called Falls
grade of a slope). For example, a grade of 10% would mean a grade
of 10 units of rise divided by (per) 100 units of distance (i.e., 10 in. THINK | How does stream erosion shape the
per 100 in., 10 m per 100 m).
About It landscape?

along the river margins, or banks. Floodplains OBJECTIVE Describe erosional and mass wastage

develop when alluvium accumulates landward of the processes at Niagara Falls and calculate the rate at
river banks, during floods ( FIGURES 11.1C and 11.1D ). which the falls is retreating upstream.

However, most flooding events do not submerge the
entire floodplain. The more abundant minor flooding PROCEDURES
events deposit sediment only where the water barely 1. Before you begin , read about Mass Wastage at
overflows the river’s banks. Over time, this creates Niagara Falls. Also, this is what you will need :
natural levees ( FIGURE 11.1C ) that are higher than the ___ ruler, calculator

rest of the floodplain. If a tributary cannot breach a ___ 30-cm (12-inch) length of string
river’s levee, then it will become a yazoo tributary that ___ Activity 11.5 Worksheet (p. 306 ) and pencil
flows parallel to the river ( FIGURE 11.1C ). with eraser

Still farther downstream, the gradient decreases 2. Then follow your instructor’s directions for
even more as discharge and load increase. The stream completing the worksheet.
valleys develop very wide, flat floodplains with sinuous

292 ■ L ABOR ATORY 11