Geology Reference
In-Depth Information
Broad, fl at fl oodplains adjacent to channels are the sites
of oxbow lakes, which are simply abandoned meanders.
An alluvial deposit at a river's mouth is a delta. Some
deltas conform to the three-part division of bottomset,
foreset, and topset beds, but large marine deltas are much
more complex and are characterized as stream-, wave-, or
tide-dominated.
Alluvial fans are fan-shaped deposits of sand and gravel
on land that form best in semiarid regions. They form
mostly by deposition from running water, but debris
fl ows are also important.
Rivers and streams carry runoff from their drain-
age basins, which are separated from one another by
divides.
Sea level is ultimate base level, the lowest level to which
streams or rivers can erode. Local base levels may be
lakes or where streams or rivers fl ow across resistant
rocks.
Graded streams tend to eliminate irregularities in their
channels, so they develop a smooth, concave profi le of
equilibrium.
A combination of processes, including downcutting, lateral
erosion, sheetwash, mass wasting, and headward erosion,
are responsible for the origin and evolution of valleys.
Stream terraces and incised meanders usually form when
a stream or river that was formerly in equilibrium begins
a new episode of downcutting.
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Important Terms
abrasion (p. 308)
alluvial fan (p. 313)
alluvium (p. 309)
base level (p. 319)
bed load (p. 308)
braided stream (p. 310)
delta (p. 311)
discharge (p. 306)
dissolved load (p. 308)
divide (p. 318)
drainage basin (p. 318)
drainage pattern (p. 318)
fl oodplain (p. 310)
graded stream (p. 320)
gradient (p. 305)
hydraulic action (p. 308)
hydrologic cycle (p. 303)
incised meanders (p. 323)
infi ltration capacity (p. 304)
meandering stream (p. 310)
natural levee (p. 311)
oxbow lake (p. 310)
point bar (p. 310)
runoff (p. 303)
stream terrace (p. 322)
superposed stream (p. 324)
suspended load (p. 308)
valley (p. 321)
velocity (p. 305)
Review Questions
1. Base level is
a. ______ the lowest level to which a stream can erode;
b. ______ the distance from a river's headwaters to
where it discharges into the sea;
c. ______ a stream profi le with numerous irregulari-
ties such as lakes and waterfalls;
d. ______ a type of river with a single, sinuous
channel;
e. ______ the amount of water moving past a given
point in a specifi ed amount of time.
2. A board, fl at area adjacent to a river is its
a. ______ oxbow lake;
b. ______ fl oodplain;
c. ______ natural levee;
d. ______ delta;
e. ______ point bar.
3. Running water carrying sand and gravel effectively
erodes by
a. ______ solution;
b. ______ sheetwash;
c. ______ deposition;
d. ______ piracy;
e. ______ abrasion.
4. The type of drainage that is likely to develop on a com-
posite volcano is
a. ______ incised;
b. ______ meandering;
c. ______ radial;
d. ______ alluvial;
e. ______ solutional.
5. A ridge of alluvium along the banks of a river that
forms during fl oods is a/an
a. ______ tide-dominated delta;
b. ______ natural levee;
c. ______ alluvial fan;
d. ______ incised meander;
e. ______ drainage basin.
6. Drainage basins are separated from one another by a/an
a. ______ bed load;
b. ______ infi ltration capacity;
c. ______ base level;
d. ______ divide;
e. ______ natural levee.
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