Meaning “of the river,” referring to different and
diverse aspects of rivers. The term may be used to refer to the
environment around rivers and streams or, more commonly,
to refer to sediments deposited by a stream or river system.
Sediments deposited by rivers tend to become finergrained
and more rounded with increasing transport distance
from the eroded source terrain, typically an uplifted
mountain range. The sediments also tend to become more
enriched in the more stable chemical components such as
quartz and micas and depleted in chemically vulnerable particles
such as feldspars.
Stream channels are rarely straight, and the velocity of
flow changes in different places. Friction makes the flow
slower on the bottom and sides of the channel, and the
bends in the river make the zone of fastest flow swing from
side to side. The character of channels changes in different
settings because of difference in slope, discharge, and load.
Straight channels are very rare, and those that do occur have
many properties of curving streams. The thalweg is a line
connecting the deepest parts of the channel. In straight segments,
the thalweg typically meanders from side to side of
the stream. In places where the thalweg is on one side of the
channel, a bar may form on the other side. A bar (for example,
a sand bar) is a deposit of alluvium in a stream. Most
streams move through a series of bends known as meanders.
Meanders are always migrating across the floodplain by the
process of the deposition of the point bar deposits and the
erosion of the bank on the opposite side of the stream with
the fastest flow. The erosion typically occurs through slumping
of the stream bank. Meanders typically migrate back and
forth, and also down-valley at a slow rate. If the downstream
portion of a meander encounters a slowly erodable
rock, the upstream part may catch up and cut off the meander.
This forms an oxbow lake, which is an elongate and
curved lake formed from the former stream channel. Braided
streams consist of two or more adjacent but interconnected
channels separated by bars or islands. Braided streams have
constantly shifting channels, which move as the bars are
eroded and redeposited, during large fluctuations in discharge.
Most braided streams have highly variable discharge
in different seasons, and they carry more load than meandering
River and stream channel deposits tend to be composed
of sands and gravelly sands that exhibit large-scale threedimensional
ripples, shown in cross section as cross-bedding.
These cross-bedded sands are common around the inner
bends of channels and mark the former positions of point
bars, and they are commonly interbedded with planar bedded
sands marking flood stage deposits and gravelly sands
deposited during higher flood stages. The tops of channel
deposits may be marked by finer-grained sands with smallscale
ripples and mud drapes forming flaser-bedding,
interbedded with muds, and grading up into overbank floodplain
deposits. This upward-fining sequence is characteristic
of fluvial deposits, especially those of meandering streams. In
contrast, braided stream deposits show less order and are
characteristically dominated by bed load material such as
gravel and sand. They include imbricated gravels, gravels
deposited in shallow scours, and horizontally bedded sands
and gravels deposited in bars.
Fluvial channel deposits form a variety of geometric patterns
on a more regional scale. Shoestring sands form an
anastomosing pattern of river channels enclosed in overbank
shales and muds, formed by meandering and anastomosing
river channels. Sheets and wedges of fluvial sediments form in
front of uplifted mountain chains in foreland and rift basins
and may pass basinward into deltaic or shallow marine sediments
and mountainward into alluvial fan deposits. The type
of tectonic setting for a basin may be deduced by changes or
migration of different fluvial facies with stratigraphic height.
Fluvial sediments are widely exploited for hydrocarbon
deposits and also are known for placer deposits of gold and
other valuable minerals.
See also CLASTIC ROCKS; DRAINAGE BASIN; FLOOD;RIVER SYSTEM