44
Meandering Rivers
Meandering river systems develop one principal, relatively narrow channel of high sinuosity (> 1.5). They
are dominated by mixed load or predominantly suspended load; their overall sand content therefore often
averages 20% to 40%. If meandering rivers are associated with a wide floodplain, the channel sediments
may be restricted to a comparatively narrow zone
within the flood basin where they form a meander belt
(Fig. 2.17a).
The different architectural elements of the fluvial
sediments, shown in Figure 2.10, can be observed best
in meandering systems. In a sinuous channel segment,
one can distinguish the following morphological features and depositional subenvironments (Fig. 2.17b):
- Channels and channel fills.
- Point bars and lateral accretion complexes.
- Chute bars.
- Channel plugs (oxbow lakes).
Levee and crevasse splay deposits.
- Alluvial floodplain deposits.
The channel Door is usually covered by lag sediments
consisting of the coarsest material transported by the
river during peak flood. This channel lag mayaiso
contain mud elasts or blocks eroded from the banks.
Lag sands and gravel usually accumulate between
scour pools and form flat, elongate bars displaying
either imbrication of gravel or crudely laminated and
planar cross-bedded gravelly sand.
Point bars accumulate on the inner sides of river
bends, while on the outer side material from the bank
is eroded. In this way, the curvature of the meander
tends to become increasingly exaggerated until the
river produces short-cuts, leaving behind abandoned
channel segments (oxbow lakes, Fig. 2.17a). Most of
the point bar material is eroded from the upstream
channel banks. It is deposited in areas of lower velocity turbulence. Because sediment moves up and out of
the channel onto the bar, cross sections of point bars
often show fining-upward sequences, with sands on
top of channellags (Fig. 2.17c and d). Similarly, the
internal structures grade from horizontal bedding (upper flow regime) to large- and small-scale trough
cross-bedding (lower flow regime). The most distinctive feature of point bars is lateral accretion (low-angle, "epsilon" cross-bedding) which may be visible at
the surface by the development of a ridge-and-swale
topography (Fig. 2.17b). The swales can be filled with
mud, and older portions of the point bar are covered by
levee sands and silts or floodplain sediment.
This idealized point bar architecture is often modified in nature by chute channels cutting into the point'
bars and producing chute bars, small terraces associated with different water stages in the main channel, or
other irregularities. The resulting internal structures of
Chapter 2 Continental Sediments
such a "lateral accretion complex", a term which is
preferred by some experts on fluvial sediments, is less
regular and more variable than the major architectural
units ofthe fluvial deposition shown in Figs. 2.10 and
2.17.
Chute bars result from chute channels which direct
part of the river flow across the surface of a point bar
during flood stage. In this way relatively coarsegrained bedload material can be deposited as lag or
chute bar on the eroded top of a lateral accretion complex (Fig. 2.17b and d). While channellag deposits are
common on the upstream part of the chute, the downstream part is often characterized by imbricated pebble
sheets and large-scale planar and trough cross-bedded
sands.
Channel plugs are infillings of cutoff meander segments (oxbow lakes). Since the further influx of
bedload is terminated, the abandoned channel segments are slowly filled with fine-grained material
washed in from the neighboring floodplain (Fig.
2.17f). In humid elimates, organic matter (ineluding
peat) may accumulate in the lake or swamp.
Levee and crevasse splay deposits. Many meandering channels are accompanied by flat ridges or
dams sloping away from the channel into the
floodplain. These levees are built up during moderate
floods which just re ach the elevation of the channel
bank or ridge. Due to decreasing flow velocity, sand is
deposited along the channel banks, grading into silt
somewhat farther away. Locally, channel water may
spill over the levees into the floodplain, forming crevasse splays. The fallout of sand and silt usually extends farther into the floodplains than the levees, but
such crevasse splays can also contribute to the buildup
of the levees. The prevailing internal structures of
these sand sheets may resemble those of thin sandy
turbidites, showing some grading, horizontal lamination and small-scale ripple cross-bedding, ineluding
elimbing ripples and occasional convolute or contorted
cross-bedding. These structures are, however, often
mashed or destroyed by the roots of vegetation.
Whereas distal crevasse splays become interbedded
with fine-grained floodplain deposits, levee sands often tend to be reworked by subsequent channel migration.
Floodplain deposits accumulate during rare inundations. They consist predominantly of suspended
load, i.e., silt and mud, though fine sand mayaiso be
present in areas where the peak flood currents are sufficiently strong to transport this grain size (Fig. 2.17e).
The deposits from individuallarge floods reach thicknesses of only a few millimeters or, locally, a few centimeters. Such thin beds may be either somewhat
graded, or internally finely laminated or cross-bedded.
Aseries of flood layers can show distinct lamination.
Floodplains may be wetlands and backswamps or areas
of dessication and calcrete development. Therefore,
primary sedimentary structures are often destroyed by
Meandering Rivers
Meandering river systems develop one principal, relatively narrow channel of high sinuosity (> 1.5). They
are dominated by mixed load or predominantly suspended load; their overall sand content therefore often
averages 20% to 40%. If meandering rivers are associated with a wide floodplain, the channel sediments
may be restricted to a comparatively narrow zone
within the flood basin where they form a meander belt
(Fig. 2.17a).
The different architectural elements of the fluvial
sediments, shown in Figure 2.10, can be observed best
in meandering systems. In a sinuous channel segment,
one can distinguish the following morphological features and depositional subenvironments (Fig. 2.17b):
- Channels and channel fills.
- Point bars and lateral accretion complexes.
- Chute bars.
- Channel plugs (oxbow lakes).
Levee and crevasse splay deposits.
- Alluvial floodplain deposits.
The channel Door is usually covered by lag sediments
consisting of the coarsest material transported by the
river during peak flood. This channel lag mayaiso
contain mud elasts or blocks eroded from the banks.
Lag sands and gravel usually accumulate between
scour pools and form flat, elongate bars displaying
either imbrication of gravel or crudely laminated and
planar cross-bedded gravelly sand.
Point bars accumulate on the inner sides of river
bends, while on the outer side material from the bank
is eroded. In this way, the curvature of the meander
tends to become increasingly exaggerated until the
river produces short-cuts, leaving behind abandoned
channel segments (oxbow lakes, Fig. 2.17a). Most of
the point bar material is eroded from the upstream
channel banks. It is deposited in areas of lower velocity turbulence. Because sediment moves up and out of
the channel onto the bar, cross sections of point bars
often show fining-upward sequences, with sands on
top of channellags (Fig. 2.17c and d). Similarly, the
internal structures grade from horizontal bedding (upper flow regime) to large- and small-scale trough
cross-bedding (lower flow regime). The most distinctive feature of point bars is lateral accretion (low-angle, "epsilon" cross-bedding) which may be visible at
the surface by the development of a ridge-and-swale
topography (Fig. 2.17b). The swales can be filled with
mud, and older portions of the point bar are covered by
levee sands and silts or floodplain sediment.
This idealized point bar architecture is often modified in nature by chute channels cutting into the point'
bars and producing chute bars, small terraces associated with different water stages in the main channel, or
other irregularities. The resulting internal structures of
Chapter 2 Continental Sediments
such a "lateral accretion complex", a term which is
preferred by some experts on fluvial sediments, is less
regular and more variable than the major architectural
units ofthe fluvial deposition shown in Figs. 2.10 and
2.17.
Chute bars result from chute channels which direct
part of the river flow across the surface of a point bar
during flood stage. In this way relatively coarsegrained bedload material can be deposited as lag or
chute bar on the eroded top of a lateral accretion complex (Fig. 2.17b and d). While channellag deposits are
common on the upstream part of the chute, the downstream part is often characterized by imbricated pebble
sheets and large-scale planar and trough cross-bedded
sands.
Channel plugs are infillings of cutoff meander segments (oxbow lakes). Since the further influx of
bedload is terminated, the abandoned channel segments are slowly filled with fine-grained material
washed in from the neighboring floodplain (Fig.
2.17f). In humid elimates, organic matter (ineluding
peat) may accumulate in the lake or swamp.
Levee and crevasse splay deposits. Many meandering channels are accompanied by flat ridges or
dams sloping away from the channel into the
floodplain. These levees are built up during moderate
floods which just re ach the elevation of the channel
bank or ridge. Due to decreasing flow velocity, sand is
deposited along the channel banks, grading into silt
somewhat farther away. Locally, channel water may
spill over the levees into the floodplain, forming crevasse splays. The fallout of sand and silt usually extends farther into the floodplains than the levees, but
such crevasse splays can also contribute to the buildup
of the levees. The prevailing internal structures of
these sand sheets may resemble those of thin sandy
turbidites, showing some grading, horizontal lamination and small-scale ripple cross-bedding, ineluding
elimbing ripples and occasional convolute or contorted
cross-bedding. These structures are, however, often
mashed or destroyed by the roots of vegetation.
Whereas distal crevasse splays become interbedded
with fine-grained floodplain deposits, levee sands often tend to be reworked by subsequent channel migration.
Floodplain deposits accumulate during rare inundations. They consist predominantly of suspended
load, i.e., silt and mud, though fine sand mayaiso be
present in areas where the peak flood currents are sufficiently strong to transport this grain size (Fig. 2.17e).
The deposits from individuallarge floods reach thicknesses of only a few millimeters or, locally, a few centimeters. Such thin beds may be either somewhat
graded, or internally finely laminated or cross-bedded.
Aseries of flood layers can show distinct lamination.
Floodplains may be wetlands and backswamps or areas
of dessication and calcrete development. Therefore,
primary sedimentary structures are often destroyed by