Avulsion in Fluvial Systems and Its Effect on Alluvial Stratigraphy
317
Iow �lying areas on the depositional surface (Fig.
1 0.7). The triggering process seems to be instability
caused by channel aggradation, with the main channel moving to occupy low-lying areas traversed by
old, former channels, or tributaries of the Kosi system (Fig. 10.8). Wells and Dorr (1987) noted several
other examples of similar lateral shifting of distribu�
taries on modern distributary systems, but it is unknown whether the very regular one-way (westward)
sweeping of the Kosi distributaries is typical.
Bluck (1979, 1980) proposed a model for the regular, autogenic, pendulum-like sweeping of an alluvial fan by its major distributary, based on some
observations of modern fans and Devonian fan deposits. Viseras and Fernandez (1994) described a
Pliocene succession in Spain which they suggest fits
this model. The mechanism for the model, suggested
by Bluck (1979, p. 201), involves the preferential
building of bars on one side of the distributary, with
bank erosion and channel widening occurring preferentially on the opposite bank. There is some
stratigraphic evidence for this, as reported by Bluck
(1980) and Viseras and Fernandez (1994), in the
form of bar accretion sets dipping preferentially in
one direction at a given stratigraphic level. However,
the cause of this preferential lateral migration of the
channel remains unclear. As discussed in Sects. 6.6
and 8.1, bar growth and cutbank erosion are related
to meander development both in braided and meandering systems, and this, in turn, is related to the
turbulent properties of flowing water. Therefore, it
seems unlikely that lateral bar accretion and cutbank
erosion would take place on the same sides of a river
for more than one meander length. It seems more
likely that a process of channel aggradation and
avulsive reoccupation of low�lying areas is the main
cause of lateral channel movement in fans, as demonstrated (above) for large fans such as the Kosi. A
possible tectonic mechanism may also be sought for
the lateral migration of channels and the preferential
preservation of accretion sets dipping in one direction, and just such a mechanism is described in Sect.
11.2.1.1.
10.3 Avulsion in Fluvial Systems
and Its Effect on Alluvial Stratigraphy
Fluvial style is governed mainly by a range of
extrabasinal controls, as discussed in Sect. 8.1. However, given a normally graded fluvial system flowing
in a steadily subsiding basin under conditions of
dynamic equilibrium, normal autogenic processes
will lead to the gradual or sudden and episodic shifting (avulsion) of the channel, with consequences for
the ongoing development of the alluvial stratigraphy
(Schumm 1977; Richards et al. 1993).
There are two kinds of avulsion, that which occurs
within the active channel system, and that which
displaces flow into a new channel position. The simplest example of the first case is where the natural
migration of a river meander results in one reach of
the meander bend cutting into another reach, resulting in a neck cutoff. This process has been known
and understood for a very long time. For example, it
was described and illustrated in Lyell's (1830) great
textbook (Fig. 2.2). Chute cutoff occurs where a flood
channel develops across the top of a point bar (Fig.
8.58). The complex of channels, cutoffs, crevasse
splays and levees created by this process may become slightly elevated fr om the main floodplain, and
is termed an alluvial ridge.
The avulsive shifting of braided channels is an
integral part of the braiding process. Several autogenic mechanisms occur. They are discussed in Sect.
6.3, where the emphasis is on the development of
gravel bars and bedforms. Avulsion is also the main
process whereby channels in anastomosing systems
evolve, as described in Sects. 7.2 and 8.2.10.
Maps of meander migration in sinuous rivers may
be produced by observing the relict topography of
abandoned channels and of the scroll bars on pointbar surfaces. Such maps began appearing in textM
books in the early part of this century, as exemplified
by those in Chamberlin and Salisbury (1909; Figs. 2.7
and 2.8 of this book). Maps of this type, or schematic
versions of them, are popular as generalized illustrations of sedimentary processes (e.g., see the cover of
Walker and )ames 1992). They are easily made from
aerial photographs, in which patterns of ancient
meanders are commonly quite spectacular. Current
work on meander evolution and migration and its
stratigraphic results is discussed in Sect. 6.7. Cutoffs
in modern rivers were reviewed by Lewis and Lewin
(1983). Within-channel avulsion in gravel-bed
braided rivers was modeled by Ashmore (1991) and
Leddy et al. (1993) and is discussed in Sect. 6.3.
The second, more stratigraphically significant,
form of avulsive channel shifting is where the entire
river changes position, abandoning its original
course and flowing in a different part of the valley.
This process is most important in the case of meandering rivers, because these normally occupy only a
small part of their total valley width, so that when a
shift occurs it has a profound effect on the resulting
317
Iow �lying areas on the depositional surface (Fig.
1 0.7). The triggering process seems to be instability
caused by channel aggradation, with the main channel moving to occupy low-lying areas traversed by
old, former channels, or tributaries of the Kosi system (Fig. 10.8). Wells and Dorr (1987) noted several
other examples of similar lateral shifting of distribu�
taries on modern distributary systems, but it is unknown whether the very regular one-way (westward)
sweeping of the Kosi distributaries is typical.
Bluck (1979, 1980) proposed a model for the regular, autogenic, pendulum-like sweeping of an alluvial fan by its major distributary, based on some
observations of modern fans and Devonian fan deposits. Viseras and Fernandez (1994) described a
Pliocene succession in Spain which they suggest fits
this model. The mechanism for the model, suggested
by Bluck (1979, p. 201), involves the preferential
building of bars on one side of the distributary, with
bank erosion and channel widening occurring preferentially on the opposite bank. There is some
stratigraphic evidence for this, as reported by Bluck
(1980) and Viseras and Fernandez (1994), in the
form of bar accretion sets dipping preferentially in
one direction at a given stratigraphic level. However,
the cause of this preferential lateral migration of the
channel remains unclear. As discussed in Sects. 6.6
and 8.1, bar growth and cutbank erosion are related
to meander development both in braided and meandering systems, and this, in turn, is related to the
turbulent properties of flowing water. Therefore, it
seems unlikely that lateral bar accretion and cutbank
erosion would take place on the same sides of a river
for more than one meander length. It seems more
likely that a process of channel aggradation and
avulsive reoccupation of low�lying areas is the main
cause of lateral channel movement in fans, as demonstrated (above) for large fans such as the Kosi. A
possible tectonic mechanism may also be sought for
the lateral migration of channels and the preferential
preservation of accretion sets dipping in one direction, and just such a mechanism is described in Sect.
11.2.1.1.
10.3 Avulsion in Fluvial Systems
and Its Effect on Alluvial Stratigraphy
Fluvial style is governed mainly by a range of
extrabasinal controls, as discussed in Sect. 8.1. However, given a normally graded fluvial system flowing
in a steadily subsiding basin under conditions of
dynamic equilibrium, normal autogenic processes
will lead to the gradual or sudden and episodic shifting (avulsion) of the channel, with consequences for
the ongoing development of the alluvial stratigraphy
(Schumm 1977; Richards et al. 1993).
There are two kinds of avulsion, that which occurs
within the active channel system, and that which
displaces flow into a new channel position. The simplest example of the first case is where the natural
migration of a river meander results in one reach of
the meander bend cutting into another reach, resulting in a neck cutoff. This process has been known
and understood for a very long time. For example, it
was described and illustrated in Lyell's (1830) great
textbook (Fig. 2.2). Chute cutoff occurs where a flood
channel develops across the top of a point bar (Fig.
8.58). The complex of channels, cutoffs, crevasse
splays and levees created by this process may become slightly elevated fr om the main floodplain, and
is termed an alluvial ridge.
The avulsive shifting of braided channels is an
integral part of the braiding process. Several autogenic mechanisms occur. They are discussed in Sect.
6.3, where the emphasis is on the development of
gravel bars and bedforms. Avulsion is also the main
process whereby channels in anastomosing systems
evolve, as described in Sects. 7.2 and 8.2.10.
Maps of meander migration in sinuous rivers may
be produced by observing the relict topography of
abandoned channels and of the scroll bars on pointbar surfaces. Such maps began appearing in textM
books in the early part of this century, as exemplified
by those in Chamberlin and Salisbury (1909; Figs. 2.7
and 2.8 of this book). Maps of this type, or schematic
versions of them, are popular as generalized illustrations of sedimentary processes (e.g., see the cover of
Walker and )ames 1992). They are easily made from
aerial photographs, in which patterns of ancient
meanders are commonly quite spectacular. Current
work on meander evolution and migration and its
stratigraphic results is discussed in Sect. 6.7. Cutoffs
in modern rivers were reviewed by Lewis and Lewin
(1983). Within-channel avulsion in gravel-bed
braided rivers was modeled by Ashmore (1991) and
Leddy et al. (1993) and is discussed in Sect. 6.3.
The second, more stratigraphically significant,
form of avulsive channel shifting is where the entire
river changes position, abandoning its original
course and flowing in a different part of the valley.
This process is most important in the case of meandering rivers, because these normally occupy only a
small part of their total valley width, so that when a
shift occurs it has a profound effect on the resulting