3!8
Fluvial Depositional Systems and Autogenic Sedimentary Controls
stratigraphic architecture. However, av ulsion is important in the case of other fluvial styles as well, as
discussed later in this section.
On coastal plains avulsion may lead to the abandonment of a delta lobe and to the initiation of a new
locus of deposition laterally displaced by tens of
kilometers. Maintenance of a new channel in such
cases is commonly facilitated by the fact that the
diversion results in the occupation of a shorter,
steeper route to the sea. This avulsion process is
analogous in some ways to that which occurs on
alluvial fans.
10.3.1 The Development of Meander Belts
Much work on avulsion in meandering systems has
been carried out by geomorphologists and civil engineers, because of the importance of the process from
the point of view of navigation and flood control.
The review by Schumm (1977, pp. 297-305) is particularly useful in this regard.
There are two types of avulsive process in meandering rivers: firstly, high discharge, or aggradation
of the channel by a large sediment load, may lead to
the channel overtopping its banks, causing crevassing and the diversion of flow into the floodplain.
Under certain conditions, this can evolve into longterm channel diversion. Secondly, avulsion can
occur when normal, gradual, meander migration
results in the channel encountering a pre-existing,
older channel at a lower elevation, resulting in the
reoccupation of that channel. Such earlier channels
may have developed as a result of the first of these
two avulsive process.
Documented cases of channel avulsion in modern
rivers are fairly limited. In some river valleys the
existence of older channel belts has been reconstructed fr om archeological or historical data in
areas of early human civilizationi for example,
Holmes (1965) referred to the lateral shifting of the
Euphrates River in Mesopotamia (Iraq) by a distance
of some 16 km, around the position of the ancient
city ofUr. MacKay (1945; quoted by Schumm 1977)
documented the rise and fall of ancient cities,
including those in Mesopotamia, whose prosperity
depended on the proximity to a navigable river.
Russell (1954) noted the existence of "old" and
present courses of the Meander River in Turkey, and
summarized historical evidence relating to the
change in position of the meander belt. According to
Russell (1954) this river was mentioned in the work
of Herodotus and Strabo, and its name became used
as a general term for highly sinuous rivers because of
its characteristic winding pattern. Anderson (1961)
document�d shifts in the Rufiji River in Tanzania.
Schumm (1977) illustrated old and new courses of
the Murrumbidgee River, Australia, using aerial
photographs, and discussed the climatic changes
that accompanied this avulsive shifting. Holmes
(1968) described the shifting of the modern Indus
River.
By far the best-known history of channel avulsion
is that of the Mississippi River, which was investigated in detail by Fisk (1952). Numerous changes in
the course of this river have been documented, and
many of these triggered the growth of new delta lobes
along the Gulf Coast (Fig. 10.9). Fourteen separate
delta lobes are known to have developed in the past
7500 years, since the last deglaciation (Frazier 1967).
All of these required a major channel avulsion to
occur. Other avulsive events which occurred far up
the Mississippi valley, such as at Cairo and Thebes
Gap (Fisk 1944), are probably not associated with
deltaic diversion. According to Fisk (1952), the Mississippi has followed 20 distinct courses during the
past 2000 years, meaning a change in course somewhere in the lower alluvial valley on average every
100 years. Not all workers agree with this reconstruction. For example, Saucier (1974) recognized only
five main meander belts (one shift every 400 years).
Leeder (1978) concluded that in major rivers such as
the Mississippi "realistic mean avulsion periodicity"
is in the order of one to two events per 1000 years. In
the hierarchical classification of this book this would
assign meander-belt avulsion to group 6 or 7 (Table
9.1).
The case of the Atchafalya River, a distributary
branching off the Mississippi (Fig. 10.9), may be
taken as a type example of channel avulsion in a
meandering system (details fr om Fisk 1952; summary in Schumm 1977). The Atchafalaya is an old,
former course of the Mississippi, known from historical records to have been an active distributary in
the early 1500s (Shlemon 1975). !t was intersected by
the modern river as a result of channel migration in
the late 1940s. Its course fr om this point to the Gulf is
225 km long. The modern channel is much longer, at
515 km, partly because of the lengthening that has
occurred at the mouth by delta construction. By 1951
the Atchafalaya was carrying 21% of the fl ow of the
Mississippi, and by 1956 it was the third largest of all
rivers flowing into the sea from the United States.
Diversion of the flow was gradual initially, because
of the originally small size of the channel, but natural
widening and deepening occurred, facilitated by the
Fluvial Depositional Systems and Autogenic Sedimentary Controls
stratigraphic architecture. However, av ulsion is important in the case of other fluvial styles as well, as
discussed later in this section.
On coastal plains avulsion may lead to the abandonment of a delta lobe and to the initiation of a new
locus of deposition laterally displaced by tens of
kilometers. Maintenance of a new channel in such
cases is commonly facilitated by the fact that the
diversion results in the occupation of a shorter,
steeper route to the sea. This avulsion process is
analogous in some ways to that which occurs on
alluvial fans.
10.3.1 The Development of Meander Belts
Much work on avulsion in meandering systems has
been carried out by geomorphologists and civil engineers, because of the importance of the process from
the point of view of navigation and flood control.
The review by Schumm (1977, pp. 297-305) is particularly useful in this regard.
There are two types of avulsive process in meandering rivers: firstly, high discharge, or aggradation
of the channel by a large sediment load, may lead to
the channel overtopping its banks, causing crevassing and the diversion of flow into the floodplain.
Under certain conditions, this can evolve into longterm channel diversion. Secondly, avulsion can
occur when normal, gradual, meander migration
results in the channel encountering a pre-existing,
older channel at a lower elevation, resulting in the
reoccupation of that channel. Such earlier channels
may have developed as a result of the first of these
two avulsive process.
Documented cases of channel avulsion in modern
rivers are fairly limited. In some river valleys the
existence of older channel belts has been reconstructed fr om archeological or historical data in
areas of early human civilizationi for example,
Holmes (1965) referred to the lateral shifting of the
Euphrates River in Mesopotamia (Iraq) by a distance
of some 16 km, around the position of the ancient
city ofUr. MacKay (1945; quoted by Schumm 1977)
documented the rise and fall of ancient cities,
including those in Mesopotamia, whose prosperity
depended on the proximity to a navigable river.
Russell (1954) noted the existence of "old" and
present courses of the Meander River in Turkey, and
summarized historical evidence relating to the
change in position of the meander belt. According to
Russell (1954) this river was mentioned in the work
of Herodotus and Strabo, and its name became used
as a general term for highly sinuous rivers because of
its characteristic winding pattern. Anderson (1961)
document�d shifts in the Rufiji River in Tanzania.
Schumm (1977) illustrated old and new courses of
the Murrumbidgee River, Australia, using aerial
photographs, and discussed the climatic changes
that accompanied this avulsive shifting. Holmes
(1968) described the shifting of the modern Indus
River.
By far the best-known history of channel avulsion
is that of the Mississippi River, which was investigated in detail by Fisk (1952). Numerous changes in
the course of this river have been documented, and
many of these triggered the growth of new delta lobes
along the Gulf Coast (Fig. 10.9). Fourteen separate
delta lobes are known to have developed in the past
7500 years, since the last deglaciation (Frazier 1967).
All of these required a major channel avulsion to
occur. Other avulsive events which occurred far up
the Mississippi valley, such as at Cairo and Thebes
Gap (Fisk 1944), are probably not associated with
deltaic diversion. According to Fisk (1952), the Mississippi has followed 20 distinct courses during the
past 2000 years, meaning a change in course somewhere in the lower alluvial valley on average every
100 years. Not all workers agree with this reconstruction. For example, Saucier (1974) recognized only
five main meander belts (one shift every 400 years).
Leeder (1978) concluded that in major rivers such as
the Mississippi "realistic mean avulsion periodicity"
is in the order of one to two events per 1000 years. In
the hierarchical classification of this book this would
assign meander-belt avulsion to group 6 or 7 (Table
9.1).
The case of the Atchafalya River, a distributary
branching off the Mississippi (Fig. 10.9), may be
taken as a type example of channel avulsion in a
meandering system (details fr om Fisk 1952; summary in Schumm 1977). The Atchafalaya is an old,
former course of the Mississippi, known from historical records to have been an active distributary in
the early 1500s (Shlemon 1975). !t was intersected by
the modern river as a result of channel migration in
the late 1940s. Its course fr om this point to the Gulf is
225 km long. The modern channel is much longer, at
515 km, partly because of the lengthening that has
occurred at the mouth by delta construction. By 1951
the Atchafalaya was carrying 21% of the fl ow of the
Mississippi, and by 1956 it was the third largest of all
rivers flowing into the sea from the United States.
Diversion of the flow was gradual initially, because
of the originally small size of the channel, but natural
widening and deepening occurred, facilitated by the