Controls on Channel Style
(a)
(b)
��
(c)
11
12
8
13
4
\\®
'���
je�� »
J�/1
1 // �
5
10
14
F ig. 8.6. T he �ange of alluvial channel patterns. a bed load
chanriels; b mixed-load channels; c suspended-load channels. (Schumm 1981)
intermediate sinuosity range. Lowest sinuosities are
those of ((sttaight" rivets. Schumm and Khan (1972)
showed that this conditiOn deVelops at Very low
slopes (Fig. 8.7)) such as near the mouths of some
delta distributaries.
For a variety of reasons, the geometric arid hydraulic characters of a river may not precisely reflect
local conditions. For example, iocal bedrock controls may slow down erosion and set back the response of a river to climate change. Many rivers are
((underfit
)) , that is) their discharge is too small to be
correlated with existing channel characteristiCs
(Dury 1964). For example, many rivers have yet to
adjust to the reduced discharge that accompanied
the postglacial climate changes of the Holocene. In
the case of many of the world's very large rivers, the
sediment load and discharge may partially reflect the
climate) relief, and sediment-yield characteristics of
i9 7
a source area many hundreds of kiioinetets distant
(Potter 1978). For example, the Ganges, Brahmaputra, and Mekong; all of which are partly braided,
have headwaters located in the Himalayan Mountains. Rivers in the Amazon BaSin show widely vary ..
ing flUvial style, reflecting the location of their
catchment areas. Those headirtg in the high Amazon
are characterized by strongly seasonal discharge artd
derive sediment ftom relict Pleistocene fluviogladal
deposits. They are of moderate sinuosity and par�
tially braided. Rivers sourced within the lowland
areas carry much smaller sediment loads and are
highly sinuous streams (Baker 1978a). Climatic controls oh channel patterns and fluvial style ate dis- .
cussed at greater length in Chap. 12.
Several channel patterns illustrated ih Flg. 8.6 are
not amongst the classic suite of fluvial styles (the
four that became most well knoWn in the 1970s;
Table 8.2), but are receiving greater attention as
sedimentological techniques improve. Fot example,
the "wandering" style (pattern 4 in Fig. 8.5, pattern 3
ih Fig. 8.6) is intermediate between braided and
meandering (Church 1983). Such rivers have a
single) relatively stable, dominant channel of
intermediate to high Sinuosity, but alsci contain
short braided reaches with bars a:rid islands. This
style is shown by sorrie gravel-bed tivers, as discussed later in this chapter. The causes of the Wandering style are not clear, although Brierley and
Hickin (1991) described a downstream change
within a single rivet front braided to. Warideting
to meandering as slope and sediment grain-size
decrease. It may, therefore, simply represent a trahsitionai condition.
Pattern 2 in Fig, 8.6 shows a straight channel with
a sinuous thalweg, with altethate bats developing on
the insides of the meanders. This pattern occurs in
delta distributaries of low slope. Crowley (1983) also
suggested that it develops as an intermediate form
between low-sinuosity braided and meandering systems in soine sand-bed rivers. The major deposi..:
tiOnal forms are alternate bars, the charaCteristics of
which ate discussed in Sect. 6. 7. The overall fluvial
style is described later in thls chapter.
Pattern 14 in Fig. 8.6 is that of the anastomosed
river, as described in the definitive work of D.G.
Smith (1973, 1983) and D.G. Smith and N.D. Smith
(1980). These rivers are characterized by a network
of relatively stable, interconnected channels of low
to high sinuosity. They commonly show low siopes
and high aggradation rates. D.G. Smith's work suggests that the anastomosed pattern is favored by high
rates of aggradation. This can occur where the river
flows across a basin that is undergoing rapid subsi-
(a)
(b)
��
(c)
11
12
8
13
4
\\®
'���
je�� »
J�/1
1 // �
5
10
14
F ig. 8.6. T he �ange of alluvial channel patterns. a bed load
chanriels; b mixed-load channels; c suspended-load channels. (Schumm 1981)
intermediate sinuosity range. Lowest sinuosities are
those of ((sttaight" rivets. Schumm and Khan (1972)
showed that this conditiOn deVelops at Very low
slopes (Fig. 8.7)) such as near the mouths of some
delta distributaries.
For a variety of reasons, the geometric arid hydraulic characters of a river may not precisely reflect
local conditions. For example, iocal bedrock controls may slow down erosion and set back the response of a river to climate change. Many rivers are
((underfit
)) , that is) their discharge is too small to be
correlated with existing channel characteristiCs
(Dury 1964). For example, many rivers have yet to
adjust to the reduced discharge that accompanied
the postglacial climate changes of the Holocene. In
the case of many of the world's very large rivers, the
sediment load and discharge may partially reflect the
climate) relief, and sediment-yield characteristics of
i9 7
a source area many hundreds of kiioinetets distant
(Potter 1978). For example, the Ganges, Brahmaputra, and Mekong; all of which are partly braided,
have headwaters located in the Himalayan Mountains. Rivers in the Amazon BaSin show widely vary ..
ing flUvial style, reflecting the location of their
catchment areas. Those headirtg in the high Amazon
are characterized by strongly seasonal discharge artd
derive sediment ftom relict Pleistocene fluviogladal
deposits. They are of moderate sinuosity and par�
tially braided. Rivers sourced within the lowland
areas carry much smaller sediment loads and are
highly sinuous streams (Baker 1978a). Climatic controls oh channel patterns and fluvial style ate dis- .
cussed at greater length in Chap. 12.
Several channel patterns illustrated ih Flg. 8.6 are
not amongst the classic suite of fluvial styles (the
four that became most well knoWn in the 1970s;
Table 8.2), but are receiving greater attention as
sedimentological techniques improve. Fot example,
the "wandering" style (pattern 4 in Fig. 8.5, pattern 3
ih Fig. 8.6) is intermediate between braided and
meandering (Church 1983). Such rivers have a
single) relatively stable, dominant channel of
intermediate to high Sinuosity, but alsci contain
short braided reaches with bars a:rid islands. This
style is shown by sorrie gravel-bed tivers, as discussed later in this chapter. The causes of the Wandering style are not clear, although Brierley and
Hickin (1991) described a downstream change
within a single rivet front braided to. Warideting
to meandering as slope and sediment grain-size
decrease. It may, therefore, simply represent a trahsitionai condition.
Pattern 2 in Fig, 8.6 shows a straight channel with
a sinuous thalweg, with altethate bats developing on
the insides of the meanders. This pattern occurs in
delta distributaries of low slope. Crowley (1983) also
suggested that it develops as an intermediate form
between low-sinuosity braided and meandering systems in soine sand-bed rivers. The major deposi..:
tiOnal forms are alternate bars, the charaCteristics of
which ate discussed in Sect. 6. 7. The overall fluvial
style is described later in thls chapter.
Pattern 14 in Fig. 8.6 is that of the anastomosed
river, as described in the definitive work of D.G.
Smith (1973, 1983) and D.G. Smith and N.D. Smith
(1980). These rivers are characterized by a network
of relatively stable, interconnected channels of low
to high sinuosity. They commonly show low siopes
and high aggradation rates. D.G. Smith's work suggests that the anastomosed pattern is favored by high
rates of aggradation. This can occur where the river
flows across a basin that is undergoing rapid subsi-