194
6 DEPOSITIONAL SYSTEMS
via ephemeral, braided river channels. The steady perennial discharge, characteristic
of meander channels, did not occur until the colonization of the land by vegetation.
Braided and meandering alluvial depositional systems are now described in turn.
6.3.2.2.2 Alluvium of braided rivers
Braided channel systems are characterized by a network of constantly shifting low sinuosity anastomozing courses. Modern braided river systems occur on alluvial fans in semiarid and arid climates, along many mountain fronts, the edges of ice caps, and the
"snouts" of glaciers. They are characterized by relatively higher gradients and coarser
sediments than meandering river systems. Discharge is seasonal and ephemeral in glacial and mountainous realms, and still more sporadic in desert climates. With their
coarse grain size and erratic flow, braided channels are generally overloaded with sediment (Fig. 6.6). Continuous formation of channel bars causes thalwegs to diverge until
they meet up with another channel course.
Braided alluvial plains thus consist essentially of a network of channels with no clearly
defined overbank terrain. Fine suspended load sediment can only come to rest in rare
abandoned channels and in the pools of active channels when a flood abates. Channel
abandonment occurs by channel bars enlarging until they block a course and cause it to
divert. Alternatively, the headward erosion of channel gullies in soft detritus results in
capture of a previously active channel course. The alluvium of braided river systems
consists, therefore, largely of channel lag gravels and of cross-bedded channel bar and
braid bar sands. Laminated, cross-laminated, and desiccation-cracked fine sands and
silts occur in rare shoe-string forms infilling abandoned channels (Fig. 6.7).
Accounts of mountain front, glacial, and arid braided alluvial deposits, ancient and
Fig. 6.6. Modern river, Tunsbergdabru in northern Norway, showing braided channels separated by longitudinal gravel bar. (Courtesy of J. M. Cohen.)
6 DEPOSITIONAL SYSTEMS
via ephemeral, braided river channels. The steady perennial discharge, characteristic
of meander channels, did not occur until the colonization of the land by vegetation.
Braided and meandering alluvial depositional systems are now described in turn.
6.3.2.2.2 Alluvium of braided rivers
Braided channel systems are characterized by a network of constantly shifting low sinuosity anastomozing courses. Modern braided river systems occur on alluvial fans in semiarid and arid climates, along many mountain fronts, the edges of ice caps, and the
"snouts" of glaciers. They are characterized by relatively higher gradients and coarser
sediments than meandering river systems. Discharge is seasonal and ephemeral in glacial and mountainous realms, and still more sporadic in desert climates. With their
coarse grain size and erratic flow, braided channels are generally overloaded with sediment (Fig. 6.6). Continuous formation of channel bars causes thalwegs to diverge until
they meet up with another channel course.
Braided alluvial plains thus consist essentially of a network of channels with no clearly
defined overbank terrain. Fine suspended load sediment can only come to rest in rare
abandoned channels and in the pools of active channels when a flood abates. Channel
abandonment occurs by channel bars enlarging until they block a course and cause it to
divert. Alternatively, the headward erosion of channel gullies in soft detritus results in
capture of a previously active channel course. The alluvium of braided river systems
consists, therefore, largely of channel lag gravels and of cross-bedded channel bar and
braid bar sands. Laminated, cross-laminated, and desiccation-cracked fine sands and
silts occur in rare shoe-string forms infilling abandoned channels (Fig. 6.7).
Accounts of mountain front, glacial, and arid braided alluvial deposits, ancient and
Fig. 6.6. Modern river, Tunsbergdabru in northern Norway, showing braided channels separated by longitudinal gravel bar. (Courtesy of J. M. Cohen.)