As we mentioned earlier, many present-day alluvial fans grade laterally into the alluvial plain of a braided stream or river; that is, one characterized by an interlacing, vein-like network of low-sinuosity channels with constantly shifting midchannel bars. (Figure 1,
Block diagram model of a braided stream system in a semiarid environment and Figure 2, Shifting channels in a braided river course -Durance River- near Avignon, southern France).
Streams and rivers tend to braid when three main factors conspire: (1) high (though possibly seasonal) discharge, (2) relatively steep slopes, (3) large amounts of coarse sediment.
This style of deposition is imposed basically because more coarse sediment exists within the environment than the river or stream can carry at any one time. As shown by Figure 3 (Plan view of cross section of a single canyon and the alluvial fan which developed at its mouth, Van Horn Sandstone, Precambrian (?) Texas.
Shown are the downfan decrease in slope and grain size, as well as the higher concentration of sedimentary structures in the distal portions. The overall progression grades from massive conglomerate/gravel of the proximal fan, through alternating trough and foreset crossbedded sand of the midfan, to finer-grained, more thinly bedded crossbedded sands of the distal fan.), braided streams often begin on the fan itself. Full-sized braided rivers occur in a variety of specific settings, including broad semiarid plains, valleys longitudinal to mountain fronts, and in glacial environments, for example at the edges of ice caps.
In the rock record, braided channel systems grade into fanglomerates at their proximal end. Distally, they may terminate in desert lakes, sand dune complexes, meander channel flood-plains, or fan deltas building into lakes or seas.
