Figure 13. Wide, shallow ephemeral stream channel with a nearly horizon-
tal, planar bed on Wild Burro Wash near Marana, Arizona.
headward over time. Immediately downstream of the headcuts, the channels are
narrow, with steep vertical banks and erosional beds. Discontinuous gravel lags
are found on the bed of the erosional channels (Field 1994). Flows will rarely
overtop the banks just downstream of the headcuts, as overland flow is more
likely to be generated where bank heights are much lower or nonexistent. Incised
reaches of washes in Arizona contain channels floored with coarse-grained
sediments and flanked by berms of fine-grained sediment (Wells 1977).
Downstream of the erosional channels (Fig. 12), the channels widen,
promoting deposition of the sediments supplied from the eroding reach upstream
(Patton and Schumm 1981). The bed topography in these reaches is subdued,
with braided flow possible. The beds of single-thread channels in arid climates
are nearly horizontal and planar (Fig. 13), as opposed to the more-pronounced
poolriffle morphologies characteristic of more humid regions (Reid and Frostick
1997). Bar forms are flat-topped and rise only 1020 cm above the channel
thalweg (Leopold et al. 1966), because secondary flow cells that would
encourage bar building are suppressed in the wider channels (Reid and Frostick
1997). The tail end of a flood will carve wide, shallow channels across the bars
as the waning flows occupy the lowest portions of the channel bottom.
As water spreads out onto the sheetflood zones (Fig. 14), gravel trains can
form behind obstructions (usually vegetation) and scour at the sides and down-
stream end of the obstruction (Blair 1987, Field 1994). Distributary channels