Chapter 3. Geomorphology
55
Figure 14. Unconfined sheetflood zone on Wild Burro Wash near Marana,
Arizona. Note the organic debris in the foreground deposited by the most
recent sheetflood.
form at the apex of the sheetflood zones and continue to branch out (becoming
smaller and smaller and less distinct as they do) before terminating in elongated
sand tongues of low positive relief. Particle sizes decrease rapidly downstream,
from sand to silt and clay, as the sediment load drops out with the rapidly
decelerating flow. Since flow is more easily diverted at lower velocities, the
flood waters become reconcentrated into broad, gentle swales before ending at
headcuts carved by the reconcentrated sediment-starved flow. Reconvergence of
flow has been observed during natural sheetfloods (McGee 1897, Packard 1974)
and at the toe of distributary areas on experimental fans (Schumm et al. 1987).
Additionally, floodouts described from Australia appear similar to sheetflood
zones and exhibit the reconcentration of flow at their downstream ends (Tooth
1999, 2000). [Floodouts are sedimentary basins formed at the endpoints of
primary or distributary fluvial systems where channeled flow ceases and flood-
waters spill across adjacent alluvial surfaces (Bourke and Zimbelman, 2001).]
McGee's (1897) first description of sheetfloods in Sonora, Mexico, and southern
Arizona is very similar to the processes described here for discontinuous ephem-
eral streams and fits Hogg's (1982) definition of sheetfloods as "a sheet of
unconfined flood water moving down a slope."
Previous Page
