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ERDC TR-04-1
Arid-region river channels, especially those with sandy banks, are often very
responsive to large flows and recover slowly from them because of the limited
vegetation growth and the large interannual variability in peak discharges (Cooke
et al. 1993, Tooth 2000). Nonexistent or poor armoring of ephemeral stream beds
(Reid and Laronne 1995) increases the sensitivity of the river channel to a range
of flow events and hinders the ability of the river to "hold" any one pattern.
Consequently, desert rivers are often in a perpetual state of change--working to
recover from a large flood but unable to "heal" completely before the next
extreme event widens the channel and renews the process (Cooke et al. 1993,
Tooth and Nanson 2000a). Unambiguous morphological features indicative of
the OHWM are unlikely under these conditions since channel morphology is in a
state of flux, with high water marks being constantly reworked. The faster the
rate of revegetation and the longer the time interval between threshold-crossing
extreme events, the greater the likelihood that the recovery process will result in
the development of reliable OHWM indicators. Conversely, the slower the
recovery process, the greater the likelihood that the high water mark indicators
present along the channel are the result of extreme events. In certain instances,
recovery is delayed even further because the effects of extreme events control the
processes that occur during subsequent smaller events (Graf 1983b, Bourke and
Pickup 1999). Regardless of the rate of recovery, dramatic temporal and spatial
changes in channel morphology and position should be considered the norm,
rather than the exception, along compound channels. A stable channel form is
never really achieved because the rate of recovery is slower than the return
interval of extreme events that alter the landscape.
Alluvial Fans
Alluvial fans are depositional landforms with a conical shape that develop
where confined streams emerge from upland areas into zones of reduced stream
power (Fig. 17) (Harvey 1997). While alluvial fans are found in almost all
climates (Nilsen and Moore 1984, Rachocki and Church 1990), they are an
especially important landscape element in the southwestern United States, where
an estimated 31% of the land surface is covered by alluvial fan deposits (Antsey
1966). Alluvial fans are characterized by a distributary flow pattern (Bull 1977,
Tooth 2000) with debris flow (Beaty 1963, Blair and McPherson 1992, Whipple
and Dunne 1992), sheetflood (Blair 2000), and streamflow (Blissenbach 1954,
Field 2001) processes predominating; all three processes are not necessarily
active on all fans, however. Debris flows are intermediate between landslides and
water flooding, with sediment concentrations ranging from 70 to 90% by weight;
sediment and water move together as a slurry at the same velocity (Ritter et al.
1995). Poorly channelized braided flow is characteristic of streamflow processes
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