Chapter 3. Geomorphology
Figure 19. Five channel morphologies observed on alluvial fans in southern
Arizona. Note that a single channel reach can go through each phase of
channel development through time, and each stage of development can be
observed at different places on a fan at the same time. (Modified from Field
accompanying an avulsion and the subsequent slow backfilling of the channel due
to a series of low-flow events. As the channels evolve through the morphological
steps, they become increasingly unstable, explaining why avulsions preferentially
occur where banks are low (Field 2001). The dramatic temporal and spatial
changes in channel morphology and position associated with channel avulsions
should be considered the norm, rather than the exception, on alluvial fans.
Large portions of many piedmonts in the Southwest, the gently sloping plains
between mountain ranges and valley bottoms, have been isolated from alluvial
fan flooding for 10,000 years or more (Field and Pearthree 1997). These inactive
alluvial fans do not need to be managed as flood-prone zones and form by multi-
ple processes including long-term tectonic quiescence (Eckis 1928, Harvey
1987), tectonic uplift (Beaty 1961, Hooke 1967), climate change (Hunt and
Mabey 1966, Silva et al. 1992), and base-level fall (Drew 1873, Wasson 1977).
Geomorphological analysis and mapping can be used to delineate these active
areas and thus identify those areas most susceptible to flooding (Fig. 20) (Rhoads
1986, Kenny 1990, French et al. 1993, Field and Pearthree 1997). A number of
in-situ weathering, erosional, and pedogenic features can be used to distinguish
flood-prone areas from inactive zones (Table 7), including desert pavement, rock
varnish, and salt-split cobbles (Fig. 21).