Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 76

the Colorado River in southern Utah and record low annual rainfall at many stations.
The reasons for switching of interdecadal periods between generally wet and generally
dry conditions remains speculative, but research centers on hemispheric-scale, low-
frequency oceanic processes in the North Pacific and North Atlantic Oceans. 19 In 2005,
despite predictions of a 30 year drought beginning in 1996, record winter rainfall
occurred in the desert Southwest, further complicating our abilities to understand and
predict interannual variation in precipitation.
4.4 Geomorphology
Several types of channels can be defined in the Southwest, depending on the substrate
under the channel and flow regime. Bedrock channels occur in canyons that span a
complex array of geomorphic configurations. Some canyons, notably those carved by
Kanab Creek and the Virgin River, consist of a thin veneer of alluvium over bedrock.
Most “bedrock canyons” have a relatively thick alluvial fill that has both coarse-grained
and fine-grained terraces. For example, Grand Canyon, although usually considered to be
a bedrock canyon, has considerable alluvial fill beneath the channel. 20 Alluvial channels
form in deep alluvial fills with little or no bedrock constraints on lateral channel migration
or vertical downcutting. This general class of channels has more variation than bedrock
canyons owing to the complex interactions among surface water, groundwater, and
subsurface geology. Most alluvial channels in the Southwest are arroyos, or channels that
deeply incised into alluvium at the end of the nineteenth century (Figure 4.3). The depth
of these channels is dependent on regional base-level control, which regionally is sea level
in the Gulf of California but locally can be a variety of geologic structures or units and
usually those that affect the major rivers.
Because of their geomorphic history, alluvial channels develop terraces at varying
heights and distances away from the channel thalwegs, or deepest points. The heights of
terraces reflect both their depositional age and stability. Whether streamflow is perennial
or not in alluvial channels is dependent on bedrock structure, the age of alluvial terraces,
variation in the particle size of the fill sediment, and regional groundwater flow. Faults
create discontinuities in bedrock and alluvial aquifers and can force groundwater to the
surface. Cementation of alluvial terraces, a common occurrence with increasing age, or
where groundwater has high concentrations of calcium carbonate, can restrict downward
water movement. Fine-grained alluvial fills can also restrict downward movement of
groundwater, locally raising water level above the regional water table.
Over most of the Southwest, and particularly in the Basin and Range, bedrock canyons
become alluvial channels at the mountain front (Figure 4.2). On the Colorado Plateau, the
complex structural geology creates a variety of transitions between alluvial and bedrock
reaches. On the Escalante River, an alluvial channel transitions into a bedrock canyon
downstream from Escalante, Utah, creating an abrupt change in the type and stability
of riparian ecosystems. 21 This type of transition is less common in the Basin and Range,
but smaller-scale examples occur in the middle reach of the San Pedro River and along
several reaches of the Gila River upstream from the confluence with the San Pedro River
and downstream from the confluence with the Salt River (Figure 4.1). The lower Colorado
River represents a large-scale example of a river with alternating bedrock canyons and
alluvial fills.