Chapter 2. Hydrology
37
fans found at the mountain front. Mixed lithologies such as dolomite, quartzite,
and shale produce the clay-to-boulder-sized range of particles necessary to
produce debris flows during periods of high rainfall intensity in steep drainage
basins. Debris flows are short-lived events (e.g., minutes to hours) that can plug
deep channels (>7 m) during a single storm event, redirecting flow into new
channels or over unchannelized surfaces. The viscous nature of debris flows can
lead to the development of low berms along their flanks.
The occurrence of channelized flow, sheetfloods, and debris flows is
spatially and temporally variable, with all three potentially occurring on the same
stream system during the same flow event or at the same place through time. The
characteristics and distribution of these flow types is further detailed in Chapter
3. In addition, these processes are active during only a small percentage of time,
with no flow occurring in many watersheds in the Southwest more than 95% of
the time. While sheetfloods and debris flows are infrequent, they exert a strong
influence on the morphology of some fluvial landforms such as alluvial fans
(Field 1994, Blair and McPherson 1992). On the other hand, the most frequent
process, channelized flow, may have limited morphological impact. Attempts to
define "ordinary" as it relates to identification of the Ordinary High Water Mark
must recognize that commonly observed physical features are potentially the
result of uncommon processes and that the effects of the most common condition
(channelized flow or no flow at all) do not persist on the landscape.
Influence of Boundary Conditions
Runoff flowing through alluvial material (sediment deposited by the river
itself) is capable of adjusting channel dimensions (width and depth) to accom-
modate the runoff, so the channel morphology along alluvial reaches reflects the
flow conditions in the stream channel. Where the streambanks consist of easily
erodible sand material, the channel adjusts its width more rapidly than depth, so
shallow, braided channelized flows or sheetfloods are more likely than deeper
channelized flows. Where banks are richer in clay and silt, channel depth adjusts
more rapidly, so deeper channelized flows are more common.
Where runoff passes through nonalluvial material (i.e., bedrock or sediments
that the river cannot move), the flow is not capable of adjusting channel dimen-
sions, so the channel morphology is more reflective of the resisting forces of the
bank material rather than the flow itself. For example, adjustments in channel
width cannot occur in bedrock canyons, so the flow width of an extreme event
may be very similar to that of a more-frequent (e.g., annual) event. Given that the
extent of flow may be similar, regardless of flow magnitude, the delineation of
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