Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 272

152 businesses, and caused more than $40 million in damage. The late Gilbert White, a
well-known hazards expert, warned about flooding in the city of Boulder, at the base of
the Front Range. In fact, residents are reminded of the danger of fast and widespread
flooding of creeks on the first of each month as the siren warning system is tested. The
construction of paved streets and roads in canyons provides impermeable channels for
flash flood waters to move swiftly downslope. Whereas large amounts of water previ-
ously infiltrated the rocky ground, lessening the potential for damage in lowlands, much
more of the water is now directed to the foot of slopes. Intelligent and effective land-use
planning is required to mitigate such problems.
There has been disagreement about when streams accomplish the most work. It
has long been believed that major floods and catastrophic events can accomplish more
erosion in a short time than day-to-day flow can accomplish in centuries. A number
of other studies indicate that much of the work of streams is accomplished in only a
few days of a decade or century, rather than on a more regularly spaced annual basis
(Rapp 1974). However, measurements of several streams indicate that most erosion is
accomplished by normal flow and by infrequent floods rather than by the rarer cata-
strophic floods (Leopold et al. 1964). Infrequent but larger events in mountains appear
to have more validity compared to lowlands. Mountain streams, which contain a bed-
load of cobbles and boulders, need catastrophic events to move such material (Nanson
1974). It is during these events that the truly awesome potential of running water is
demonstrated. In the future, as glaciers continue to recede, the frequency of catastroph-
ic events may increase, as discussed in Chapter 4.
Drainage Features: Braided Channels and Alluvial Fans
A number of drainage features are well developed in mountains. These include braided
channels, alluvial fans, and asymmetric valleys. The basic drainage patterns, when
viewed from the air or on a map, are also diagnostic of geomorphic processes and reflect
the presence of certain types of rocks, structures, and past geologic events.