Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 269

may be a more important variable. Local geology and surface deposits have important
influences on the delivery of fine-grained sediment to mountain streams. Dedkov and
Moszherin (1992) found that specific sediment yields vary from 100 t km −2 a −1 for ig-
neous rocks to as high as 1,800 t km −2 a −1 for loess. Specific sediment yield is positively
correlated with drainage area. Most often, a positive correlation occurs in regions with
vegetated slopes, where erosion is minimal compared to channel erosion. For basins
that have little established vegetation and high surface erosion, however, specific sedi-
ment yield is inversely related to increasing basin size.
Mountain watersheds have great erosional potential. The upper Indus and Kosi rivers
draining the Himalaya show a regional denudational rate of 1.0 mm yr −1 (Hewitt 1972).
Other mountain ranges may also approach this rate. The Alps are estimated to be erod-
ing at between 0.4 and 1.0 mm yr −1 (Clark and Jäger 1969). The Canadian Rockies and
the mountains of Alaska are being denuded at a rate of up to 0.6 mm yr −1 (McPherson
1971a; Slaymaker 1974; Slaymaker and McPherson 1977), while the rate in the Rocky
Mountains of Colorado and Wyoming is somewhat lower: 0.1 mm yr −1 (Caine 1974).
Characteristics of Mountain Streams
Mountain streams are similar to other streams in most respects, but they do have spe-
cial attributes. Water flows down steep slopes through highly varied terrain with great
local relief; the moisture supply is highly variable, depending on rainfall and melting
snow and ice, and the debris delivered to the streams is often too large to be trans-
ported effectively. The physical behavior of water in mountains, however, is no different
from that in any other natural environment. When the gradient or volume increases,
so do the velocity and the ability of the stream to transport material. Since all of these
factors change rapidly in mountains on spatial and temporal scales, the system fre-
quently exhibits pulsation. Velocity varies greatly between steep slopes and flat valleys,
between interconnecting pools or lakes, within the diurnal sequence, and with the sud-
den addition of rain from thunderstorms.
Smaller streams in high mountains are ephemeral, flowing only at certain times of
the year (Leopold et al. 1964). Perennial streams are found at lower elevations where
there is a greater drainage network. Discharge also increases downstream, making
these regions more susceptible to erosion, although this is counteracted somewhat by
well-developed vegetation.
The typical mountain stream is steepest in its upper reaches and gradually flattens
downstream to form a concave longitudinal profile. Rocky ledges and other abrupt
changes in the local relief called nickpoints create waterfalls. In semiarid mountains,
the channels of ephemeral streams often consist of a series of steps composed of larger
rocks interspersed with sand patches. Apparently, this is a response to intermittently
heavy runoff and flooding which first scour and then refill the bed. Downstream, chan-
nel width increases and the bed materials tend to become finer (McPherson 1971b).
High mountain streams typically have small discharges and low velocities despite
their steep slopes. They are consequently able to transport little coarse-grained materi-
al. The characteristic clearness of the water in high mountain streams, even at full-bank
flow, is evidence of this. The beds of mountain streams, originating from present or past
weathering, consist of gravel or boulders. The water flows around and between these