Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 270

boulders, which are really more a part of the channel than of the bedload. Maximum in-
cision occurs when sediment supply is moderate since sediment promotes abrasion but
also limits the extent of bedrock exposure (Sklar and Dietrich 2001). Glacial meltwater
streams are an exception to the typical clear rushing brooks, as they have a continuous
supply of fine sediment, called glacial flour. It is hard to believe that the “pure” glacial
meltwater contained in some bottled water was once filled with suspended sediment!
A common characteristic of most lowland streams is the presence of alternating deep
and shallow areas. Typically, deep areas exist on one side of the stream, with a gravel
bar or shallow area on the opposite side. These bars tend to alternate from one side
of the channel to the other. Smooth-water pools often form over the deeper areas and
riffles (or rapids) over the bars, features well known to trout anglers. There is evid-
ence, however, that many high mountain streams, especially those under semiarid re-
gimes, lack pools and riffles, apparently because of the dominance of coarse bed mater-
ial. The typically small discharges are unable to move the larger material. In the Rock-
ies, streams do exhibit pools and riffles, but their spacing is much more variable than
in lowland streams (Leopold et al. 1964). Step-pool sequences are typical in mountain
streams that contain gravel bed with gradients steeper than 2-3 percent. The steps are
areas of high gradient and increased velocity because of accumulations of wood, bed-
rock, cobbles, or boulders. Pools have fine-grained bed material with low gradient and
slow flow. Floodplains and natural levees are less common in high mountains than in
lowlands, since streams tend to be confined to bedrock channels and do not overflow
their banks. There may be occasional valley flats, but these are often formed by other
factors, such as moraine or beaver dams, landslides, and avalanches, rather than being
strictly a result of stream action. Mountain streams with bedrock channels display little
meandering except in stretches of low-gradient mountain meadows. Increased stream
complexity in mountain regions increases the storage potential compared to urban and
agricultural streams (Gooseff et al. 2007). Sediment sinks are important controls on
mountain drainage. For instance, 50 percent recovery occurred within 2-4 km of a sink,
but full recovery was not reached within 20 km downstream (Arp et al. 2007).
Discharge Regime
Mountain streams show periodicity on a daily as well as a seasonal basis. Snowmelt is
reduced at night and increases during the day. Consequently, streams supplied by melt-
water carry their greatest volume during the afternoon and early evening, and their low-
est volume during the early morning. Hydrographs typically display a high daily fluctu-
ation in flow during the summer. Snow stays in the high country until temperatures rise
in the spring. Many high mountain streams freeze, and the flow decreases to a trickle,
except in tropical or midlatitude coastal mountains where temperatures are not as cold
and rainfall or wet snow is common throughout the winter. In the summer, however, wa-
ter stored in the form of snow is released by melting (Fig. 5.18). In fact, 90 percent of
mountain stream variability in the Green Lakes Valley in Colorado is due to snowmelt
(Caine 1996). This may result in nearflood conditions for short periods in the spring;
rain falling on snow in the summer could also induce flooding (Rothlisberger and Lang
1987). In some years, great damage is done to bridges and highways in the lower stream
channels. Discharge data from the Rocky Mountains show that winter flows have in-