|
Leaf
<br />HYDROLOGY
<br />Alpine / Subalpine Climatic Zone
<br />This zone ranges from approximately 9,500 feet
<br />above sea level to the highest peaks above t:imberline.
<br />The forested portion extends from 8,500 feet; to 1.1,500
<br />feet along the entire length of the Continental I)ivide
<br />and includes lodgepole pine, spruce-fir, aspen, and
<br />Douglas Fir.
<br />Total precipitation is primarily snow, most of,which
<br />is released as runoff during spring melt. On average,
<br />the peak snowpack is equivalent to about 15 anches of
<br />water, and during the spring about five more inches of
<br />water falls as snow and/or rain. Summer and early
<br />fall rains equal 8 to 10 inches. Of this 28 to 30-inch
<br />input, about 12-15 inches becomes streamflow.
<br />Of considerable significance to the issue uf channel
<br />maintenance flows in this zone is that little, if any,
<br />overland flow of water appears at any seaso:n. Numer-
<br />ous forest hydrologists including Anderson et al.
<br />(1976), Hoover (1969), Troendle (1987), Garstka et al.
<br />(1958), Leaf (1975), and Troendle and Leaf (1980)
<br />have reported that the primary runoff genei•ation pro-
<br />cess is from subsurface flow for both snovvmeXt and
<br />rainfall. During the summer months, rairifall rates
<br />rarely exceed about 1.5 inches per hour. Even under
<br />these conditions, the well protected, deep ;permeable
<br />soils characteristic of this zone are capable of infil-
<br />trating the water even above timberline (Retzer,
<br />1962). Virtually all of the water is retained on-site
<br />and subsequently vaporized through the evaporation
<br />process (ZYoendle, 1987).
<br />ponderosa pirie type. Annual precipitation varies from
<br />about 12 to perhaps 16 inches. This represents the
<br />norm in a fluctuating and complex hydrolog,y.
<br />The rainfall-dominated hydrology of the Montane
<br />zone has considerable significance to the issue of
<br />channel maintenance flows. High intensity rainfall is
<br />common in the ponderosa pine type. For example,
<br />highest rainfall intensities measured at the Monitou
<br />ExperimentFil Forest near Colorado Springs have
<br />exceeded 4 inches per hour for a 10-minute period
<br />(Gary, 1975). The Montane zone has a long history of
<br />intensive rainfall causing infrequent but major floods
<br />(Follansbee and Sawyer, 1948; Hansen, 1973). Flood-
<br />ing can result from high intensity rainfall below 7,000
<br />feet and extending eastward to the plains for a dis-
<br />tance of perhaps 50 miles. Such storms are concen-
<br />trated on relatively small areas and last for only a
<br />short time.
<br />Another equally devastating scenario can occur
<br />during May or June from general-type unslope storms
<br />in which the regional precipitation can vary from 2 to
<br />an extreme of 20 inches over a period of several days.
<br />At the higher elevations, precipitation from spring
<br />storms can fall as deep accumulations of snow. These
<br />deep snows retard runoff initially but when melt
<br />occurs coupled with rains, runoff can be extensive and
<br />rapid triggering extensive geologic processes such as
<br />landslides and mudflows (Hansen, 1973).
<br />In contrast to the Subalpine zone, streamflow gen-
<br />eration can be the result of a significant overland flow
<br />component - particularly during high intensity
<br />storms and/or rapid snowmelt in the ponderosa pine
<br />type (Gary, 1975).
<br />Montane Climatie 7,one
<br />This zone includes the eastern foothills region from
<br />southern Wyoming to Canon City, Colorado, and is
<br />bounded on the east by high plains. To the west, this
<br />zone extents to about 8,500 feet above sea level.
<br />Between 8,500 feet and 9,500 feet is a transition zone
<br />between the Montane and Subalpine zones. Forest
<br />cover in the transition zone is primarily imixed pine
<br />and fir. From 6,000 to 9,000 feet forest cover is pon-
<br />derosa pine interspersed with grasslands in large
<br />parks and along the lower reaches of major streams.
<br />Soils in the ponderosa pine type are shallow, unpro-
<br />tected, and potentially unstable (Gary, 1975). Precipi-
<br />tation occurs mostly as snow and/or rain in late
<br />spring and as afternoon thunderstorms ciuring the
<br />summer and fall.
<br />Normally, the Montane Zone is a moistuire-stressed
<br />environment. Runoff averages about 2.5 inr,hes in the
<br />SEDIMENT SOURCES AND YIELDS
<br />The sediment load in a river or stream comes from
<br />two sources: from the bed and banks of the channel
<br />itself, and from outside the channel. As discussed
<br />above, in the Alpine/Subalpine zone, the predomi-
<br />nance of subsurface and deep seepage in runoff gener-
<br />ation, with virtually no surface runoff from rainfall,
<br />results in negligible sediment introduced from outside
<br />the channel. Rates of sediment yield on a watershed
<br />basis range frorn 20 tons per square mile to less than
<br />1 ton per square mile (Leaf, 1966; Caine, 1974). Thus,
<br />flow induced channel erosion is the primary sediment
<br />source in the Alpine/Subalpine zone (Anderson et al.,
<br />1976; Retzer, 1962; Morris and Moses, 1987; Leaf,
<br />1966; Patric, 1976; Stednick, 1987).
<br />In contrast to the Alpine/Subalpine zone, sheet and
<br />gully erosion resulting from rainfall on steep sparsely
<br />vegetated soils can be significant sources of sediment
<br />JAWRA 866 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
|