The western slope of Colorado
=s Rocky Mountains begins at the continental divide, the apex of the high peaks. Rivers east of the divide flow eastward to the Gulf of Mexico and the Atlantic Ocean, while rivers west of the divide flow into the Pacific Ocean.
In the high country, the vegetation of the East and West slopes are quite similar. The high mountain vegetation is discussed in the East Slope Ecosystems Display. Below the subalpine zone, the vegetation of the West Slope fans out into a diversity of types. While many people associate the west slope with an arid climate and desert vegetation, this region is also dissected by lush riparian corridors and, in places such as the Grand Mesa, it is speckled with wetlands. Such ecosystem diversity creates a mosaic of habitats and contributes to the overall richness and splendor of the Colorado landscape.
Many types of rock, such as limestones, shales, and volcanics, are found in the mountains of Colorado, however, the dominant type are the granites and metamorphized granites. Starting at the foot of the Rockies and reaching to the Sierra Nevada Mountains in California, is the province known as the intermountain region. Nearly one-fifth of Colorado lies in this area. The geology of the intermountain region is due to the Colorado Plateau, a massif that was uplifted during the Pliocene. The Colorado Plateau is predominantly composed of sandstones which have been exposed by constant erosion through the ages. The softness of this sedimentary rock, the aridity of the climate, and the sculpting of rivers have created a landscape that is one of the most awe inspiring in the state and one found nowhere else on Earth.
The climate of the west slope is primarily influenced by the Pacific air mass which flows over the Sierra Nevada and Cascade Mountains. As the Pacific air mass descends these mountains it loses much of its moisture. When it then flows across the intermountain region, the air mass acts like a sponge drawing most of the scant moisture present out of the air. This is what produces the extremely arid environment of the intermountain region. In fact, the overwhelming characteristic of the intermountain portion of the West Slope climate is aridity. Typically this area receives less than 10 inches (25.5 cm) of precipitation annually.
Unexpectedly, the reverse is true for the mountainous portions of the West Slope. The western side of the mountains receive much more precipitation than elsewhere in the state because
once the Pacific air mass has reached the mountains it has once again become laden with moisture. As this moist air rises over the mountains it cools and precipitation is formed. This Aorographic@ precipitation is responsible for the world-class skiing found in Colorado.
Several major rivers begin on Colorado
=s West Slope, including the Colorado and Rio Grande, but numerous smaller channels and tributaries are also found coursing throughout this region. Associated with each river is a strip of vegetation, which can be as little as a few feet, to more than a mile wide. The vegetation of riparian corridors is considerably more lush than in surrounding areas due to the additional water supplied by the river. Riparian areas are frequently dominated by cottonwood forests or woodlands which have an understory of shrubs, such as willows, forbs, grasses and sedges. In areas frequently disturbed, mature cottonwoods may not be present and the upper canopy dominated by willows. Due to changes in river flow and disturbance regimes, a weedy tree, salt-cedar (Tamarix ramosissima) is replacing many of the native riparian species. The control of this species is the subject of much current research.
The shrublands of the West Slope can broadly be placed into one of two categories
C the sagebrush lands or the saline shrublands. Sagebrush lands cover extensive areas of Colorado being best developed below 7,000 feet. They tend to occur on fairly coarse soils that are well drained. Grazing and fire have an important influence on this vegetation type, with grazing favoring sagebrush and fire favoring a grassy understory.
The saline shrublands also cover vast areas. They occur in areas in which the soil is rather clayey and moderately to very saline. Characteristic of this vegetation is widely spaced shrubs with little or no vegetation between them. Shrubs can be spaced from about 3 to 10 feet apart in these shrublands, with the spacing depending on the aridity and salinity of the area.
A number of woodland types can be found on the western slope, however, the three most common are the riparian, pinyon-juniper, and oak brush. Riparian woodlands are discussed in the riparian section of this display. The pinyon-juniper woodland becomes best developed between 6,000 - 8,500 feet. Typically it is found where soils are thin, the plants rooting into crevices in the fractured bedrock. A sparse, shrubby understory is generally present, consisting of big sagebrush, rabbitbrush, mountain mahogany, and others. Grasses such as blue grama and western wheat grass comprise the field layer.
Oak brush woodlands are found between 7,000 and 8,000 feet, and frequently mix with other vegetation types such as sagebrush and pinyon-juniper woodland. Gambel oak is the dominant of these woodlands and grows in distinct clumps or continuous thickets. Unlike their large eastern relatives, gambel oak only reaches about ten feet in height and about three inches around.
GEOLOGY
CLIMATE
SHRUBLANDS
WOODLANDS
RIPARIAN