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Environmental Conditions of the Upper Colorado River Planning Area

Discussed in this section is vegetation, riparian protection through the Arizona Water Protection Fund Program, instream flow claims, threatened and endangered species, protected public lands and unique waters.

Click to view Figure 4.0-9


Four of Arizona’s six ecoregions are represented in the Upper Colorado River Planning Area: the Mojave Desert, Sonoran Desert, Colorado Plateau Shrublands and the Arizona Mountains Forests. (Figure 4.0-9) The planning area is diverse in terms of biotic communities, ranging from lower Colorado River Sonoran desertscrub to pine forests.  Much of the area vegetation is Mohave and upland Sonoran desertscrub and semidesert grassland and Great Basin conifer woodland in the northeastern portion.  The largest yucca species, the Joshua tree, characterizes the Mojave Desert ecoregion, a transitional desert between the higher and cooler Great Basin Desert and the lower, hotter Sonoran Desert. The Sonoran Desert ecoregion occurs in the southern part of the planning area where the saguaro is the characteristic plant and biodiversity is quite high.  The Colorado Plateau Shrublands and Arizona Mountains Forests ecoregions are characterized by chaparral, conifer woodlands and higher elevation grasslands.

Rocky Mountain (Petran) and Madrean montane conifer forests commonly occur between about 7,200 to 8,700 feet in Arizona. In the planning area, most of this community is below 8,000 feet in elevation where ponderosa pine is the predominant species in areas that receive about 18 to 26 inches of annual precipitation. About half of the precipitation occurs during the growing season, which permits forests to exist on less than 25 inches of annual rainfall, making them some of the driest forests in North America (Brown, 1982). In the planning area these forests exist in only a few relatively small areas: the Hualapai Mountains south of Kingman; the northeast part of the Bill Williams Basin; and the northeast part of the Peach Springs Basin.

Great Basin conifer (piñon-juniper) woodlands cover large areas below the ponderosa pine forest at elevations between about 5,000 and 7,500 feet that receive about 12 to 20 inches of annual precipitation. Extensive stands exist in the Peach Springs Basin and the eastern part of the Big Sandy Basin as shown on Figure 4.0-9.

Great Basin desertscrub occurs in northern Arizona mostly at elevations of 4,000 to 6,500 feet with average rainfall of about 7 to 12 inches.  This vegetative community is dominated by multi-branched, aromatic shrubs with evergreen leaves, primarily sagebrush, blackbrush and shadscale and grasses. Great Basin desertscrub is found only in relatively small areas of the Peach Springs Basin.

Great Basin Conifer

Great Basin conifer woodlands in the Peach Springs Basin

At similar elevations to Great Basin desertscrub (4,000-6,000 feet), interior chaparral is found in areas that receive 13 to 23 inches of annual precipitation.  Chaparral consists of dense shrubs that grow around the same height with occasional taller shrubs or small trees.  Chaparral communities typically are a mix of several shrubby species such as mountain mahogany, shrub live oak, and manzanita and commonly include cactus, agave, and yucca. Chaparral plants are well adapted to drought conditions.

Plains and Great Basin grasslands, primarily composed of mixed or short-grass communities, are found in the Peach Springs Basin and small areas of the Big Sandy Basin at elevations above about 4,000 feet that receive between 11 and 18 inches of annual precipitation. Semi-desert grasslands are more extensive and occur in valleys between the desert and woodlands or chaparral at elevations between 3,500 and 5,000 feet that receive 10 to 15 inches of annual precipitation.  Semi-desert grasslands are found are found primarily in the Hualapai Valley, Big Sandy and eastern portion of the Bill Williams basins. Desert grasslands often contain a mixture of grasses, shrubs and small trees.

Mohave Desert Scrub

Mohave Desertscrub, Detrital Valley Basin.

The boundary between Mohave desertscrub and Arizona Upland and Lower Colorado River Sonoran desertscrub is often difficult to discern.  While many of the same plants found in the other deserts occur here, some are indicative of the Mohave Desert such as the Joshua tree and certain cacti and endemic ephemeral plants, most of which are winter annuals (Brown, 1982).  The community is shrub-dominated and creosote bush and bursage are often dominant species. Mohave desertscrub covers most of the Detrital Valley, Lake Mohave and Sacramento Valley basins at elevations below about 3,500 feet that receive 5 to 11 inches of annual rainfall.

Two subdivisions of the Sonoran desertscrub region exist in the planning area-the Lower Colorado River Valley subdivision and the Arizona Upland subdivision. The Lower Colorado River Valley subdivision is the hottest and driest of the Sonoran desertscrub subdivisions. It covers most of the Lake Havasu Basin and smaller areas of adjacent basins (Figure 4.0-9). Intense competition for water results in widely spaced plants and more concentrated vegetation along drainage channels. In some areas the soil is covered by a single layer of tightly packed pebbles known as “desert pavement” that restricts plant types to ephemeral species.  Characteristic plants include creosote bush, bursage, saltbush, and mixed, more diverse vegetation along washes including blue palo verde, ironwood and jojoba.  Also commonly found in the subdivision are several types of cholla and other cacti. (Brown, 1982)

The Arizona Upland subdivision occurs primarily on slopes and sloping plains at elevations of 980 to over 3,000 feet where it merges with interior chaparral or semidesert grassland. This subdivision receives between 8 to 16 inches of average annual precipitation. It is the dominant biotic community in the Bill Williams Basin. Vegetation is scrubland or low woodland in appearance with blue and foothill palo verde, ironwood, mesquite and cat-claw acacia as common tree species.  Cacti are extremely important in this subdivision including saguaro, organ pipe, cholla and barrel cacti. (Brown, 1982) 

Riparian vegetation has been mapped along some perennial watercourses in the planning area including the Colorado, Bill Williams, Big Sandy and Santa Maria rivers and along smaller watercourses including Date, Trout and Burro creeks

(Figure 4.0-10).

Webb and others (2007) studied changes in riparian vegetation along a number of watercourses in the southwestern United States. Watercourses studied in the Upper Colorado River Planning Area include Lake Mead, Lake Mohave, Lake Havasu, the Bill Williams River, Big Sandy River, and the Santa Maria River. Historically, locally lush riparian vegetation existed  along reaches of the Colorado, particularly at major tributary confluences, although most of the now submerged river corridor was either barren sand or bedrock (Webb and others, 2007).  With construction of dams on the river, new habitat has formed including cottonwood and willow, and tamarisk along reservoir margins.  Fluctuating reservoir elevations and high salinity favor tamarisk.

Bill Williams

Vegetation along the Bill Williams River.

The mouth of the Bill Williams River at the Colorado River historically supported a considerable amount of riparian vegetation including cottonwood-willow. Lake Havasu now inundates the mouth of the river, supporting a 2,300 acre riparian zone including a cottonwood-willow forest and 500 acres of cattail marshes designated as the Bill Williams National Wildlife Refuge that extends 12 miles upstream. This area is also supported from releases of water from Alamo Dam, which completely regulates flow in the river downstream from the dam. Beaver dams are now common and riparian vegetation has increased substantially in many places.

The floodplain of the Big Sandy River upstream from Wikieup supports dense riparian vegetation including cottonwood and tamarisk. Downstream from Burro Creek, native and non-native vegetation have increased from historic observations. At the confluence of the Santa Maria River and the Big Sandy River, riparian vegetation, including tamarisk, has increased but also native species, particularly cottonwood and black willow.  (Webb and others, 2007)


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