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Lower Colorado River Environmental Conditions - Vegetation

Environmental conditions reflect the geography, climate and cultural activities in an area and may be a critical consideration in water resource management and development.  Discussed in this section is vegetation, protection of riparian areas through the Arizona Water Protection Fund Program, threatened and endangered species, public lands protected from development as national monuments, wildlife refuges and wilderness areas, and managed waters.  No instream flow claims (a non-diversionary appropriation of surface water for recreation and wildlife use) have been filed in this planning area.

Click to view Figure 7.0-9


Information on ecoregions and biotic (vegetative) communities in the planning area are shown on Figure 7.0-9.  With the exception of a very small area of Chihuhuan desert and Sierra Madre Occidental pine-oak forest along the southeastern boundary, the entire planning area is within the Sonoran Desert ecoregion. Biotic communities range from Lower Colorado River Valley Sonoran desertscrub to Madrean evergreen woodland. Most of the planning area is covered by Lower Colorado River Valley and Arizona Uplands Sonoran desertscrub.

Madrean evergreen woodland occurs at the highest elevations of the San Simon Wash Basin in the Baboquivari Mountains where mean annual precipitation exceeds 16 inches.  The woodland consists of evergreen oaks, alligator bark and one-seed junipers, and Mexican pinyon transitioning to semidesert grassland at lower elevation.  Cacti of the semidesert grassland may extend well into the woodland. (Brown, 1982)

Interior chaparral occupies mid-elevation foothills, mountain slopes and canyons in small areas along the boundary of McMullen Valley and Butler Valley basins and along the McMullen Valley/Harquahala/Tiger Wash basin boundaries.  Interior chaparral is found in areas between about 3,500 and 6,000 feet in elevation that receive 15 to 25 inches of annual precipitation (Brown, 1982). Chaparral consists of dense shrubs that grow around the same height with occasional taller shrubs or small trees.  Typical shrubby species are mountain mahogany, shrub live oak, and manzanita. Chaparral plants are well adapted to drought conditions.

The western limit of the semidesert grassland community occurs in the eastern part of the planning area.  A small area adjoins the Madrean evergreen woodland community in the Baboquivari Mountains and smaller areas exist in the central part of the San Simon Wash Basin along the Lower Gila/Western Mexican Drainage/San Simon Wash basin boundaries, and near Aguila in the McMullen Valley Basin.  Semidesert grasslands receive between about 10 to 17 inches of annual rainfall.  Grasses were originally perennial bunch grasses with intervening areas of bare ground.  Where heavily grazed, grasses have shifted to annual species where summer rainfall is low, or to low growing sod grasses where rainfall is moderate to heavy.  Shrubs, cacti and herbaceous plants are commonly found in the semidesert grassland community. (Brown, 1982)

Two subdivisions of the Sonoran desertscrub region exist in the planning area-the Lower Colorado River subdivision and the Arizona Upland subdivision. The Lower Colorado River subdivision is the hottest and driest of the Sonoran desertscrub subdivisions. There is intense competition for water, with plants widely spaced and more concentrated 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.  High concentrations of sodium in the soil below the pavement may also restrict plant growth. Sand dunes occur near Yuma and Parker. Characteristic plants include creosote bush, bursage, saltbush, and mixed, more diverse vegetation along washes and other areas with more water.  These areas may include blue palo verde, ironwood and jojoba.  Also commonly found in the subdivision are several types of cholla and other cacti. (Turner and Brown, 1982)

Sonoran desertscrub in Gila Bend Basin

Sonoran desert scrub in the Gila Bend Basin

The Arizona Upland subdivision borders the Lower Colorado River subdivision and 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 more precipitation than the other Sonoran desertscrub subdivisions with average annual precipitation between 8 to 16 inches.  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. (Turner and Brown, 1982) 

Bufflegrass (Pennisetum ciliare), was introduced to the United States in the 1930s as livestock forage, and since the 1980s it has spread rapidly and can now be found on the edges of roads in most of southern Arizona.  It is problematic in the Sonoran Desert because it grows densely, crowding out and competing for water with native plants and it is a fire-prone perennial that alters the natural fire regime. (ASDM, 2007b)  When wildfires occur, the densely growing grass spreads fire rapidly and it thrives after fires, unlike native species (Brooks and Pyke, 2002).

Some efforts to control the spread of bufflegrass have been successful.  Organ Pipe Cactus National Monument undertook a large eradication effort through yearly weeding efforts and has managed to control and largely prevent its proliferation in the area (Burns, 2007).

Riparian vegetation along Colorado River

Riparian vegetation in the backwaters of the Colorado River, Lower Gila Basin

Riparian vegetation exists at locations along the Colorado and Gila rivers as shown on Figure 7.0-10.  Along the Gila River in the vicinity of Gillespie Dam, primarily tamarisk, but also cattail, occurs. Downstream from Gillespie Dam to Painted Rock Reservoir, irrigated agriculture adjacent to the river may support native and nonnative riparian vegetation. Below Painted Rock Dam, the Gila River is mostly dry until irrigation return flows within the Wellton-Mohawk Irrigation District add some flow to the river. In the area near Dome, return flow supports riparian vegetation consisting of a narrow line of cottonwood along the channel with dense tamarisk behind (Webb and others, 2007)

The riparian corridor of the lower Colorado River was historically a mixture of cottonwood and willow trees with backwater wetlands.  These habitats were maintained by the natural flow regime consisting of spring floods that washed salts from the banks, supported germination of tree seeds, and created seasonal wetlands (University of Arizona, 2003).  Although the river has been altered by dams and water delivery infrastructure, riparian ecosystems exist along most of the reach of the Colorado upstream of Imperial Dam. Floods no longer occur so the composition of woody riparian vegetation has changed with native species and tamarisk predominant.

Downstream from Parker Dam, non-native date palm, giant reed and fan palm are found with mesquite and arrowweed found further from the river. Downstream of Headgate Rock Dam (Figure 7.6-5), the river corridor widens. Riparian vegetation in this area was mapped in 1962 and covered 108,000 acres of primarily mesquite bosque with some reaches of native riparian vegetation among stands of tamarisk. The All American Canal at Imperial Dam diverts much of the flow of the Colorado River to California. Black willow, cottonwood and tamarisk are found in the abandoned river channel in this area. Through Yuma, flood control and bank protection have narrowed the river channel but has also provided more stable hydrologic conditions, resulting in an increase of riparian vegetation, primarily arrowweed. (Webb and others, 2007)

In Mexico, the Colorado River Delta was historically two million acres in size and was a maze of lagoons and thickly forested.  Today, only about 420,000 acres of riparian, wetland and intertidal habitat remain.  This habitat is largely maintained by the delivery of irrigation drainage water from the Wellton-Mohawk Irrigation District in Arizona. This water has flowed to the eastern side of the delta since 1979, creating the largest wetland in the Sonoran Desert, the Cienega de Santa Clara (Glenn and others, 2004).



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