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Lower Colorado River Geography

The Lower Colorado River Planning Area encompasses about 17,200 square miles (sq. mi.) and includes the Butler Valley, Gila Bend, Harquahala, Lower Gila, McMullen Valley, Parker, Ranegras Plain, San Simon Wash, Tiger Wash, Western Mexican Drainage and Yuma basins.  Basin boundaries, counties and prominent cities, towns and places are shown in Figure 7.0-2.  The planning area is bounded on the north by the Bill Williams Basin in the Upper Colorado River Planning Area, on the east by the Phoenix, Pinal and Tucson Active Management Areas (AMA), on the south by the international boundary with Mexico and on the west by the State of California and the international boundary. 

The planning area includes all or part of four watersheds, which are discussed in Section 7.0.2.  The Cocopah Indian Reservation (10 sq. mi.) and the Gila Bend Indian Reservation (16.3 sq. mi.) are entirely within the planning area.  Approximately 86% (391 sq. mi.) of the CRIT, 57% (2,471 sq. mi.) of the Tohono O’odham Indian Reservation, and 4% (3 sq. mi.) of the Fort Yuma-Quechan Indian Reservation are also located within the planning area (Figure 7.0-1). The Gila Bend and Tohono O’odham reservations are two of the four land bases that make up the Tohono O’odham Nation.  Comparable in size to the state of Connecticut, the Nation is the second largest Indian reservation in the United States.

Click to view Figure 7.0-2

Figure 7.0-2 Lower Colorado River Planning Area

The entire planning area is within the Basin and Range physiographic province characterized by northwest-southeast trending mountain ranges separated by broad alluvial valleys (Figure 7.0-3).  The planning area is relatively low elevation – generally less than 3,500 feet.  Higher elevation mountain ranges occur along part of the northern boundary and in the Baboquivari Mountains that form the southeastern boundary where elevations rise to over 7,700 feet.  The lowest elevation is about 70 feet where the Colorado River enters Mexico at the Southerly International Boundary (SIB) in the Yuma Basin.  The basin with the largest elevational range is the San Simon Wash Basin with a range of 1,650 to 7,730 feet.

Figure 7.0-3

Figure 7.0-3 Physiographic Provinces of Arizona

A unique geographic feature of the planning area is its aridity, which has shaped its topography and surface water characteristics.  In the more arid western part of the planning area, the geography consists of widely-scattered, small mountain ranges of mostly barren rock and broad, flat valleys (or plains).  A number of groundwater basins in the planning area take their name from this geographic feature, e.g. Butler Valley, McMullen Valley and Ranegras Plain.  Other examples of major valleys and plains are the Mohawk Valley in the Lower Gila Basin and the La Posa Plain in the Parker Basin.  Relatively large areas of sand dunes occur south of Yuma and west of the Gila and Tinajas Altas Mountains in an ancient river terrace.  To the southeast, the terrain contains more numerous mountain ranges and narrower valleys with higher rainfall and more plant diversity and density (ASDM, 2007a).  With the exception of the Colorado River, there are no perennial streams in the planning area. The Gila River was historically perennial for most of its length but by the beginning of the 20th century the effects of farming and construction of dams both upstream and within the planning area caused cessation of perennial flows (Tellman and others, 1997).  Broad sandy washes are the main surface water feature in the planning area, flowing only in response to significant precipitation events


water drop  Continue to Section 7.0.2 Hydrology-Groundwater


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