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Securing Arizona's Water Future

Western Plateau Planning Area Geographic Features

The Western Plateau Planning Area covers about 13,700 square miles and includes the Coconino Plateau, Grand Wash, Kanab Plateau, Paria, Shivwits Plateau and Virgin River basins.  Basin boundaries, counties and prominent cities, towns and places are shown in Figure 6.0-2.  The planning area is bounded on the north by the State of Utah, on the east by the Eastern Plateau Planning Area, on the south by the Central Highlands and Upper Colorado River planning areas and on the west by the State of Nevada (Figure 6.0-1).  The planning area includes parts of three watersheds, which are discussed in Section 6.0.2.  The Kaibab-Paiute Indian Reservation (188 square miles) and the Havasupai Indian Reservation (294 square miles) are located entirely within the planning area.  In addition, the planning area includes the western portion of the Navajo Indian Reservation (1,177 square miles) and the northeastern portion of the Hualapai Indian Reservation (741 square miles) (Figure 6.0-1).

Click to view Figure 6.0-2

Figure 6.0-2 Western Plateau Planning Area

Physiographic provinces

Figure 6.0-3 Physiographic Regions of Arizona

Almost all of the planning area is within the Colorado  Plateau physiographic province characterized by generally horizontally stratified sedimentary rocks that have eroded into numerous incised canyons and high desert plateaus (Figure 6.0-3).  The extreme western part of the planning area, encompassing the western portions of the Virgin River and Grand Wash basins, extends into the Basin and Range physiographic province, which is characterized by northwest-southeast trending mountain ranges separated by broad alluvial valleys.  The Coconino Plateau Basin contains the largest elevational range in the planning area with elevations ranging from 1,400 feet where the Colorado River exits the basin in the Grand Canyon to over 12,000 feet in the San Francisco Peaks at the southeastern edge of the basin.

A unique geographic feature of the planning area is the Grand Canyon, incised by the Colorado River and its tributaries over a 5-6 million year period.  The average depth of the canyon is 4,000 feet over its 277 mile length, and 6,000 feet at its deepest point. Its average width is 10 miles.  The geologic record at the Grand Canyon is unique in the variety of rocks and their exposure in the canyon walls, with nearly half of the earth’s 4.6-billion-year history displayed (NPS, 2005).

Most rocks in the Grand Canyon date from the Paleozoic Era (550-250 million years ago) but there are scattered remnants of Precambrian Vishnu Schist as old as 2 billion years in age in the inner gorge.  Western Plateau Plateau Planning Area geology including the location of Precambrian rocks in the Grand Canyon is shown in Figure 6.0-4.  With the exception of Kaibab Limestone, younger Mesozoic and Cenozoic rocks (250 million years old to the present) are largely missing at Grand Canyon, having been either never deposited or worn away. The different rock layers in the canyon respond differently to erosion leading to the canyon’s distinctive shape (NPS, 2005). Lava flows ranging in age from 1,000 to 1 million years old are found in the western part of the canyon.

The Grand Canyon and the Colorado River form a significant physical barrier between the Arizona Strip and the rest of the planning area and the state.  Highway 89A at Navajo Bridge and Highway 89 at Glen Canyon Dam are the only highways that span the Colorado River linking the Arizona Strip to the rest of the state.  By contrast, there are a number of road links between the Arizona Strip and Utah.  As a result, the Arizona Strip has strong historic, cultural and economic ties to Utah.

South and east of the Colorado River, the Coconino Plateau marks the southern edge of the Colorado Plateau which covers 130,000 square miles across southeastern Utah, northern Arizona, northwestern New Mexico, and western Colorado. The Coconino Plateau stretches east toward the Colorado River surface water divide and south to the Mogollon Rim, which is less well defined to the northwest, and defines the southern boundary of the Coconino Plateau Basin.  Most of the Coconino Plateau is above 5,000 feet in elevation and consists of low hills, mesas, broad valleys and lava flows in the southern portion. The plateau is defined by large elevational changes along its margins, notably the south rim of the Grand Canyon (Bills and others, 2007).

In the northwest corner of the planning area, the Virgin River cuts through the Beaver Dam Mountains creating the Virgin River Gorge.  West of the gorge, the topography abruptly changes to a broad alluvial valley with numerous washes that drain the upland and mountain areas.  The Virgin Mountains, south of the river, form the southwest edge of the Colorado Plateau.

Other significant geographic features are numerous high plateaus, steep cliffs, deeply incised canyons and few surface water features. 

Colorado River through the Grand Canyon

Colorado River through the Grand Canyon.  The Grand Canyon and the Colorado River form a significant physical barrier between the Arizona Strip and the rest of the planning area and the state

 

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