Saturday, February 14, 2009

A New Conception of My Home-Watershed: the Gila River Basin

It took me three years of living and traveling extensively around Tucson to understand the local watershed. I grasped early on the basin and range topography of parallel North-South mountain ranges, but these ridges served to confuse rather than clarify the overall drainage, since I did not know how the meandering rivers and dry washes of the bottomlands connected. Some went north, others flowed south. Where was the continental divide? Where did they meet the sea?

It was only by fortuitous circumstance that the last year found me exploring the full scope of an area I finally recognized as my home watershed. I new conception of the Gila River Basin emerged from these vast peregrinations, a new understanding of the lay of the land and a meshing of medium and large scale aspects:

The Sonoran desert in Arizona exists in the Gila Basin, surrounded on three sides (north, east, and south) by concentric rings of grassland valleys and mesas, woodland hills and forested mountains. The Gila, which rises on the continental divide to the east (in NM), is joined by tributaries from the north and south, representing, respectively, the Mogollon Rim divide and another divide which roughly approximates the U.S./Mexico border.

This general schema is interrupted, of course, by the basic and range, which further divides ecotones and watersheds, creating a kind of comb effect wherein numerous drainages flow parallel before their confluence with the Gila. The Gila itself is also indistinct, since it virtually disappears in an amorphous wash in the middle of its basin. And, since it flows through sparsely inhabited parts of the countryside, it is even easier to ignore its central role in ordering the landform. Most people living in the large metropolitan areas of Phoenix and Tucson do not realize their place along the cusp of the Sonoran lowlands, or the importance of the placements of their cities at the ecotone divide between grassland and desert, where rivers [used to ] bring water from the mountains before evaporating in the desert. [Theoretically,] the Gila eventually flows into the Colorado, before that crosses the U.S. Mexico border, reaches its estuary, and flows into the Sea of Cortez (Gulf of California). The Colorado rises from the western rocky mountains and the high colorado plateau to the north of the gila basin. To the south, in Mexico, streams flow directly off the Sierra Madre into the Sea of Cortez, without joining together in a large basin such as the Gila. To the east, on the other side of the continental divide, are the Chihuahan desert and the Rio Grand. These areas combined form the Southwest, and a large part of three out of four of the N. American deserts.

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