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The Great Basin covers most of both Nevada and Utah, large portions of Oregon and California, and corners of Idaho and Wyoming, and Baja California.  The Basin-and-Range physiographic province is predominant over most of the area, but the defining feature is interior drainage.

The rivers and streams of this vast tract of the American West drain into saline lakes (most notably the Great Salt Lake), a great many intermittent lakes, and innumerable playas (ephemeral lakes where surface water is only briefly seen). This is a generally very arid region, but it is both rimmed and punctuated by high mountain ranges with heavy snowfall like the Sierra Nevada. Lake Tahoe is the largest Great Basin Lake by volume, draining through the Truckee River into Pyramid Lake northeast of Reno. 

  

Like most geographic definitions, the "Great Basin" is clearest at a distance, but becomes murkier on close inspection. Interior drainage depends on lake levels, river flows and diversions, long-term climate variations, and the endlessly shifting levels of landforms.

Goose Lake offers an example. Straddling the Oregon-California border, it is a typical Great Basin Lake most of the time, virtually dry in some years. But, at high water levels, it spills into the Pit River, thence into the Sacramento and the Pacific. In a longer time frame, the Great Salt Lake is a remnant of the very much larger Pleistocene Lake Bonneville. About 17,000 years ago glacial meltwater raised Lake Bonneville so far above its present (fluctuating) level that it drained north into the Snake-Columbia River system to the Pacific through Red Rock Pass in southern Idaho. The Great Basin was for that brief time much smaller.

Conversely, the Colorado River has repeatedly drained into the below-sea level Salton Depression instead of into the Gulf of California. This was the result of the river's natural levee-building process, shifting climate cycles, and local subsistence due to the opening Gulf of California rift. (Most recently, a badly-engineered irrigation diversion was the immediate cause). During those episodes, the hydrologic Great Basin included the entire Colorado River Basin, more than doubling its current size.

Finally, the south end of California's San Joaquin Valley no longer drains north to the ocean, and it is contiguous with the Great Basin's Mojave Desert. Irrigation diversions and groundwater pumping play a major role here, and the region is no one's idea of the Great Basin, but the point can be argued.

Geographic definitions should be approached with caution. Maps, however, can be embraced wholeheartedly.


 

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