A strict hydrologic definition of the Great Basin would move its boundaries back to the headwaters of all streams draining into it. Following the Sevier, Provo, and Bear rivers would extend the Basin deep into the high plateaus and the Uinta Mountains. A strict structural definition might place the Basin boundary at the major fault lines, usually buried in piedmont alluvium of the prominent mountains and plateaus facing the Basin. Generally, some physiographic compromise is most serviceable in dealing with the history of man in the region.
Three distinct natural environments are encountered in crossing the Great Basin. Playas are undrained mud or salt-encrusted flats resulting the from deposit of sedimentary material as the lowest part of the basins fill. During wet seasons they may shimmer with shallow, ephemeral lakes. Climbing upgrade, alluvial fans of sand and gravel are deposited by runoff from above according to the speed and volume of water available. The many canyon exits merge their fans into extensive bajadas and piedmonts. The source of the material is the mountains--uplifted, tilted fault blocks usually with a steep front and more gentle backslope. Bare rock and cliff may still outcrop along the crest as a mountain is gradually reduced to alluvium.