Great Basin Nevada The hydrographic Great Basin, whose unique inward-draining characteristics were first recognized by John C. Frémont as early as 1846, represents an area covering most of Nevada and much of western Utah and portions of southern Oregon and southeastern California. The region consists primarily of arid, high elevation, desert valleys, sinks (playas), dry lake beds, and salt flats. The Great Basin is characterized by the fact that all surface waters drain inward to terminal lakes, sinks or playas. Portions of Nevada which are excluded from the Great Basin include the extreme north-central portion of the state, where surface waters drain northward into the Snake River Basin, thence to the Columbia River and finally to the Pacific Ocean, and the south-eastern portion of Nevada where surface waters drain into the Colorado River Basin, thence to the Gulf of California (Mexico) and the Pacific Ocean. Within the Great Basin, major river drainage systems located wholly or partially in Nevada include: (1) the Truckee River, whose source is Lake Tahoe (Basin) in the Sierra Nevada and located partly in California and Nevada and whose terminus is Pyramid Lake in western Nevada, (2) the Carson River, whose west and east forks originate along the eastern slopes of the Sierra Nevada in California and whose terminus is the Carson Sink (Playa) in west-central Nevada, (3) the Walker River, whose west and east forks also originate along the eastern slopes of the Sierra Nevada in California and whose terminus is Walker Lake in western Nevada, and (4) the Humboldt River, the only major river wholly contained within Nevada and whose principal source is the Ruby, Jarbidge and Independence Mountains in eastern Nevada and whose terminus is the Humboldt Sink in west-central Nevada. Pyramid Lake and Walker Lake in western Nevada represent the only lake remnants of the ancient Lake Lahontan, an Ice Age lake that covered a considerable portion of northwestern Nevada during much of the Pleistocene Epoch of some two million to 10,000 years before present. At its peak elevation, this lake joined all these river systems of western and northern Nevada. The Great Salt Lake in western Utah, the last major lake remnant of the ancient Ice Age Lake Bonneville, which once covered a large portion of northwester Utah and spilled over into eastern Nevada, is also contained within the Great Basin and serves as the terminus for surface water drainage from the western slopes of the Wasatch Range in north-central Utah.