flank of Two Oceans Plateau in Yellowstone National Park, western Wyoming, the Snake starts out flowing west and south into Jackson Lake. Its first run through Jackson Hole, a wide valley between the Teton Range and the Gros Ventre Range. Below the tourist town of Jackson, the river turns west and flows through Snake River Canyon, cutting through the Snake River Range and into eastern Idaho. It receives the Hoback and Greys Rivers before entering Palisades Reservoir, where the Salt River joins at the mouth of Star Valley. Below Palisades Dam, the Snake River flows through the Snake River Plain, a vast arid physiographic province extending through southern Idaho southwest of the Rocky Mountains and underlain by the Snake River Aquifer, one of the most productive aquifers in the United States. Southwest of Rexburg, Idaho, the Snake is joined from the north by Henrys Fork. The Henrys Fork is sometimes called the North Fork of the Snake River, with the main Snake above their confluence known as the "South Fork". From there, it turns south, flowing through downtown Idaho Falls, then past the Fort Hall Indian Reservation and into American Falls Reservoir, where it is joined by the Portneuf River. The Portneuf River Valley is an overflow channel that in the last glacial period carried floodwaters from pluvial Lake Bonneville into the Snake River, significantly altering the landscape of the Snake River Plain through massive erosion. From there, the Snake resumes its journey west, entering the Snake River Canyon of Idaho. It is interrupted by several major cataracts, the largest being Shoshone Falls, which historically marked the upriver limit of migrating salmon. A short distance downstream it passes under the Perrine Bridge. Near Twin Falls, the Snake approaches the southernmost point in its entire course, after which it starts to flow west-northwest. The Snake continues through its canyon, receiving the Malad River from the east near Bliss and then the Bruneau River from the south in C.J. Strike Reservoir. It passes through an agricultural valley about southwest of Boise and flows briefly west into Oregon, before turning north to define the Idaho–Oregon border. Here the Snake River almost doubles in size as it receives several major tributaries – the Owyhee from the southwest, then the Boise and Payette rivers from the east, and farther downstream the Malheur River from the west and Weiser River from the east. North of Boise,