The Snake River Plain and the gap between the Sierra Nevada and Cascade Range together formed a moisture channel, opening the way for Pacific storms to travel more than inland to the headwaters of the Snake River. When the Teton Range uplifted about 9 million years ago along a detachment fault running north-south through the central Rockies, the river maintained its original course and cut through the southern end of the mountains, forming the Snake River Canyon of Wyoming. About 6 million years ago, the Salmon River Mountains and Blue Mountains at the far end of the plain began to rise; the river cut through these mountains as well, forming Hells Canyon. Lake Idaho, formed during the Miocene, covered a large portion of the Snake River Plain between Twin Falls and Hells Canyon, and its lava dam was finally breached about 2 million years ago.