three main phases or "adaptive systems". The first he calls "Broad Spectrum Foraging", dating from 11,500 to 4,200 years before present. During this period people drew upon a wide variety of food resources. The second period, "Semisedentary Foraging", dates from 4,200–250 years before present and is distinctive for an increased reliance upon fish, especially salmon, as well as food preservation and storage. The third phase, from 250 to 100 years before present, he calls "Equestrian Foragers". It is characterized by large horse-mounted tribes that spent long amounts of time away from their local foraging range hunting bison. In the eastern Snake River Plain there is some evidence of Clovis, Folsom, and Plano cultures dating back over 10,000 years ago. Early fur traders and explorers noted regional trading centers, and archaeological evidence has shown some to be of considerable antiquity. One such trading center in the Weiser area existed as early as 4,500 years ago. The Fremont culture may have contributed to the historic Shoshones, but it is not well understood. Another poorly understood early cultural component is called the Midvale Complex. The introduction of the horse to the Snake River Plain around 1700 helped in establishing the Shoshone and Northern Paiute cultures. On the Snake River in southeastern Washington there are several ancient sites. One of the oldest and most well-known is called the Marmes Rockshelter, which was used from over 11,000 years ago to relatively recent times. The Marmes Rockshelter was flooded in 1968 by Lake Herbert G. West, the Lower Monumental Dam's reservoir. Eventually, two large Native American groups controlled most of the Snake River: the Nez Perce, whose territory stretched from the southeastern Columbia Plateau into northern Oregon and western Idaho, and the Shoshone, who occupied the Snake River Plain both above and below Shoshone Falls. Lifestyles along the Snake River varied widely. Below Shoshone Falls, the economy centered on salmon, who often came up the river in enormous numbers. Salmon were the mainstay of the Nez Perce and most of the other tribes below Shoshone Falls. Above the falls, life was significantly different. The Snake River Plain forms one of the only relatively easy paths across the main Rocky Mountains for many hundreds of miles, allowing Native Americans both east and west of the mountains to interact. As a result, the Shoshone centered on a trading economy. According to legend, the Nez Perce tribe was first