By "salines" Jefferson meant salt flats, salt marshes, salt pans, salt springs, and rock or "fossil" salt deposits. Most would contain a variety of salts, including sodium chloride, which was an extremely valuable resource for a nation whose citizens already were moving westward. The explorers made firsthand observations, and collected secondhand reports of salines such as the Salt River (now Salt Creek), a tributary of the Platte River in Nebraska.
There was one objective not listed by Jefferson in his instructions to Lewis, but believed by many people then, and even mentioned in the official description of the Louisiana Territory. That was the supposed existence, about a thousand miles up the Missouri River, of a mountain of rock salt 180 miles long and 45 wide. On that matter the expedition's diarists were mute.
They did, however, observe that the Shoshones were the first Indians they had met who would taste salt. "These Indians all appear very fond of salt," wrote Private Whitehouse,"and eat it with their meat &ca. This we judged from some Captain Lewis gave them." The officers had previously given salt to Indian chiefs, but we don't know if the gift was appreciated or not. Most Indians on the Plains as well as to the east rarely used it. In the West, tradition suggests that the Tillamooks and other Indians south of the Columbia may have used it, but the Chinooks to the north did not.
Indians and Salt
Virgil J. Vogel, American Indian Medicine (Norman: University of Oklahoma Press, 1963).
Alfred L. Kroeber, "Salt, Dogs, Tobacco," Anthropological Records, Vol. 6, No. 1 (Berkeley, University of California Press, 1941).