As they returned to camp on the morning of their stroll to the heights to view their surroundings, the captains "boar a little to the left and discovered a handsome little river falling into the N. fork on Lard. side [the west side; their left, looking "Maria's River"] about 1-1/2 ms. above our camp." Clark first called it the "Tanzey." Apparently Lewis dubbed it Rose River, for he noted that "the wild rose which grows here in great abundance in the bottoms of all these rivers is now in full bloom, and adds not a little to the beauty of the cenery."
We have no clue as to who gave it its present name, Teton, nor when, much less any logical reason for it.
With the name "Tanzey," Clark referred to a plant he saw in great quantities along this tributary that presumably resembled a familiar one back in Virginia, the common tansy (Tanacetum vulgare L.), a perennial herb that had been introduced into colonial gardens from Europe, probably for medicinal purposes.
The common tansy spreads aggressively by means of short rhizomes, or underground stems, and by its attractiveness to a wide variety of insects for pollination purposes. It is now considered a weed, especially in pastures, since it is unpalatable to cows and horses. It seems highly unlikely, of course, that it could have spread fast enough to have established a presence on the Tansy-Rose-Teton River by 1805. Yet the other members of the sunflower family with which botanists figure Clark might have confused it – pineapple weed and dog fennel – are so obviously different in one aspect or another that we cannot determine what plant he was really remarking upon.