An important portion of The Corps of Discovery's journey occurred in southwest Montana. On July 19, 1805, Lewis and his men were struggling up the Missouri between present-day Upper and Lower Holter lakes. "from the singular appearance of this place I called it the gates of the rocky mountains." The next day, searching for "the three forks of the Missouri," they entered the first of southwest Montana's big valleys – Helena's Prickly Pear Valley.

Arriving at the meeting of the rivers on July 27, Lewis stated, "the country opens suddonly to extensive and beatifull plains and meadows which a...