Grand Fall, June 17, 1908
Montana Historical Society
I took my position on the top of some rocks about 20 feet high opposite the center of the falls," Lewis wrote on 13 June 1805. He continued,
this chain of rocks appear once to have formed a part of those over which the waters tumbled, but in the course of time has been seperated from it to the distance of 150 yards lying prarrallel [sic] to it and forming a butment against which the water after falling over the precipice beats with great fury; this barrier extends on the right to the perpendicular clift which forms that board of the river but to the distance of 120 yards next to the clift it is but a few feet above the level of the water, and here the water in very high tides appears to pass in a channel of 40 yds. next to the higher part of the ledg of rocks; on the left it extends within 80 or ninty yards of the lard. Clift which is also perpendicular; between this abrupt extremity of the ledge of rocks and the perpendicular bluff the whole body of water passes with incredible swiftness.
Nineteen-eight was one of those exceptional years when the spring runoff usurped that "channel of 40 yds" (bottom of photo), which Lewis walked across. It was long remembered as the year of "The Mother Flood." On 14 June 1908, the streamflow at Fort Benton, thirty-five miles downriver, was measured at 140,000 cubic feet per second. By the seventeenth, when this photo was taken, the flow had slackened to 120,000 cfs. For comparison, the daily mean flow at a gauge upriver from the falls, during a forty-six-year period in the late 20th century, was 5,971 cfs.1