Thanks to careful surveying and engineering, contractors could build flumes to float logs across the Divide by starting on high slopes and running the water (and logs) through a lower pass. ACM contracted with A.A. McCune & John Caplice in the 1883 for the initial construction and operation of a flume from the California Creek area on the Big Hole side and down Mill Creek on the Clark Fork side. This operation was expanded by William R. Allen c. 1906-1911. Here's a historical photo from 1906 of a portion of the flume in French Gulch (Mansfield Library photo):
Each year, more than 600 men and nearly a hundred teams of mules stripped the hills of timber. By the early 1900s, hundreds of millions of board feet had been clear-cut, and the resulting outrage over environmental damages led President Theodore Roosevelt to create the Big Hole Forest Reserve (now the Beaverhead National Forest) in 1906.
Here's a contemporary view of a portion of the flume, more or less across the center of the photo: