The lower Bull Run River changed dramatically in 1906, when the Mount Hood Railway and Power Company (MHR&P) began work on the Bull Run Hydroelectric Project. The project included a powerhouse on the Bull Run River at RM 1.5 (RK 2.4), and a diversion dam on one of its largest tributaries, the Little Sandy River, from its confluence with the Bull Run River. Water from the Little Sandy Dam diverted much of the Little Sandy's flow through a wooden flume about long to a reservoir called Roslyn Lake and from there to the powerhouse.