Around the 4:30 mark of the film they mention about the tons of rock that constricted the river. I wasn't aware of that, so off to Wikipedia I went for the history of Hell's Gate.....
The first recorded history of Hell's Gate is found in the explorer Simon Fraser's journal, 1808. There he describes this narrow passage as an "awesome gorge" He also says that "surely this is the gate of hell". On June 26, 1808 Fraser passed along the cliffs on a series of bridges and ladders built by local Nlaka'pamux people.
Construction of the Canadian Northern Railway in 1914 blasted thousands of tons of rock into the river below the railroad grade which further constricted the river and damaged sockeye salmon runs. Thirty years of scientific planning and several years' construction have not completely repaired the damage. Hell's Gate's fishways, built by a joint Canadian-American Commission, were completed in 1946.
The route of the present Trans-Canada Highway through the Fraser Canyon parallels, roughly, the fur brigade trail of the Hudson's Bay Company, which was built over the shoulder of the Cascade Mountains high above the east bank of Hell's Gate, as the route north from Kequaloose (opposite Spuzzum) was completely impassable, even for mules. Beginning with the Fraser Canyon Gold Rush of 1858 Canyon a usable mule trail was built through the Canyon towards the 'Eye of Morello', a route which the new colonial government invested in heavily to build the Cariboo Wagon Road. The Cariboo Road was completed in 1864 but destroyed by CPR construction in the 1880s. A road through the canyon was not opened again until 1922 as the Cariboo Highway.
The Canadian Pacific Railway runs through the canyon. Construction through the canyon took four years and was completed in 1884. Across the river is the Canadian National Railway. Originally called the Canadian Northern Railway, this stretch was completed in 1914. Rockslides during construction narrowed the channel just above Hell's Gate, resulting in the need for the present fishways.