The White Pass railway to Carcross and Skagway, 1966

The ExploreNorth Blog explains what caused the exposed silt: "Lewis Lake was named for WP&YR railway engineer A. B. Lewis, not for a success but for a significant screw-up at the lake. On September 11, 1899, construction crews had reached the foot of the lake, and Lewis decided that if they drained the lake by 10 feet, a much easier grade could be located. On September 14th, crews began cutting a 4-foot-wide ditch acros the sandy 300-foot-long ridge that held the lake back. Opening that ditch to the lake at 2:00 that afternoon, it rapidly became apparent that things were not going to go as planned. The water from the lake quickly washed away the sand, opening the ditch to 100 feet wide and 10 feet deep. That evening, a huge collapse of the ridge occurred, and the ensuing torrent flooded the valley right down to Lake Bennett, 15 miles south. The lake level was lowered by 70 feet, not the planned 10 feet. To cross the canyons that now existed, the White Pass had to build two bridges, each one nearly 50 feet in height and 600 feet long. Getting supplies to build those bridges in before winter arrived was a huge challenge. Materials did arrive, though, crews worked through the winter building the bridges, and on May 10th, the rail line across them opened."