To Dmac844; the snowshed @ the East portal of Stampede Tunnel does very little to direct the waterfall. The waterfall was diverted by a small dam above the portal during the original construction of the tunnel, and actually crosses under the tracks just at the exit of the tunnel. This is the creek you can see & hear on the North side of the tracks @ Martin. The original snowshed at this site was wooden, and was torched by vandals in the early - 1990's during the lines' embargo. The fire burned so hot that it warped the girders of the original bridge, requiring its' replacement during the rebuilding. The current concrete snowshed is simply an extension of the Tunnel portal, and prevents slides from blocking the portal. There are wooden structures at both portals of Tunnel 4 that serve the same purpose.
Regarding the original question, the Great Northern was forced to build many miles of wooden & concrete snowsheds on their original line over Stevens Pass. The continued expense & maintenance of these sheds was the impetus for the digging of the current Cascade tunnel. Come on up next summer & see for yourself; the Iron Goat Trail offers many views of the remains of both types of sheds as it follows the old GN line from trailheads at Scenic, Martins Creek, & Tye/Wellington.