I can't be 100% sure, but I think Jim probably hit the nail on the head about those being wooden fenders to protect her from banging her brains too badly.

I tried to find a bit more on the subject, so I ran to my library. I found the same photo reproduced in both Graham Wilson's book, "Paddlewheelers of Alaska and the Yukon'' published by Wolf Creek Books, Whitehorse, YukonTerritory in 1999 and "Yukon River Steamboats; A Pictorial History" by Stan Cohen, published by Pictorial Histories Publishing Co. of Missoula, Montana, sixth printing 2005. Nothing was mentioned about the mysterious fenders, but the photo is of the Str. CLIFFORD SIFTON and was taken on July 24, 1900. As stated by others, she's in Miles Canyon which is between Whitehorse, and Carcross, YT.

The amazing thing is that these rapids would have been traveled by steamboats traveling to and from the boom town of Bennett, British Columbia.