This is an issue that spans beyond ecology and into safety.
Much of the Fraser River (the river you'll see during most of your drive from Vancouver to Williams Lake) is fast moving and hazardous to swimmers. Growing up in Prince George (about 150 miles north of Williams Lake), we'd float down part of the Nechako River, which merges into the Fraser... and about once a year, we'd hear of a missing person who went too far, ended up in the Fraser and the body was recovered (sometimes) around Vancouver in a couple days.
Not to mention, we're talking about next to no visibility in a lot of the river -- a lot of sediment is carried down to form the Fraser River Delta on which Vancouver Airport and other important Lower Mainland areas are built upon.
If you find somewhere to do it safely and responsibly, please let me know -- it does sound very interesting.