I doubt that current flows will remain until October 15, but it is possible. Typically when farmers start harvesting sugar beets, (around 1st week in Oct) they pull the plug, (or put the plug in as it were...) The bible is right, the canyon is class IV all the time, even at low flows, (except for high flows when it is class V). Nevertheless, It can be boated down to 200 cfs.
The upper part of the canyon is not flow dependent because it is uopstream of the Power plant, which receives its water from a tunnel from the dam. They leave enough water in it to maintain fish habitat and it will likely stay the same level until next seasons high water. It is fun, low-volume creeky pool drop stuff. Below the power plant the river reaches full volume and has more of a big water feel. If the weather is good and you are passing through with your boat you should check it out.