There is a meandering run below the lake. There is one old logjam, not dangerous but you will want to use canoes or duckies that can be easily portaged/dragged over the logs. Not sure of the miles but it is a good long daytrip. The fishingg is also pretty good. The takeout is a couple miles above cascade Reservour. Cabarton is a great classIII run. Put in off Cabarton rd, takeout at smiths ferry. Run the rapid under the RR bridge (Trestle rapid) on the right. After rainbow bridge (auto) run middle right to avoid one hole on the left. Follow the current tongue thru Howards Plunge. Doesnt really matter if you are in the boat or out, or riding a giant pooltoy for that matter.... as long as you dont go into the hole on the right.
The SF payette Garden valley run is also a good family trip, or the middle for Payette from the FS boundary to Crouch when the water is high (Before july).