There are just two ways that I know of, you can park by the substation and walk around the gate and across the dam, which is what most people do. Or just a little further up the road is an access down a fence line. The other day that access wasn't accessible but by snowmobile. Most of the reservoir is inaccessible due to private property. There's a boat ramp access just past the hwy 20 bridge that I guess you could fish the upper end but I've never tried that because a lot of the time it isn't frozen up there. The road right at the frosttop drive in is what takes you over to the dam. Just past the river bridge take the road on the right.
