Genaro, you can see the way I went when I rode up to GMR - note that I went the WRONG way (had to ride on a little dirt road for a bit) in that park by the dam going north, but got it right going south. There's a little guard shack looking thing at the end of the dam and the path starts back up over to the right (I missed it). Mix and match as appropriate
http://app.strava.com/activities/16750453
Of course, if you're getting off on Arrow highway you need to know none of this. And you're crazy. But yeah, you just cross over Arrow and head up to the top of the dam if you want to keep going north.