There looks to be a lot of dirt behind Prado Dam. Are any of those trails rideable or are they a sandy mess? Where would you park?
There are some trails in Prado Dam Regional Park. So far I have not been bored enough to drive that far to check them out. If things looked a little more inviting on Google Earth I would give it a try...one of these days I will go explore.
Just be careful, there are some gun ranges out there. You could probably park by the construction zone near the end of the south 71. People used to ride chino hills from that parking lot.
@rojomas just looking for something different. @Gary IFF I will have to keep the gun ranges in mind if I go out there!
About 10-12 years ago I had the opportunity to become a bit familiar with the Prado Dam area. It hasn't changed much since. The area is under control of the Army Corps of Engineers, and you shouldn't have any problem accessing the land. For the most direct access, from Euclid, take Pine east, then Chino-Corona Rd. south past the women's prison. You'll pass the model airplane airport on the left, then an abandoned olive orchard. Park anywhere you're comfortable. Go too far south, and you'll intrude on a yard where construction equipment is stored and you'll likely be very unwelcome. There's a n/s creek on to the east. If you cross it, you'll be encroaching on SC Village (paintball) or Raahauge's (real guns). So don't cross it. I suspect that besides prisoners, paintballers, hunters, and swampland, you'll find little of interest. Disclaimer: My description of this area doesn't in any way indicate legality. If it's legality you seek, go to the County Park on Euclid. There are trails that go from the lake over to the area behind the dam. I'm sure the park ranger would be willing to lend advice.