Rio Grande from Lajitas to Presidio on FM170, the River Road, said to be one of the most scenic drives in North America.
Lajitas is the eastern entrance to the Big Bend Ranch State Park, contiguous with the western boundary of Big Bend National Park.
For more information see http://www.tpwd.state.tx.us/state-parks/big-bend-ranch

For more on Ben Bend National Park see maps here http://www.nps.gov/bibe/planyourvisit/maps.htm
and Wikipedia articles on Big Bend National Park and the Rio Grande River.