Take Mexico State Road 20 east from Hermosillo for about 140 Km.  You will reach the small village of La Estrella.  There is a bridge that crosses the river on the east side of town.  Continue on SR 20 east for another 1-2 Km until the road nears the river.  There are several primitive access points in this area.  You could also possibly access the river from the bridge but there are some casas and other structures near there.