American Whitewater has an article about the Río Chínipas in its January/February 2010 edition written by Rocky Contos. It is located to the northwest of Divisadero. It is a class IV run that is 102 miles in length with access in the middle of the run at the town of the eponymous name.
In the summer of 2006, I and some others did several first descents in the Copper Canyon region. These were:
Rio Huevachic-Oteros-Chinipas (one trip: 8 days, ~220 km; tons of class V-VI on Huevachic with many mandatory portages, mostly IV on Oteros, III-IV on Chinipas)
Rio Choix (4 days: ~80 km; class IV-V; one mandatory portage)
Rio Tutuaca (4 days: ~200 km; class III-IV; one gorge with some class V)
Rio Papigochic, upper (3 days: ~80 km; class III; runout on Rio Sirupa to Puente Sirupa)