Hi Richard,
Sorry I didn't see this sooner. I published a book detailing all the whitewater runs on the Conchos and the Copper Canyon area (see GUIDEBOOK at
www.sierrarios.org). In general, if you want to do the Conchos, the closest section to the Rio Grande is Caños Pegüis - mostly a flat beautiful float but with a few IIs and one V/P. It's pretty much the last canyon before the Rio Grande. There are other runs above here that are dam controlled and a lot (the nicest) that are free-flowing.
You might also consider going down to the Oriental - around San Luis Potosi/Ciudad Valles. You'll find some fine class II-III sections of river to float, like the lower Moctezuma, the lower Santa Maria (Tampaon; one P), the Gallinas, and the Guayalejo (a few P).
Rocky
James "Rocky" Contos, Ph.D.
Director, SierraRios
rocky@sierrarios.orgwww.sierrarios.org