“In Texas, I had crossed over the Pecos River, which I had read of in many stories. You could have easily walked across the Pecos River without getting your feet wet.”
Now, now, let’s be fair. Depends on the season and the location. The Pecos feeds a reservoir close to the Texas-New Mexico state line, where a hydroelectric plant pumps out the electrons daily. There’s even whitewater canoeing thereabouts, and usually enough water all the rest of the way to the Rio Grande without any walking necessary. It could be belly-deep on a horse in the late 1850s.
Late spring, early summer and mid-to-late fall are the best times for good flow.
Via Alexandria.
THE TEXAS SCRIBBLER 
For me Pecos River is associated with the Judge Roy Bean movie. Somewhere at the beginning of that romp the river is mentioned. As for Rio Grande – as far as I remember, there are a lot of places where you just wade through to the other side, not even wetting your knees.