Park 183 Rio Grande National Scenic River- Big Bend Park

This is actually part of the Big Bend National Park, but counts as a visit- 

Here is the description from the website:

A River Wilderness

The Rio Grande flows from its headwaters in the San Juan Mountains of southern Colorado for 1,865 miles to the Gulf of Mexico near Brownsville, Texas. For 1,250 miles, the Rio Grande is the boundary between the United States and Mexico. In a remote stretch in west Texas, the river makes a curve to the northeast to form the “big bend.” It is here that the wild character of the river lives on.

There are three stamps at various locations in the park, I got one on the first visit at the Castolon VC: