Great Sand Dunes National Park and Preserve, Colorado (https://www.nps.gov/grsa/index.htm) has the highest sand dunes in North America. Nestled against the 13000 foot peaks of the San Juan Mountains in the San Luis Valley south of Denver, the dune field is at 8200 feet and the highest dune is 750 feet. Between the mountains and the dunes, the Medano River runs in the spring. This is a very shallow river that "pulses" - see web site. The weather is often extreme with summers being very hot and winters very cold so check it out before you go.
On my last trip in May, it snowed the night before we arrived and the dunes were wet. Most of my photos were taken on a summer trip but you will notice a few where you can tell by the patterns on them that they are still wet. The dunes don't have trails on them. You just wander as you choose. There are other hikes in the park that you may want to check out. They have a campground with many of the sites having views of the dunes.