Related to the environment:
* Rocks: Emphasizing the rocky terrain of deserts.
* Rugged: Describing the harsh and unforgiving nature of the landscape.
* Red: Referring to the color of the sand or the sunbaked earth.
* Rift: Could refer to a geological feature or a sense of isolation.
Related to wildlife:
* Rattlesnake: A common desert creature.
* Roadrunner: A bird known for its speed and agility.
* Reptiles: Highlighting the diverse reptilian life in deserts.
Related to experiences:
* Ride: Evoking the image of a camel or jeep ride through the desert.
* Rough: Describing the difficult conditions of a desert journey.
* Rest: A moment of peace amidst the harsh environment.
Other options:
* Rare: Highlighting the unique and precious nature of the desert ecosystem.
* Remembrance: Connecting the desert to memories or stories of exploration.
Ultimately, the best word for your "R" will depend on the overall theme and tone of your acrostic poem.