The reservoir and surrounding terrain are characterized by eroded, gently rolling brushland crossed by silted stream valleys. The land was formed during the Cenozoic Era (the period following the extinction of the dinosaur) by accumulating sediments from seas that once covered south Texas. Ancient rivers flowing to the southeast dumped their sediments into what was then part of the Gulf of Mexico, producing new land. Seas intermittently covered the newly formed land with river-carried sediments, which eventually dominated. These sediments are generally composed of volcanic ash, claystone, siltstone, tuff, shale and shaley limestone. The present location of Choke Canyon Dam is near the ancient Gulf shoreline of about 30 million years ago. Erosion of these sediments and subsequent deposits of river silt eventually produced the present terrain.
Both Calliham and South Shore have a wide variety of wildlife that inhabits dense thickets of mesquite and blackbush acacia. Choke Canyon is the site of the westernmost common occurrence of the American alligator. Rio Grande turkey, white-tailed deer, javelina, coyote, opossum, fox squirrel, raccoon and various skunks are among the most common animals. The crested caracara (Mexican eagle) can also be seen in the area. The following fish are in the reservoir: largemouth bass, white bass, striped bass, white crappie, bluegill, longear sunfish, green sunfish, flathead, channel and blue catfish, carp, freshwater drum and gar.
As part of a joint project of the Bureau of Reclamation and the American Birding Association, Choke Canyon Reservoir has been recognized as a place of special importance for birds and birdwatchers. Large numbers and varieties of birds are attracted to the water and to the adjacent upland habitats. Also, many typically Mexican species of birds approach the northern limits of their range here, making this one of Texas' finest places to watch birds.