The pinion-juniper forest offers wildlife viewing of many bird species including pinion jay and roadrunners as well as deer, elk, and a variety of smaller animals.
Pinion-juniper woodland and short and mixed grass prairies occupy park uplands. The pinion-juniper woodland community occurs on rocky outcrops and slopes over one-third of the park. Pinion pine, one-seed juniper and Rocky Mountain juniper dominate the overstory, while scrubland species such as Gambel oak, mountain mahogany and serviceberry dominate the understory.
Mixed grass prairie is dominated by western wheatgrass, needle and thread grass and Indian ricegrass. Shortgrass prairie is dominated by blue grama, galleta and side oats grama. Saline bottomland scrubland, consisting of rabbitbrush, four-wing saltbush and alkali sacaton grows in areas disturbed in the past by ground work and earthmoving activities.
Diverse wetland, riparian, and aquatic plant communities have become established around the reservoir, in the Purgatoire River Valley above the reservoir and below the dam. Riparian scrubland is dominated by sandbar willow and typically grows adjacent to cottonwood forests.
Believed to be one of the best examples in the world; the park is home to the K-T or K-Pg Boundary marking the boundary between the Mesozoic and Cenozoic geologic eras.
The Purgatoire River cuts through Upper Cretaceous Trinidad Sandstone, which caps the valley rim, and Pierre Shale, which is exposed throughout the park. This combination of buff-colored sandstone and dark gray siltstone and shale provides a subtle backdrop for the lake and river valley.