The Little Missouri River is a prominent feature in all three units of Theodore Roosevelt National Park, and its floodplain is an important feature that contributes to the diversity of habitat, plants, and animals in the park. The sediment-heavy Little Missouri River constantly winds within its banks, slowly changing course over the years. Periodically, the river overflows its banks and onto its floodplain. This periodic flooding has helped trees such as the cottonwood to become established on the floodplain.
The cottonwoods on the floodplain provide valuable habitat for other plants and for animals, too. White-tailed deer prefer the heavily wooded habitat of the cottonwood forests. Numerous species of birds are attracted to the forests in the Little Missouri floodplain because of the diverse and abundant food sources and easy access to water. These large forests are home to year-round feathered residents such as black-capped chickadees, white-breasted nuthatches, and great-horned owls, and provide seasonal homes for nesting Baltimore orioles, waxwings, and a variety of warblers. Porcupines and beavers also use the cottonwood trees themselves for cover and food.
The flat, grassy areas on the floodplains also provide grazing opportunities for the park's numerous grazing mammals, including bison, deer, horses, and elk. Prairie dog towns are also located in some areas on the floodplain.