The San Antonio River Basin spans five ecoregions:
1. Edwards Plateau
2. Post Oak Savannah (Cross Timbers)
3. Blackland Prairie
4. South Texas Plains (Brush Country)
5. Gulf Prairies and Marshes
These ecoregions are natural regions defined by their individual characteristics of plant and animal communities as well as their geology, hydrology, topography and soils. Some species of plants and animals can only be found in a particular ecoregion, whereas others utilize multiple ecoregions.
The Edwards Plateau ecoregion is located in Central Texas and is often called the Texas Hill Country. Known for its abundant fresh water springs, rocky hills and canyons; it supports a variety of unique plants and animals. The soils are often very thin and overlay limestone with numerous cracks, holes, caverns and caves. Plant communities were historically open grasslands and savannahs with denser shrubs and trees present within canyons and riparian areas. Today, the vegetation consists primarily of juniper-oak woodlands, grasslands, and Plateau live oak or Mesquite savannahs.
The Post Oak Savannah ecoregion is often called the Cross Timbers and is characterized by strips of oak forest areas crossing prairie grasslands. The dominant oak species are Post oak (Quercus stellata) and Blackjack oak (Quercus marilandica). Soils vary from loams to clays and sands.
The Blackland Prairie ecoregion is characterized by deep, black clay soils that historically supported tallgrass prairie plants and animals. Much of the fertile soils have been converted to agriculture or other land uses. Prairie remnants still remain and can be identified by unique plants such as Big bluestem (Andropogon gerardii), Indian grass (Sorghastrum nutans), Texas Gayfeather (Liatris punctate var. mucronata) and Prairie penstemon (Penstemon cobaea).
The South Texas Plains ecoregion is characterized by a mixture of thorny scrub and trees, and is often called the South Texas Brush Country. A variety of thorny plants are found here including Mesquite species (Prosopis spp.), Acacia species (Acaciella, Senegalia and Vachellia spp.) and Prickly pear species (Opuntia spp.). Soils are predominantly loams and clays.
The Gulf Coast and Marshes ecoregion is characterized by flat plains and is composed of both marine and terrestrial habitats including sandy beaches/dunes, estuaries, marshes, prairie/grasslands and wetlands. The San Antonio River provides important fresh water flows to the estuaries along San Antonio Bay which includes the Aransas National Wildlife Refuge which is habitat for many unique wildlife species such as the endangered Whooping Crane (Grus americana) and endangered Attwater’s Prairie Chicken (Tympanuchus cupido attwateri).