Boundaries

This ecoregion stretches from eastern New Mexico to southeastern Texas, covering most of central Texas. The ecoregion is defined primarily by the watersheds of Matagorda Bay, as well as the Brazos and Colorado rivers and their numerous tributaries.

Topography

The headwaters of the Colorado and Brazos fall within the Llano Estacado in the southern High Plains in northwestern Texas and New Mexico. This is a large gently sloping mesa with elevations reaching 1500 m. The rivers descend across the Central Lowlands and Coastal Plain formations in their middle and lower portions. To the southwest lies the Edwards Plateau, which exhibits little relief except along the stream valleys of the Colorado and Brazos. The southeastern edge of the Plateau is marked by high relief in the Balcones Escarpment (Connor & Suttkus 1986).

Freshwater habitats

The Edwards Plateau lies in the southwestern portion of this ecoregion, and also covers part of the West Texas Gulf [139] and the Lower Rio Grande/Río Bravo del Norte [135] ecoregions. This is a karst area characterized by the Edwards Aquifer, a body of groundwater that has a distinct biota associated with its caverns and springs. Freshwater and intertidal marshes are along the coast.

Terrestrial habitats

A broad ecotone of eastern pine-hardwood forests is the dominant vegetation in the central and southeastern portion of the ecoregion. The Edwards Plateau is dominated by juniper-oak savanna and mesquite-Acacia savanna (Küchler 1964). Short and mixed grass prairie dominate the upper Brazos and Colorado drainages (Connor & Suttkus 1986). Along the coast tallgrass coastal prairie was once the primary plant community from southeastern Louisiana to the mouth of the Rio Grande River (Smeins et al. 1991). Today, less than one percent of this original community type remains.

Description of endemic fishes

In the East Texas Gulf ecoregion, there are around 100 fish species, of which few are considered endemic. Examples include the burrhead chub (Macrhybopsis marconis) and smalleye shiner (Notropis buccula), in the Brazos River drainage (Connor & Suttkus 1986).

Justification for delineation

Ecoregion boundaries are modified from Abell et al. (2000), which based its units on subregions defined by Maxwell et al. (1995).  Modifications to this ecoregion were made following recommendations from the Endangered Species Committee of the American Fisheries Society. This ecoregion was considered unique because of distinct faunal breaks that result from differences in drainage size and physiographic complexity. There are marked differences in richness as one moves east to west across the Gulf Slope region, as well as distinct species clusters at the 80% level among the Nueces-San Antonio drainages (in [139]), Colorado-Brazos drainages (in [140]), and Galveston-Sabine-Calcasieu drainages (in [141]).