Boundaries

Part of the Rio Grande complex, the Río San Juan stretches from southeastern Coahuila through central Nuevo León, and northwestern Tamaulipas to the Rio Grande (Río Bravo) River.

Topography

Headwaters of the Río San Juan originate in the Sierra Madre Oriental (mostly within Área Natural Protegida Cumbres de Monterrey). The topography throughout the lower course is hilly in the piedmont and grades to plains toward the confluence with the Río Grande/Río Bravo.

Freshwater habitats

Freshwater habitats in the ecoregion include springs, mountain creeks, plains rivers, and a few lagoons. There are some semi-isolated springs (such as Apodaca, Mezquital, Infiernillo, Huasteca, and others that have dried out) with strictly Neotropical species. However, most headwaters are Nearctic, with only one or two Neotropical species.

Terrestrial habitats

The ecoregion includes the following terrestrial ecoregions: Sierra Madre Oriental pine-oak forests, Chihuahua desert, Tamaulipan mezquital, and Tamaulipan matorral.

Description of endemic fishes

Like many xeric regions, the Río San Juan exhibits high endemism among its freshwater fauna. The only strict endemic to the ecoregion is Membras vagrans, although the Mexican red shiner (Cyprinella rutila), Salinas chub (Gila modesta), and Monterrey platyfish (Xiphophorus couchianus) are near-endemic. There are no known endemics from the Río Álamo sub-basin, although there is one distinctive form of ghost shiner (Notropis buchanani), as well as the northernmost population of shortfin molly (Poecilia Mexicana).

Justification for delineation

Ecoregion delineations were based on qualitative similarity/dissimilarity assessments of major basins, using the standard administrative hydrographical regions of the Mexican federal government. The delineation of this ecoregion was based on several endemics, as well as a high presence Notropis species.

Level of taxonomic exploration

Excellent for southern tributaries, but poor for northern ones.