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Frio, Nueces, and Devils rivers, which pass from the Plateau Province directly across the Balcones scarp line into the Coastal Plain, are complex, presenting entirely distinct topographic characters in the two provinces. On the plateau summit their ultimate heads are gentle, waterless draws, like those of the Llano Estacado. These lead suddenly down into the deep, wide-bottomed box canyons indenting the margin of the plateau, also similar to those of the eastern border of the Llano Estacado, and at the bottom of which water begins to flow from gravity springs. The permanently flowing water of these canyons makes streams of great beauty. The headwater streams continue oily short distances, and are succeeded by waterless gravel. On passing the Balcones scarp line into the lower-lying country of the Rio Grande embayment the character of the streamways entirely changes. Here the beds are only slightly indented below the general level of the country and have no steep canyon walls. Furthermore, in this portion of their courses some of them, like the Frio and Nueces, except in time of flood, are normally dry gravel beds without water. Lower down in their courses, however, permanent water again appears.

Rivers of the Southern and Eastern provinces.-Within the Southern and Eastern provinces, which collectively make the Coastal Plain, two distinct systems of streams have originated and occupy the relatively higher surfaces between the through-flowing streams. The longest and oldest rivers of these systems rise along the eastern margin of the Black Prairie Subprovince and at the Balcones scarp line; those of the second begin at the western margin of the Coast Prairie, upon which they are established. To the first class belong the Sabine, Sulphur, Neches, and Angelina. The second includes a number of short streams (creeks and bayous) which in their coastal extent are sluggish and brackish. In that portion of the Coastal Plain south of the Colorado there are streams of another category, which have their origin in remarkable fissure springs that break out at the foot of the Balcones escarpment. Among these spring rivers may be mentioned the San Marcos, Comal, San Antonio, Las Moras, and San Pedro.

In résumé it may be said that the rivers of the mountains are complex, composite streams. The rivers of the Coastward Slope plain consist of four distinct systems of consequent streams which have developed during different epochs of geologic history, each recording distinct changes of level, accompanied by a migration of the coast line back and forth.

By provinces it may be said the drainage of the Trans-Pecos Country