RIVERS. The eleven larger rivers of Texas drain more or less narrow watersheds that are from two to five times longer than their average widths. Consequently these streams have generally developed dendritic drainage systems, a type developed by a stream flowing across or developing upon a broad, gently dipping coastal plain. The average trend of these drainage areas is north by forty-five degrees west, a general direction also parallel with the trend of the southern Rocky Mountains in West Texas. Most of the streams of Texas normally follow the continental slope towards the Gulf of Mexico. Others, like the Pecos River, flow at a considerable angle to this slope of the surface, the course defined by geological structures.
The Colorado, Brazos, and Red rivers, which have their source in the Panhandle of Texas or in New Mexico, all flow across the Permian Red Beds and, accordingly, during periods of considerable runoff bring down a crimson flood. Others like the Pecos and Rio Grande drain areas of extensive bolson deposits and carry waters having a dull gray color. Rivers having their source on the Edwards Plateau carry light-colored sediments during flood stages. Thus the color of the water during flood stage indicates the nature of some of the country through which the stream flows. In the southeast portions of the state such rivers as the Sabine, Neches, Trinity, and Angelina flow through areas covered by the forest of the timber belt and on through the Big Thicket to the Gulf. This general area has an annual rainfall of from forty-five to fifty inches, so that, with the timbered area and the thicket to retard runoff, these streams, in their lower reaches, maintain a fairly steady flow throughout the year.
The longer rivers that have their sources in Texas or eastern New Mexico have an exceedingly large spread between their maximum and minimum discharges of water. This variance is largely due to three factors: (1) the general lack of forests and other plant cover that would inhibit the runoff, (2) the absence of erosion-prevention measures and the prevalence of overgrazing, which contributes to excessive runoff during periods of considerable rainfall, and (3) the considerable amount of rainfall that comes in short periods of time. (Overgrazing and soil erosion are not so prevalent now as formerly.) This variation is typical of the Southwest and difficult to control. It is conducive to high flood stages followed by little or no flow. The shorter rivers, such as the Nueces, Guadalupe, and San Antonio, which rise in the southwestern Edwards Plateau, maintain a much more even flow because they are replenished constantly by springs that owe their existence to the Balcones Fault system.
The streamcourses of the larger rivers flowing southeast through the central part of Texas, the Colorado, Brazos, and Trinityqv, are situated much nearer the northeast margin of their drainage area than the southwest boundary, probably only one-third of the drainage area lying to the northeast of these streams. Thus these streams have their larger tributaries on the west of their watershed. The Concho, San Saba, Llano, and Pedernalesqv all flow into the Colorado from the west; the Pecan Bayouqv is its only important eastern tributary. In like manner the Little, Lampasas, Leon, and Bosque rivers join the Brazos from the southwest, the Navasotaqv being the only principal tributary flowing into it from a narrow watershed on the northeast. The Trinity River is also fed mainly from the west.
The headwaters of the Rio Grande, the Pecos, the Canadianqv, and the Red all rise in northern New Mexico or southern Colorado. The Canadian flows almost south for about 190 miles and then flows east and slightly north across the Texas Panhandle. The Rio Grande flows slightly southwest from its source for approximately 325 miles and then assumes a direction of south by forty-five degrees east along the southwestern boundary of Texas. The Red River has its source in New Mexico very close to the Canadian River and flows in the general direction of south by seventy-seven degrees east along the northern boundary of the state. Thus, the upper portions of these rivers form a muleshoe-shaped area, closed to the northwest, which surrounds and confines the other rivers of Texas and prevents additional development of their watersheds.
The valleys and floodplains of most Texas rivers are much larger and wider than the present average runoff would suggest, the Brazos bottom being an example. It is possible that during Pleistocene times, when the ice front stood near St. Louis and Kansas City and many valley glaciers were hanging on the east slopes of the southern Rocky Mountains, cold banks of air blanketed these areas, and, as the warmer southern breezes drifted north, precipitation far in excess of anything now known fell, giving the Texas rivers many times their present runoff and developing the wide floodplains so characteristic of them.