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The extensive region under discussion has a diverse drainage, its streams varying in origin, number, length, and volume with the topographic and structural conditions, rainfall, evaporation, slope, and porosity of the surface. Some parts have numerous streamways which drain the surface and lead the water to the sea. Others, like the great bolson deserts of the Trans-Pecos region and the Plateau of the Plains, are practically without surficial drainage, and the surface precipitation is disposed of by evaporation and imbibition.

There is little surficial obstruction to the direct run-off of the rainfall, whereby a constant supply of water to streamway s may be regulated, except such as the forest growth of the Eastern Province, and the absorbent character of the dry soil and rocks in certain local areas-for example, in the Cross Timbers, the Llano Estacado, and the bolson deserts of the west. The Central. Province and the Trans-Pecos Mountains are larger without either vegetal or structural obstruction to run-off, and hence they are rapidly drained after rainfall.

Some streamways, like those draining the Coast Prairie, are of a simple consequent type; they rise upon a slightly tilted plain and occupy one geographic province. Others of a similar character traverse two or more provinces, being prior relative to the one and consequent relative to the other. Still others are of a more complicated nature and origin and traverse all the provinces. (See Pl. VII.)

In the eastern portion of the State, owing to the large rainfall, streamways are numerous and continuously carry flowing water, while small streams which enter this region from the west become large within it through locally acquired laterals. Only in this and the Coastal Plain is navigation practicable, rapidity of fall and scarcity of water prohibiting it to the west.

In parts of the State, such as the summit of the Llano Estacado and the great bolson deserts of the Trans-Pecos Province, drainage channels of local origin are few in proportion to area, feebly etched, and normally without water except for a few hours after heavy rainfall. These areas consist of extensive plains of gentle slope underlain by pervious substructure, and the streamways are the products of the torrential rainfall, which is sporadic and less in quantity than the average rate of evaporation. In such runways as are developed water is normally absent, being intermittent in occurrence and consisting of occasional floods which temporarily (at rare intervals) occupy them. Furthermore, the temporary run-off is seldom of sufficient quantity to endure more than a few miles, owing to loss by imbibition in its normally