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their course several of the types described, and may be called compound.

The topography of each stream valley varies throughout its course with the structure of the country upon which it is established. Hence the members of a series of long, parallel streams flowing across different belts of country locally resemble one another in each belt.

The run-off in the streamways in the Texas region is of three kinds-intermittent, interrupted, and continuous. Intermittent run-off is sporadic in character, occurs only after rainfall, and soon ceases. Most of the drainageways of the western half of the State are of this character. Drainways of this type are termed arroyos. Interrupted drainage is that in which the continuity of the permanent flowing stream is broken by alternate stretches of dry streamway. Rivers of this character abound in the East-Central and Central provinces and are indicated by alternations of continuous lines and dots. Continuous streams are those which flow continuously from the head of permanent water to their mouths.

While some of the larger streams flow across all the different types of country and are prior thereto, each of the greater provinces we have mentioned has a distinct group of local rivers, forming a drainage system which finds outlet directly to the sea or is gathered into the larger lateral trunks. The different members of each system possess similar characteristics of origin, slope, length, and valley topography.

When a streamway gathers its drainage from one province it is of a provincial type. On the other hand, when it flows through two or more provinces, diverting a number of local drainage systems, it becomes composite. The rivers of the Coast Prairie system are of the simple type; those of the Central Province are composite. Geographically the drainage of the Greater Texas region as a whole may be classified generically as follows: Rivers of the Cordilleras, rivers of the Great Plains, rivers of the Central Province, rivers of the East-Central Province, rivers of the Edwards Plateau, and rivers of the Coastal Plain.

Rivers of the Cordilleras.-The rivers of the Cordilleras are the Arkansas, Canadian, Pecos, and Rio Grande. They receive most of their volume from the precipitation on the Colorado group of the Rocky Mountains, and gather little or no drainage from the provinces of the Texas region as they cross it. They are normally flooded in May and June, at the time the snow melts in the mountains at their headwaters.

Rivers of the Great Plains.-There are no true rivers of the Great Plains in the general region under discussion. The few faint drainageways