THE NATURAL DRAINAGE OF
The navigable rivers had a very real importance in the exploration of the region, as they were routes of travel by boat and canoe. In the great plains, where many rivers were unnavigable, the traveller was nevertheless obliged to follow the course of the river to obtain water and the cottonwood tree which supplied him with wood fuel. The main expeditions were undertaken by traders, and along the rivers they found not only their prey, the valuable beaver, but also the settlements of the Indians with whom they traded.1
The continental divide between the rivers flowing to the Atlantic and the Pacific Oceans is indicated on Fig. 13. It will be seen that most of the rivers take their rise in the Rocky Mountains and flow across either the Plains or the Interior Plateaux.
The drainage on the east of the Rocky Mountains can be divided into four systems, the upper Missouri, the Platte and the Kansas, the Arkansas and the Red, and the Rio Grande and Pecos (Fig. 13). The first three systems contain tributaries of the Mississippi, but the Rio Grande and Pecos flow independently into the 6 Gulf of Mexico.
Upper Missouri . The most important of the rivers flowing on the eastern side of the Rockies is the Missouri. This river, rising in the northern mountains, a region of heavy rainfall and snowfall, has a larger supply of water than the streams which rise farther south. The river flows through the dry plains, where the rainfall is less than 20 inches. Evaporation decreases the volume of water but the river does not disappear. The Missouri____________________