TOPOGRAPHY 37 



The Missouri and some of its lesser affluents drain the 

 whole of the eastern slope of the Divide in Montana, from the 

 Milk River in the north to the ultimate sources of the Madison, 

 over 500 miles. The Jefferson, the largest of these mountain 

 tributaries, is formed by the Big Hole River and the Beaverhead 

 and carries the waters from 250 miles of the Divide) which 

 describes a wide and deep embayment, partly known as "The 

 Big Hole" and occupying the southwestern corner of the State. 

 The Jefferson, the Madison and the Gallatin unite at Three 

 Forks to form the Missouri and from this point on to the mouth 

 of the Dearborn there are no streams of considerable size. From 

 the south the Missouri receives the Musselshell which drains the 

 Crazy and the Snowy ranges, the highlands of the central part 

 of the State. Near the eastern boundary the Yellowstone River 

 enters but its waters have their sources mainly outside of the 

 State, in the Yellowstone National Park and in northern Wyo- 

 ming, although some are supplied by the Absaroka and the Bear- 

 tooth Ranges in the southern part of Montana. 



On the western slope, from the northern boundary to the 

 head of the Bitter Root, somewhat over 300 miles, the waters 

 are gathered by the North, 'the Middle and the South Forks of 

 the Flathead, by the Blackfoot, the Bitter Root and the upper 

 source of the Clark's Fork, commonly known as the Hellgate, all 

 uniting to form the Clark's Fork which drains the whole of 

 western Montana beyond the Divide. This stream finds its out- 

 let through the narrow valley between the Coeur d'Alene and 

 the Cabinet Ranges and finally empties into Lake Pend d'Oreille. 



The main sources of these rivers lie ultimately in the winter's 

 snows. During the long winters these accumulate, more on the 

 western than on the eastern slopes, and as the warmth of summer 

 dissolves them their waters run off in torrents or, percolating 

 through the soil, feed the numberless springs and sources of the 

 small streams. In February the waters are low, but in June the 

 streams are at flood and their .volume is increased as much as 

 70 fold as measured in second feet. Similarly, on the Atlantic 

 drainage the streams are swollen by the melting snows but the 

 minimum may come in March or in August depending on the 

 location of the stream sources, and the maximum may occur ac- 

 cordingly anywhere from March to June. Measurements of flow 



