26 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



(Plate 3.) The whole valley occupied at the Boundary by the Kootenay 

 river is the easternmost and much the longest. It is a part of a single 

 Cordilleran feature easily the most useful in delimiting the Canadian 

 ranges. From Flathead lake to the Liard river, a distance of about 800 miles, 

 this feature has the form of a narrow, wonderfully straight depression lying 

 between the Rocky Mountain system and all the rest of the Cordillera. Unique 

 among all the mountain-features of the globe for its remarkable persistence, 

 this depression is in turn occupied by the headwaters of the Flathead, Kootenay, 

 Columbia, Canoe, Fraser, Parsnip, Finlay, and Kachika rivers, and is therefore 

 not fairly to be called a valley. It may for present purposes be referred to as 

 the ' Rocky Mountain Trench.' The term ' trench ' throughout this report 

 means a long, narrow, intermontane depression occupied by two or more streams 

 (whether expanded into lakes or not) alternately draining the depression in 

 opposite directions. An analogy is found in a military trench run through a 

 hilly country. (See Plate 4.) 



The first-rank valley next in order on the west is also occupied at the 

 Boundary by the Kootenay river, returning into Canada from its great bend 

 at Jennings, Montana. This valley begins on the south near Bonner's Ferry, 

 Tdaho, and is continued north of Kootenay lake by the valley of the Duncan 

 river. Recently, Wheeler has shown that the singular 40-mile trough occupied 

 by Beaver river, which enters the Columbia river at the Canadian Pacific rail- 

 way, is precisely en axe with the Duncan river valley.* The whole string of 

 valleys from Bonner's Ferry to the mouth of the Beaver, a distance of approxi- 

 mately 200 miles, forms a topographic unit that may be called the 'Purcell 

 Trench.' (Plate 5.) 



The third of the first-rank valleys is drained southward by the Columbia 

 river, expanded upstream to form the long Arrow lakes. At its northern extrem- 

 ity near the Fifty-second Parallel of latitude, this valley is confluent with the 

 Rocky Mountain Trench. The southern termination of the valley regarded as a 

 primary limit for these mountain ranges occurs about sixty miles south of the 

 Forty-ninth Parallel, where the Columbia enters the vast lava plain of Wash- 

 ington. To distinguish this orographic part of the whole Columbia valley 

 between the points just defined, it may be called the ' Selkirk Valley.' 



A glance at the map will show that the two primary trenches and the 

 Selkirk Valley are in simple mnemonic relation to three principal mountain 

 divisions of the Cordillera. They lie respectively to the westward of the Rocky 

 Mountain system, the Purcell range, and the Selkirk system. 



The fourth of the first-rank valleys carries the south-flowing Okanagan 

 river, with its various upstream expansions, including Osoyoos and Okanagan 

 lakes. The latter lies wholly within the belt of Interior Plateaus, a primary 

 Cordilleran division. Important as Okanagan lake is, no one has yet suggested 

 that the plateau belt itself be subdivided into named portions separated by the 

 lake. It appears, on the other hand, wiser to recognize in the nomenclature 



* A. O. Wheeler, The Selkirk Range, Gov't Printing Bureau, Ottawa, map in Vol. 

 •2, 1905. 



