prindle.]' GEOGRAPHIC DESCTPTION. 1 '.) 
the stream and in places as far as a mile back from it arc conspic- 
uously benched. The bench can be traced all along this portion of 
the river and is the floor of an older valley formed by the stream and 
occupied by it at a period before the present canyon was commenced. 
This feature is well shown on the map of the Fortymile quadrangle 
(PI. VII). 
The Fortymile. which varies in width from 250 to 300 feet or more, 
carries a considerable volume of water, and often flow T s in long 
reaches of quiet water, separated by ritHes caused by outcrops of bed 
rock across the stream. Navigation is rather difficult, but freight is 
carried in small boats as far as Chicken Creek on South Fork. At 
low water the stream is easily forded by pack animals. 
North Fork, which drains a large area to the west and has numer- 
ous tributaries from the north toward Glacier Mountain, has sunk 
itself in a canyon like that of Fortymile, and Hows in an easterly 
direction to the main river. South Fork, on the other hand, flows in 
i general northerly direction in a canyon which becomes more open 
oward the south and bends abruptly to the west about 15 miles above 
he forks; 5 miles beyond, it divides into Denison and Mosquito forks, 
hich drain a large area to the south and west. Because of its 
)lacers. Chicken Creek is an important tributary of the latter. 
Valker Fork heads on the Canadian side, flows in an irregular west- 
rly direction, and joins South Fork about \'2 miles south of the main 
orks; it forms the southern boundary of an area containing several 
eeks of economic importance. Fortymile, between the forks and the 
mndarv. receives from the south several small tributaries, of which 
Canyon Creek, about 5 miles Avest of the boundary, is the most im- 
ortant. A larger number of creeks are tributary from the north, 
Lit most of them are small. O'Brien Creek, however, gathers the 
carnage from an area extending 25 miles to the north, where its most 
Lstant sources are opposite those of American ('reek'. 
Mission Creek system. — -The Mission Creek system drains the area 
tween Glacier Mountain and Eagle. The headwaters of its many 
ibutaries are cutting into the plateau surface far more vigorously 
an are those of the Fortymile system, which flow in less accentu- 
ed valleys on the southern side of the divide with the decreased 
wer due to greater distance from the major stream. For the 
eater portion of their courses Mission Creek and its tributaries (low 
t >.l a northeasterly direction. At the point where Excelsior Creek 
ilins it the main stream turns nearly at right angles to its former 
ection, and flows southeast for a distance of about 8 miles to the 
IJikon. The valley through this part of its course is unsymmetrical, 
ng bounded on the north by a steep ridge and on the south by 
lope 3 or 4 miles in width, which rises gradually toward the higher 
el. The stream belongs to a smaller class than the Fortymile and is 
