104 BOTANICAL GAZETTE [FEBRUARY 
Lake; its principal inlet, Flathead River; and its outlet, the Pend — 
d’Oreille River. Flathead River is formed by the confluence of L 
three branches known as the North Fork, South Fork, and Middle 
Fork, all of which rise in the Rocky Mountains. These flow for 
their entire course in valleys in the mountains and unite to form the : 
main river near Columbia Falls. The Stillwater and Whitefish 
Rivers are the principal branches that lie in the Flathead valley. 
For about the lower half of its course through the valley, the Flat- 
head River is a broad, sluggish, navigable stream that is constantly 
depositing sediment. Especially is this the case at its mouth, where 
a delta about 3*™ in length has been formed (fig. 4). 
Flathead Lake (jig. 5) is some 40*™ from north to south and 
varies in width from 10 to 20*™, its altitude being 890™. It is a 
remnant of a former lake of much greater extent, which probably 
covered a large portion of the valley at its head. An old outlet 
near Dayton (jig. 3), some 120™ above the present level of the lake, 
and terraces at approximately the same height, indicate the former 
distribution of the waters of the lake. The Pend d’Oreille River 
(jigs. 5, 6), the present outlet, has cut its way through a huge moraine 
at the foot of the lake. This moraine no doubt acted as the dam 
that backed up the water over the low valley lying to the north. 
The river rapidly cut its way through this moraine until its present 
condition was developed. This erosive process is going on much 
more slowly today because the river channel has reached bed rock. 
The Mission Range lies to the east of the lake and the Mission 
valley. This is a distinct range separated from the other mountains 
on the east by the Swan River valley. It has an altitude of approxi- 
mately 2750™ at its southern end. From this altitude the mountains 
become gradually lower until near the north end of the lake, where | 
they merge imperceptibly into the valley. The Swan River valley | 
opens into the Flathead valley where these mountains end (map and : 
jig. 1). It is in reality only an arm of the latter, and during the time — 
of the greatest extension of Flathead Lake the water probably backed — 
up into this valley and formed an embayment. As_ the Pend 3 
d’Oreille River cut its way through the moraine at the foot of Flat- — 
head Lake, the level of the water in this embayment was lowered. | 
But some time before the present condition was reached, a moraine 
