63 
Although the differences in the elevation of the plain above the water- 
table are slight, they are enough to determine the arrangement of the plant 
cover. Lands subject to inundation, or at most only a few inches above the 
water-table, have an herbaceous vegetation ranging from semi-floating 
aquatic plants to sedges and grasses. Large areas in the lower deltas have 
nearly pure stands of the meadow sedge Carex trichocarpa var. aristata , 
or blue-joint grass Calamagrostis canadensis. On the margins of stream 
channels, abandoned or otherwise, are long lines of willows Salix spp., which 
are limited to the slightly elevated ridges peculiar to such areas. The far- 
ther toward the margins of the basin the more land is covered by shrub and 
tree growth, so that the upper deltas and the banks of the larger channels 
support a forest of Canada spruce and balsam poplar. Forest growth 
extends farthest into the lowlands along the actual margins of the streams. 
The granite hills have a scrubby timber of Canada spruce Picea glauca, 
jackpine Pinus Banksiana, and canoe 'birch Betula papyrifera var. 
neoalaskana. Not only are the positions of these major types of vegetation 
determined topographically, but also most of the lesser plant associations 
within them. An attempt to account for the phenomena of distribution 
necesitates, therefore, some knowledge of local topographic history. This 
has been treated in a general way in an earlier part of the paper, but will 
now be examined in more detail. 
The last post-Glacial lake stage in Athabaska-Great Slave Lake region, 
wdfich stood above the present levels, had its bottom at about the present 
SOO-foot contour (11). With subsequent change in drainage conditions the 
Athabaska division of this lake was lowered until it was separated from 
Great Slave lake except for Slave river ; but it still covered the Lake Claire- 
Lake Mamawi basin and had long extensions in the valleys of the main 
rivers. It received the heavily loaded waters of the Peace and Athabaska, 
and formed, in this western section of the Athabaska division, a wide settling 
basin for these rivers, serving to stop immense quantities of detritus. The 
large amount of drift timber brought from the mountains by these streams 
has greatly increased the rate of filling, due not only to the volume of the 
timber itself, but also to the part played by logs as lodgment places for 
other materials. The basin has been filled to its present condition largely 
by this process of deposit. 
The Athabaska formerly had its main outlet much farther to the west 
than it does at present, probably to the west of Embarras channel. As 
these earlier channels so filled their mouths as nearly to reach a graded 
condition (Cree [or Mamawi] creek appears to be such a channel) their 
waters were shunted farther to the eastward where they reach the main 
lake by a greater fall in level. The Athabaska seems to be doing the great- 
est amount of building at present at the mouths of the Embarras, Fletcher 
Goose Island, and Big Point channels. The southeast shores of lake Claire, 
the land between lakes Claire and Mamawi as far north as Hay (Prairie) 
river, and the south shore of lake Mamawi must have been formed in this 
way. The Peace, a larger stream, must have sent a large part of its waters, 
even from the early history of this lake, into Slave river, but it also had 
a series of channels that deposited the present north and northeast shores 
of lake Claire. The deposits from these streams meet those of the Atha- 
baska in Hay River area and in the delta of the Quatre Fourches. The 
91963 — 5 * 
