66 
Abandoned channels, either of the main streams or their tributaries f 
remain as sloughs separated by low levee ridges much like those in delta 
plains. More detailed accounts of such phenomena in specific localities 
will be given in connexion with the vegetation. 
It is evident from this brief treatment that the lowland districts present 
endless changes in the form of the land surfaces, involving the deposit of 
vast areas of new land in the deltas, local flood-plains, and on islands in 
the main streams. As these new lands come into being they are exposed for 
the acquisition of a cover of vegetation which is selected from similar 
habitats in the surroundings. The process has been going on since the whole 
area was exposed at the recession of the former lake, and by a study of 
the present program of development it is possible to gain some insight into 
the earlier stages. 
General Trend in the Development of the Vegetation 
Succession of vegetation types in the lowlands is comparatively simple 
in outline. On new land formed in delta plains the early stages are sub- 
merged or partly floating aquatic plants rooted in shallow mud flats. These 
are followed immediately by a marsh vegetation dominated by sedges and 
various grasses. On mud and sand-bars in the main streams aquatic plants 
are prevented by the current, and the first land plant to appear is usually 
some species of horsetail Equisetum. The next notable change in the suc- 
cession is the growth of various species of willows, closely followed by an 
admixture of alders Alnus incana. The balsam or black poplar Populus 
taeamahacca is the first tree to appear, and gradually gives way to spruce 
Picea glauca . The spruce forest appears, as it does on the upland, to be 
the most advanced stage attainable, and perpetuates itself if not altered by 
clearing or fire. The general outline may be indicated as follows: 
Marsh and meadow vegetation of aquatics, v 
sedges and grasses \ Salix 
Mud and sand-bar vegetation of Equ'setum / 
Salix 
Populus 
Ainu e 
tacamahac:a 
Picea 
glauca 
There are many minor stages interpolated into this outline, differing 
according to the mode of origin of the succession, but the general arrange- 
ment is almost universal for the area under discussion. The convergence 
of all successions to the spruce forest indicates that eventually this forest 
may cover the whole region, provided no major changes in climate or topog- 
raphy occur. Furthermore, most of the variations in the major trend are 
to be found in those stages that precede the willows {See diagram of suc- 
cessions, Figure 15) and are nearer to the differing physiographic origins of 
the series. 
An outline for the more or less detailed description of the vegetation 
of the lowlands may be derived from the above plan of its development. 
The first division will have to do with the delta plains, the second with local 
river deposits, and the third with those phases of the first two that are con- 
cerned with sloughs and wet meadows formed in abandoned channels, and 
warrant more elaboration due to their diversity. 
