216 BOTANICAL GAZETTE [MA RCH 
in these streams, which processes are due principally to alternating 
currents produced by seiche movements (ADAms 4) of the waters 
of LakeSuperior. The currents thus produced are quite considerable. 
At one moment there is a strong outward flow; a few minutes 
later the movement may be just as swift in the opposite direction. 
The effect of the seiche current is practically the same as that of an 
ordinary stream current. It undermines the bank in some places 
and deposits the eroded materials in others. On account of its 
alternating direction the gradational effects in a particular case 
cannot be so simply worked out, but judging from the relative 
positions of the eroding and depositing portions of the banks it 
seems probable, as one would naturally expect, that the outward 
current is the more effective. It is possible that occasional heavy 
rains may considerably increase the volume and velocity of the 
streams. The channels are not ordinarily sunk far into the sedi- 
mentary substratum. Their banks appear largely as_ vertical 
sections of the layer of plant growth which has spread over the 
delta plain. 
III. Vegetational development in the habitat 
The first vegetation upon the delta deposits enters when the 
water over them shallows sufficiently to permit the growth of 
aquatics of the pondweed type. Next come water lilies and rushes, 
and when the sediments accumulate until they reach nearly to the 
surface, sedges gain a foothold and soon form a mat. Up to this 
point the development has followed the same course as the bog 
succession. Important differences now appear. The sedge mat 
does not build out over the water, probably because of wave and 
ice action, since the bodies of water into which the streams flow 
are rather large and open. The sedge mat stage does not last long, 
but is very soon superseded by a dense growth of tall grasses, among 
which Calamagrostis canadensis (Michx.) Beauv. is by far the most 
important. It is this type of plant, growing in dense, closely placed 
stools, that forms the bulk of the stratum of plant remains which 
finally covers the plain. The grasses are followed by shrubs, 
and these by the swamp trees, which finally give way to the climax 
forest. 
