ECOLOGY OF NORTHERN MICHIGAN. 37 



agraded stream is to cut into the sides of its valley, and to build up a plain 

 at its own level by the deposition in its channel of the material derived in 

 this way. It was also pointed out that areas representing different stages 

 in this process are characterized by different biota. Since the process is 

 still going on it is necessary to consider the biota in reference to the changing 

 conditions. The nature of the changes which Carp river produces on the 

 topography is determined by its low gradient and slow current. As it meanders 

 over its flood-plain, the mud flats that are formed on the inner side of the 

 bends are extended into the stream by the deposition of material on their 

 inner margin. The quiet water and favorable substratum pn the sub- 

 merged parts of these flats afford a favorable habitat for the pond or quiet 

 water forms of life. As the deposition of the streams during floods builds 

 these flats above the ordinary level of the river, the conditions become un- 

 favorable for the aquatic forms, which are compelled to migrate outward; 

 at the same time, however, they become favorable for the amphibious forms 

 of the grass and sedge zone that gradually push out and occupy the flat as 

 it becomes dry enough. The continued deposition during floods, and the 

 accumulation of plant remains continue to raise the level of the mud flats, 

 and, as the conditions become dryer, they support successively the cassandra, 

 alder and the deciduous forest types of life. Where the river cuts into the 

 sides of the valley, the flood-plain is destroyed, and its societies are limited 

 to a single narrow zone between the deciduous forest and the river; in this 

 zone, the littoral, cassandra and coniferous societies may be entirely lacking, 

 but the alder society is nearly always represented. 



Carp Lake may be considered as the river expanded and covering its 

 flood-plain to the sides of the valley, and the changes that are going on are 

 very similar to those that are being produced by the river. The conditions 

 however, are more pond-like for the current that the river possessed above 

 the lake is lost, so that there is no lateral corrasion. Deposition is going on 

 all about the margin, owing to wash from the sides of the valley and the de- 

 posits made at the mouth of the river, so that the acpatic conditions are 

 being changed into the terrestrial in a manner analagous to the development 

 of a mud flat' into a flood-plain, and the successive zones referred to in the 

 discussion of the biota are steadily encroaching on the lake. 



Different stages in the destruction of the lake may be observed nearly 

 every where about its shores, but, owing to the dominance of different 

 factors, the encroaching societies are not always equally developed. The 

 extreme of this is shown on the north shore where, owing to the proximity 

 of the cliff, the talus slope dips into the lake, the fresh talus fragments fall 

 nearly to the water and very little soil has accumulated. There is, therefore, 

 no shelving beach at this point, the aquatic and mud flat societies are 

 absent while the alder zone is represented only by a few forms, the Hoary 

 Alder more conspicuously, and is largely mixed with the Mountain Ash, 

 White and Red Pine and Wild Cherry of the talus slope. At the east end 

 of the lake the grass and sedge zone constitutes the principal society'", for, 

 owing to the large amount of material carried by the river, the delta is built 

 up to the level of the lake much faster than it can be raised above it by the 

 accumulation of plant remains and flood deposits, while at the same time 

 owing to the shelving nature of its outer margin, the aquatic zone is also 

 extensively developed. At the west end of the lake, however, the margin 

 is shallow, deposition is slow, and the aquatic societies are the most 

 prominently developed. 



But, notwithstanding differences in the rapidity of the process, the lake 



