THE CHANGING FACE OF NATURE 15 



Mississippi discharges annually into the Gulf of Mexico as much 

 debris worn away from the land in its course as would a thousand 

 cargo ships each carrying ten thousand tons. Dip a pailful of 

 water from the Des Plaines at Riverside when the spring freshet 

 is on or from the Illinois at Starved Rock; let the suspended 

 mud settle, collect it, dry it, and then weigh it, and you will be 

 surprised to see how much soil one cubic foot of water is carrying. 

 Measure the width of the stream, its average depth, and its 

 rate of flow by getting the distance a floating chip goes in Ave 

 or ten minutes; calculate from this data the volume of water 

 that passes in an hour's time, then the weight of sediment it 

 carries, and it foots up an incredible amount. All the streams 

 that flow into Lake Michigan are ceaselessly filling in the lake. 

 The wave action along its shores tends to make it wider, but also 

 shallower since the eroded material is carried out to settle in the 

 quieter, deeper portions. The constant necessity of dredging out 

 harbors and their approaches is readily understood when one 

 appreciates this incessant deposition. 



The ultimate fate of a lake or pond is to be transformed into 

 land as it is filled up by the material washed into it from the 

 surrounding hills and by accumulating plant debris (Fig. 15)^ 

 Becoming shallower, water weeds and rushes grow farther and 

 farther out in it until it transforms into a marsh. Even then the 

 filling does not cease, for each year's crop of marsh grass and 

 water weeds piles up and, only partly decomposing, aft'ords the 

 following year a slightly higher footing for the next crop. Wolf 

 Lake and Lake Calumet are both shallow and filling rapidly. 

 Already many of their bays are marsh rather than pond areas. 

 Indeed, along the margins of these lakes it is very easy to trace 

 all stages from pond to prairie land. 



The river builds land instead of eroding it wherever its 

 current is checked. The carrying power of a stream depends 

 on its rate of flow and its volume. The swift current can keep 

 coarse material moving that is deposited when the movement is 

 sluggish. When the stream flows into pond or lake, its rate of 



