PAPERS ON BIOLOGY AND AGRICULTURE 163 



of Ogle, causing abnormally low water. This is followed by 

 the sudden release of the water, and, consequently, unusual 

 influences of stream flow are at work at times upon the 

 shore Imes of islands and mainland. Island formation has 

 been greatly influenced in places by the building of bridges, 

 as at Oregon, and the breaking of dams, as at Oregon and 

 at Grand Detour. Artificial islands have been created by 

 the gravel and detritus washed from a broken dam against 

 the piers of a bridge, as about the iron bridge at Oregon. 

 The effect of ice, when ice gorges break farther north in 

 the river in February or March and send down the river 

 great quantities of ice, has been marked on shore lines and 

 on island erosion and movement. There has beeen great 

 change in the river islands in the county within the last few 

 decades. The map issued about 1900 by the County Super- 

 intendent of Schools exhibits a number of differences from 

 the recent, still unpublished, map of the State Soil Sui*\'ey, 

 and it is possible today to trace the course of some of these 

 changes. The earlier map shows a rather large island im- 

 mediately north of Margaret Fuller's island above Oregon. 

 No island exists there now, but a very small one farther 

 up stream near the west shore shows evidence of being 

 the remains of the former larger one, since southward from 

 it the remains of stems of dead willows may be seen stick- 

 ing above the surface at low water. In a number of other 

 locations similar effects may be seen. The arrangement of 

 the islands south of the iron bridge at Oregon is strikingly 

 different now from that of the earlier map, and the reasons 

 for this are known in the events of the past fifty years or so. 

 The dynamism of the islands is probably still of a high 

 rate at times so far as their disappearance and appearance 

 is concerned, while that of their progression downstream is 

 generally that usually encountered in a stream of this char- 

 acter. The canalization near the mouth of Pine Creek near 

 the southern boundary- of the county, whereby the lower 

 end of the creek was afforded a shorter course, also pre- 

 sents an artificial rearrangement of natural conditions. It 

 should be noted that much of the industrial development on 

 the Rock assists natural causes in making the river a de- 

 positing as well as an eroding stream. The muddiness of 



