Ill 



In low water stages the flood-plain has less influence upon 

 the quantity of plankton present in the river, for the contribu- 

 tions from the lagoons and marshes are at a minimum at that 

 season, constituting but a small part of the total discharge of 

 the river. On the other hand, as the river falls and the flood- 

 plain emerges the local environments of the relict bodies of 

 water become more pronounced, and local developments of the 

 plankton more varied. The great variety of forms found in the 

 summer plankton is doubtless due in large part to the contri- 

 butions from this diversified environment. 



Four dams have been placed in the river for the purpose of 

 aiding navigation, the State of Illinois building those at Henry 

 and Copperas Creek, and the United States Government subse- 

 quently erecting the other two. The appended table gives their 

 location, date of construction, length of pool, elevation of water, 

 and the estimated increase in the volume of water in the part 

 of the river included in their respective pools. The data in- 

 cluded in the table have been in part compiled from the vari- 

 ous reports of Captain Marshall in the Annual Reports of the 

 Chief of Engineers, U. S. A., for 1890-94, and in part furnished 

 through the courtesy of Mr. G. A. M. Liljencrantz, Assistant 

 Engineer at the Chicago office, and Mr. E. J. Ward, of the Illi- 

 nois and Michigan Canal. 



The effect of the dams upon the volume of water in the 

 river is much greater at low stages than when the river is out 

 of its banks, the increase in volume resulting from them rang- 

 ing from eighty to one hundred and twenty-five per cent. The 

 increase due to the dam at Copperas Creek is less than that from 

 the other dams because of the deep reach of water from Lacon to 

 Chillicothe, and the considerable expanse under normal condi- 

 tions in Peoria Lake, both above the dam in question. As a 

 part of the biological environment the dams are of great im- 

 portance, for they check the current, delay the run-off, especial- 

 ly at low water, facilitate the deposition of silt, and double 

 the volume of water at low-water stages. All of these factors 

 tend to increase the production of plankton and are most 

 effective at low-water stages. 



