Stage-discharge relations were developed for the Brandon Road Dam on the Des Plaines River and the Dresden Island, Marseilles, Starved Rock, Peoria, and La Grange Dams on the Illinois River. A t Brandon Road Dam, streamflow is regulated by the operation of tainter gates and headgates. Tainter gates are operated to regulate streamflow at the Dresden Island, Marseilles, and Starved Rock Dams. Peoria Dam and La Grange Dam comprise timber-Chanoine wickets, which are lowered to a horizontal position on the streambed when not used for streamflow regulation. Both dams have concrete abutments housing butterfly valves that are also used for regulation.

Discharge coefficients, in equations that express discharge as a function of tailwater depth, headwater depth, and vertical height of gate opening, were determined for conditions of free-orifice, submerged-orifice, and free-weir flow under a tainter gate. A free-orifice flow coefficient was determined for the headgates at Brandon Road Dam. Stage-discharge relations for flow over sections of lowered wickets and flow between raised wickets had been developed from hydraulic model studies by the U.S. Army Corps of Engineers in 1937 and 1938. These relations were verified by discharge measurements at V.S. Geological Survey stream-gaging stations on the Illinois River near the Peoria and La Grange Dams. Discharge coefficients in equations of free-orifice and submerged-orifice flow through a butterfly valve were computed from additional measurements at the stream-gaging stations.

A total of 50 measurements of discharge that ranged from 1,730 to 86,400 cubic feet per second were used to develop stage-discharge relations at the six dams. The computed relations compared favorably with published hydraulic design criteria. The stage-discharge relations derived from tainter gate control are applicable to all conditions except extremely high streamflow conditions when submerged weir flow exists. The stage-discharge relations for wicket control are applicable to flow over no more than 20 lowered wickets. The stage-discharge relations for butterfly valve and head ate control are applicable to all streamflow conditions.