Application of AVM Measurements in the Computation of the State of Illinois' Diversion from Lake Michigan

The 1980 Supreme Court Decree limits the State of Illinois' diversion of water from Lake Michigan to 90.6 cubic meters per second (m³/s). The Water Resources Development Act (WRDA) of 1986 gave the Corps of Engineers responsibility for the computation of the State of Illinois' diversion from Lake Michigan. The USGS has been instrumental in providing acoustic velocity meter (AVM) measurements in support of the Corps effort to quantify diversion flows. Currently Illinois' diversion is measured at the USGS AVM on the Chicago Ship and Sanitary Canal (CSSC) in Romeoville, Illinois. The Romeoville AVM has been operational since 1984 and records approximately 95% of Illinois' diversion. The State of Illinois has exceeded its diversion limits. As a result, mediation talks were initiated in 1995 with the other Great Lakes states and Canada. These talks resulted in a new method of computing Illinois' diversion that is being tested during a three-year trial period. This new method, Lakefront Accounting, involves a direct measurement of the diverted components (water supply and direct diversions) and fixing the other diversion components (runoff and consumptive use). In support of this effort, the USGS has installed AVMs to measure the direct diversions (lockage, leakage, navigation makeup flow and discretionary flows) at the three lakefront structures in and around Chicago. Due to the lower velocities at the lakefront sites, the percent errors at these AVMs, to date, have been found to be greater than the error at the Romeoville gage. As the lakefront gages are better understood, the overall error is expected to be less using Lakefront Accounting because the overall discharge passing these three AVMs is considerably less than that passing Romeoville — thus improving the accuracy of Illinois' diversion.