Impact of Human Activities to Hydrologic Alterations on the Illinois River

Abstract

The Illinois River is a tributary of the Mississippi River that connects Lake Michigan and the Mississippi River. Starting in 1848 when the Illinois and Michigan Canal began to open, the Illinois River has experienced some major human activities such as the Lake Michigan flow diversion, creation of levee and drainage districts on floodplains, and construction of locks and dams on the river. This paper uses Pettitt-Mann-Whitney change-point statistical analysis to identify the hydrologic change points caused by human activities and to quantify hydrologic alterations in the system. Observed stage data from 12 U.S. Army Corps of Engineers gauges and observed flows from three U.S. Geological Survey gauges were used to analyze human effects on hydrologic and hydraulic conditions in the Illinois River. The year 1938 was identified as the change point for low flows and low stages and 1972 as the change point for high flows and high stages. The low flow and stage condition changes were due to a combination of added flow from Lake Michigan, levee and drainage district construction, and construction of locks and dams, whereas the high flow and stage condition changes were due to hydroclimatic change within the Illinois River basin. Analyses based on the Indicators of Hydrologic Alteration (IHA) have shown that the magnitudes, frequency, duration, and number of reversals during low flood conditions were greatly modified by: (1) the construction of locks and dams on the Illinois River that were completed in 1938, (2) the reduction of flow diversion from Lake Michigan, and (3) the hydroclimatic condition change around 1972. The latter change probably contributed to the loss of both soil-moist plants and submerged aquatic plants that once provided several important ecosystem services in the system. The analyses described in this paper, coupled with hydraulic and ecological models, can help with site selection and management plans for the ecosystem restoration of floodplains in regulated rivers.