Impacts of the Reversal (water levels, water quality, bottom fauna, and fish)

  Water Levels

According to a report by Stephen A. Forbes and Robert E. Richardson entitled, Some Recent Changes in Illinois River Biology (April 1919), the primary causes of change in the biological environment of the Illinois River have occurred since 1899 with the opening of the Sanitary Ship Canal in Chicago. One of these changes has to do with the increased depth and flows throughout the Illinois River valley.

During the 10 years prior to the opening of the canal in January 1900, the average flow of the river at Peoria is 8,391 cubic feet per second (cfs), but the flow of the canal in Chicago alone in 1913 is 7,193 cfs, 85.7% of the Peoria flow of the original river. Forbes and Richardson estimate that approximately half of the average flow at Peoria in 1913 comes from Lake Michigan.

This leads to a dramatic increase in the depth of the river. Water levels at Peoria increase by 5-6 feet and at Havana by 3.6 feet. This in turn, increases the surface areas of bottomland lakes, sloughs, and marshes from 54,000 to 120,000 acres. This flooding of thousands of acres of bottomland forest benefits fish and waterfowl in the short term, but also results in the loss of pin oak and pecan trees which are species sensitive to increased water levels (Havera and Bellrose 2002).

An earlier report by Forbes and Richardson shows the average depth of the Illinois River at Havana as three feet greater in the 10-year period following the opening of the canal than during the 10 years preceding 1900. Alvord and Burdick (1915) compare the gage readings 14 years after the canal opens and 10 years following to show a difference of five feet for Peoria and three feet at Grafton. The water levels increase from the canal to the mouth of the Illinois River at Grafton.

These increased water levels produce a greater expanse and depth of the lakes in the floodplain. These lakes exist at about the level of the river prior to the opening of the canal. Table 2 shows the difference in area for Thompson Lake, located near Havana, for the period 1874 to 1907. The average lake level for 26 years prior to 1900 is 429.46 above sea level, but for eight years following 1900, the average is 433.06 feet, a difference of 3.6 feet more in the summer depth. The expanse of Thompson Lake also increases dramatically from 1,943 acres before 1900 to 5,072 afterward. The area of the lake increases 2.5 times due to the opening of the canal.

Table 2: Average Levels of Thompson Lake, June, July, August, and September, 1874-1907 (Forbes & Richardson 1919)

*“Memphis datum” is 7.3 feet below sea level

There are secondary effects of the increase in depth and expanse of water:

• The permanent loss of the shallow weedy water along the banks and the temporary loss of vegetation along the shoreline of lakes. The river water typically extends to the willows, leaving a broad belt of dead trees and shrubs, particularly from Peoria south. Water quality deteriorates further as the trees fall into the water of the stagnant lakes, fouling the water as they decay.
• The deeper water cannot heat up as rapidly which retards the decomposition of the sewage and plant material in the river.
• The average rate of flow increases, as does the speed of passage from point to point.

Forbes and Richardson’s calculations show that only half the time is needed for water to flow the length of the river than the time needed prior to the opening of the canal - 13 days being sufficient for the distance between Utica and Grafton in August during the period from 1910-1918 compared with an average of 29 days for the same month in the period from 1890-99.

This increased rate of flow also has consequences. Decomposition and assimilation of the sewage now extends further downstream than previously occurred. Plankton carried in the stream also have less time to multiply, and are less likely to be consumed. Forbes and Richardson estimate that the effects to plankton are equal to the river being shortened by one-half. Finally, bacteria entering the river with the sewage in Chicago now are carried further downstream.