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This case study paper presents results on the application of the HEC-6 model to the main stem of the Kankakee River in Illinois, a distance of about 39.3 km. Modeling was performed to develop comprehensive plans for enhancing the aquatic habitats and also to forecast future sedimentation problems if specific management practices are implemented. The paper concentrates on the modeling aspects of this research. The extent of the model was from the Stateline Bridge to Kankakee Dam in Kankakee. The HEC-6 model, originally developed by the Hydrologic Engineering Center (HEC) of the U.S. Army Corps of Engineers, was adapted for this application. The model was run, calibrated, and verified for both the hydraulic and sediment components. The hydraulic component was calibrated through comparison of measured yearly hydrographs with computed values for three gauging stations on the river. The hydrologic component was verified for the same three gauging stations for two yearly hydrographs for 2 additional water years. The sediment component was calibrated with river cross-sectional data collected by the Illinois State Water Survey in 1980 and 1999. The calibrated and verified hydraulic and calibrated sediment components then were used to predict future changes in water surface elevations and thalweg elevations for a 20-year period beginning in 1999, the last date for which river cross-sectional data are available.

Water and suspended sediment inflows, transport, volume changes, and outflows determine the environment for aquatic plants and animals in a given reach of a river. Lock and Dam 19 at Keokuk, Iowa is the oldest and highest of the 27 lock and dam structures that maintain adequate depths for navigation by 2.75-m draft barge tows on the Mississippi River between St. Louis, Missouri, and Minneapolis, Minnesota. Pool 19 is used as an example for various measures of water and sediment transport and variability. The 104-year water discharge record at Keokuk, Iowa is used to demonstrate the average conditions and variability of water discharge. Major tributaries to Pool 19 illustrate the importance of local inflows even when they appear insignificant compared to the Mississippi flows. Flow duration and the annual cycle of high and low flow periods are presented. Sediment budgets are developed from suspended sediment measurements. Sedimentation survey data are used for developing historical sedimentation rates and determining the loss of volume in the pool. Combining the sedimentation survey and the suspended sediment budget analysis yields a present volume equal to 45% of the original. The flood of 1973 and the drought of 1977 are used to illustrate the impact of extreme events on the riverine ecosystem. The 1973 flood set the record for daily discharge and ranks second in annual volume. The 1977 water year ranks third lowest in annual volume and had no spring flood. The drought had a more persistent impact on the ecosystem than the flood. Though Lock and Dam 19 is the only one with crest gates and a hydropower plant, Pool 19 has water discharge and sediment transport characteristics that are typical of the Upper Mississippi River navigation pools.

The Illinois River valley has experienced a tremendous amount of sediment deposition in recent years. Many of the 53 or so backwater lakes along this major river have lost 30 to 100% of their capacity to sediment deposition. Peoria Lake, a bottomland lake, has lost 68% of its 1903 capacity, and upper Peoria Lake will eventually attain the appearance of an incised river with broad and shallow wetlands on both sides. On the average about 18.7 million metric tons of sediment is deposited annually over the entire river valley, with a deposition rate of 20.5–53.3 mm yr −1 . Recently implemented nonpoint source pollution control measures are showing their impacts on the receiving bodies of water through substantially lower concentrations of trace elements within the recently deposited sediment.

In the early 1980s, a number of research projects were conducted to evaluate the potential impact on the Illinois Waterway of proposed increased diversions from the currently authorized rate of 3,200 cfs (90.6 m³/s) to 6,600 cfs (186.9 m³/s) and 10,000 cfs (283.2 m³/s). This article summarizes some of this research. The following effects of diversion are identified: (a) Increased scour and deposition of sediment in the upper reaches of the waterway; (b) a probable increase in backwater sedimentation rates. In addition, no significant changes in sediment transport in the lower reaches except those associated with naturally increased inflow (however, the effects on resuspension, scouring, and deposition of sediments in the lower reaches are not known) were identified. No significant increase in bank erosion rates except within the constricted reaches while increased flood damage in the Alton pool compared to the upper reaches were noticed. Ground-water levels increased near the Illinois Waterway, making higher drawdown available for pumping. Pumpage costs increased for the drainage districts, ranging from 2–50% or more. Water quality improved in the upper reaches but no significant alteration of the osmotic balances were seen. Finally, there were adverse effects on vegetation, including bottomland species of trees, on some species of invertebrates, on feeding grounds of migratory individuals, and on algal blooms (because of the availability of clear water).

Upstream of St. Louis, Missouri, navigation on the Upper Mississippi River is made possible by a series of lock and dam structures. Many of the pools formed by these navigation dams have nearly reached a new equilibrium condition for scour and deposition of sediment. Several pools with extensive backwater or channel border areas are still accumulating sediment at rates similar to those for man-made lakes. The original open-water habitats in these pools are changing to aquatic macrophyte beds and then to marsh or terrestrial floodplain conditions because of sediment deposition.