HNA Summary Report
The Role of Disturbance in the UMRS Ecosystem

Rock wing dams, closing dams, and bank revetments are used to maintain the navigation channel and to reduce dredging requirements. These structures decrease bank erosion and force flow into the main river channel. In the Open River reach, channel training structures have greatly reduced the number and quality of secondary channels (Fig. 5). There has also been loss of channel area as sediment filled the area between wing dams.

Much of the floodplain south of Pool 16 on the Mississippi River and on the La Grange and Alton pools on the Illinois River has been isolated by levees. The distribution of levees as proportion of total floodplain area is about:

• 3 percent north of Pool 13;
• 50 percent from Pool 14 through Pool 26;
• 80 percent in the Open River; and
• 60 percent of the lower 160 miles of the Illinois River.

In total, more than 1.1 million acres, mostly agricultural land, are protected from moderate floods by levees.

Logging has caused significant habitat degradation throughout the river floodplains and northern parts of the basin. Logging was necessary to supply fuel-wood for steamboats and railroads, firewood for heat and cooking, and lumber to build cities. In most floodplain areas deforested land was rapidly converted to agriculture. The impact is particularly dramatic below the Kaskaskia River where the densely forested floodplain was almost completely cleared (Fig. 6). Deforestation and agricultural conversion throughout the basin increased sediment delivery to the mainstem rivers.

Urban development displaced native habitats, but also caused indirect impacts. Sewage and industrial pollution caused significant water quality problems that eradicated sensitive species downstream of large cities. The problem has subsided since the 1970s.