The three types of hazards that typically cause flooding in Rock Island County are:
- Overbank Flooding
- Local Drainage
- Ice Jams
Rock Island County has perpetually been affected by flooding on both the Mississippi and Rock Rivers. With over 5,000 parcels and 41,310 acres identified in the floodplain, flooding affects a significant portion of Rock Island County. Both of these rivers affect Rock Island County differently.
The Mississippi River is the second-longest river system in the United States. Only the Missouri River is longer. The Mississippi flows 2,340 miles (3,766 kilometers) from its source in northwestern Minnesota to its mouth in the Gulf of Mexico. And in Rock Island County, the Mississippi River runs East and West for roughly 40 miles, which can create some overbank flooding.
However flooding on the Mississippi River has been reduced in Illinois urban communities by the construction of levee systems, floodwalls and a lock and dam system by the Corps of Engineers. Some residences have been removed from the hazard area by being elevated. The greatest floods of record on the Mississippi River at Rock Island have occurred in the spring of 1965, 1969, 1973, 1975 and the summer of 1993. In 1993, 507 structures were damaged totaling $4.9 million dollars.
The Rock River is approximately 285 mi (459 km) long originating in southeast Wisconsin and traveling through Wisconsin and Illinois to Rock Island County where it joins the Mississippi River. The Rock River poses the greatest flooding threats to the floodplain residents in Rock Island County. While much of the Mississippi's shoreline is industrial in nature, the Rock River is lined with residential structures. Between 1993 and 2001 the Rock River has experienced severe flooding 6 out of 7 years. With all of the current residential structures and an ongoing trend toward development, the problems are continually changing.
Rock Island County's local drainage problems are primarily due to development and filling in wetlands. Over the years, development pressure along the Rock River has resulted in the filling of wetlands or the reduction in the value of wetlands as useful stormwater retention areas. Wetlands are a natural buffer, acting as an absorption field for stormwater prior to entrance into major waterways and retention areas for floodwaters during times of high waters.
Wetland soils and vegetation provide an effective natural mechanism for the absorption of storm and floodwater. Increased development in the watershed area increases runoff and stormwater, reducing the capabilities and natural functions of the wetlands. Elimination of these wetlands exacerbates these problems, by eliminating totally the natural and beneficial functions of wetlands.
Pieces of floating ice carried with a stream's current can accumulate at any obstruction to the stream flow. These ice jams can develop near river bends, mouths of tributaries, points where the river slope decreases, downstream of dams and upstream of bridges or obstructions. The water held back can cause flooding upstream, and if the obstruction suddenly breaks, flash flooding can then occur downstream as well.
An ice jam can occur anytime from early winter to late spring in Rock Island County, depending upon changes in temperatures that cause alternate freezing and melting of water surfaces. The most likely times are early winter before the surfaces are completely frozen and early spring when the ice cover begins to break up due to melting.
In Rock Island County we have many bridges and many bends in our rivers. Recent bridge construction was designed to help reduce occurrences of ice jams and obstructions of water flow.