Side-by-side images of the confluence of the Illinois, Missouri, and Mississippi Rivers with and without flooding (right and left, respectively).

American Bottoms: Hydrogeology

A summary of the hydrogeology of the American Bottoms Water Supply Planning Region in Illinois.

Allan E. Jones, Michael P. Krasowski, Esra Gulsen

The hydrogeology of the American Bottoms region is controlled and shaped by the Mississippi River. The banner image above shows satellite images of conditions before (left - 5 June 2018) and after (right - 7 May 2019) major flooding on the Mississippi River in 2019.

Mississippi River

The Mississippi River watershed is one of the largest in the world, and the largest on the North American continent. The watershed hydraulically connects the upper Midwest of the United States with the Gulf of Mexico and covers almost all of Illinois. As such, it is the region's most important hydrogeologic feature and provides significant agricultural, economic, and recreational benefits. Healthy river levels not only help to maintain those benefits, but also provide vital transportation to support the region's economy.

Graph of streamflow (in cubic feet per second) of the Mississippi River at St. Louis, Missouri over the last seven days. Present streamflow conditions are shown by the blue line, while the median daily discharge over the 90 years of avilable record is displayed by the yellow triangles. Data were acquired from the USGS at monitoring site 07010000.

The Mississippi River is the most important source of water for the American Bottoms region. Evidence shows that exchange between the river and the adjacent groundwater aquifer is a highly important and prominent regional process (Schicht & Jones, 1962). Along the river’s eastern edge, steep levees impede Mississippi River floodwaters from inundating the American Bottoms during high stages. These high stages may also influence groundwater levels near the river, potentially promoting recharge to the aquifer.

Graph of Mississippi River gage height (feet) at St. Louis, Missouri over the last seven days. The data source for this graph is USGS monitoring location 07010000.

The amount and quality of water in the Mississippi River is important since several local communities use the Mississippi River for their municipal water supply (e.g., East St. Louis and Sauget, IL).

Mississippi River with the St.Louis Arch in the background. (Photo credit: George Roadcap)

Mississippi Alluvial Aquifer

The American Bottoms Aquifer is a surficial alluvial aquifer that resides along (and is directly connected to) the Mississippi River in southwestern Illinois directly east of St. Louis, MO. Composed of sand, clay, and gravel sediments predominantly deposited by the ancient Mississippi River, the American Bottoms Aquifer is highly transmissive and was noted for itsexceptional water quality and quantity around the turn of the 20 th  century. As such, throughout the first half of the 20 th  century, groundwater withdrawals for industrial use steadily increased to peak near 110 million gallons per day (mgd) during the 1950s and 1960s (Schicht & Buck, 1995).

Heavy industrial activities have led to considerable contamination of groundwater resources in the region. The region houses multiple EPA Comprehensive Environmental Response, Compensation and Liability Act (CERCLA; commonly known as "Superfund") sites and several Resource Conservation and Recovery Act (RCRA) sites, along with other Illinois and U.S. Environmental Protection Agency monitored sites (United States Environmental Protection Agency, 2020a, 2020b).

The interactive map above displays the surficial geology of the American Bottoms Water Supply Planning Region. The region's alluvial aquifer borders the Mississippi River along the region's western boundary and is primarily composed of the sandy and clayey facies of the Cahokia Formation.

Surficial Geology

The formation boundaries are generalized in areas where the bedrock is covered by surficial deposits of clay, silt, sand, and gravel. The most prominent surficial geologic formations in the American Bottoms region are the Cahokia Formation and the Peoria and Roxana Silts.

Many of these formations were deposited by the ancestral Mississippi River. Oxbow lakes and river meanders are vestiges of that depositional history. Modern-day Horseshoe Lake is an oxbow lake formed as the Mississippi River meandered back on itself within the American Bottoms floodplain. The geologic features and scars throughout the region are all consequences of the Mississippi River's history.

Geologic map depicting the surficial geology of the American Bottoms Water Supply Planning Region.

Although much of the surficial geology in the region is contains clay facies of the Cahokia formation (figure above), much of the underlying unconsolidated shallow geology (i.e., above bedrock) is composed of sands and gravels. These sands and gravels are highly conductive, meaning that they enable relatively rapid and unimpeded movement of groundwater through the subsurface.

In addition, the water table elevation throughout the alluvial aquifer is near land surface, meaning that the majority of the > 100 ft thick aquifer is saturated. As such, the amount of water that may be transmitted through the aquifer is quite large (figure below). Transmissivity is a quantitative descriptor of how much water may move through a column of the aquifer, and is a result of the amount of the saturated thickness of the aquifer and the hydraulic conductivity of the aquifer. Transmissivity is fairly large for the regional alluvial aquifer.

Map of the alluvial aquifer transimissivity throughout the American Bottoms region. The sand and gravel composition of this aquifer leads to high levels of transmissivity (i.e., water moves through the aquifer relatively easily and quickly).

Bedrock Geology

The surficial aquifer is underlain by limestone bedrock of Mississippian and Pennsylvanian age. These limestone units have low permeability and poor water quality with depth. As such, these units are not a reliable water resource (Schicht, 1965), but form a fairly impermeable boundary that guide groundwater toward the Mississippi River.

Outcrops of Mississippian limestone units form the bluff along the eastern portion of the region. In fact, exposures of these limestone units along the Mississippi River in Iowa, Illinois, and Missouri are the reason that the units are grouped as the "Mississippian System." (Willman et al., 1975).

Maps depict the surficial (left) and bedrock (right) geology for the state of Illinois. The black outline surrounds the American Bottoms Water Supply Planning Region.

Summary

The entire geology and hydrology of the American Bottoms region is related to the history of the Mississippi River. Erosion from the Mississippi River carved the underlying limestone and shaped the region's limestone bluffs and topography. Deposits from the Mississippi River created the adjacent alluvial aquifer that was used extensively for industry in the early half of the 20 th  century. As the most important hydrogeologic feature in the American Bottoms region, the Mississippi River is monitored by many agencies, including the USGS, with records extending as far back as 1844 (USGS, near St. Louis, Missouri). The Mississippi River serves as the primary water supply source for municipalities in the American Bottoms region.

Supporting Documents

Funding Information

This story map is part of a series about Water Supply Planning throughout the state of Illinois. Water Supply Planning is an ongoing research effort conducted by the Illinois State Water Survey and is funded by the   Illinois Department of Natural Resources Office of Water Resources.  

Mississippi River with the St.Louis Arch in the background. (Photo credit: George Roadcap)

Geologic map depicting the surficial geology of the American Bottoms Water Supply Planning Region.

Map of the alluvial aquifer transimissivity throughout the American Bottoms region. The sand and gravel composition of this aquifer leads to high levels of transmissivity (i.e., water moves through the aquifer relatively easily and quickly).