Keokuk Water Works: Its Geological Foundations
by Jean C. Prior and Robert M. McKay
Keokuk Water Works, barely visible beyond Lock and Dam #19 on the far shore, is dwarfed by the broad expanse of the Mississippi River. Its width was scoured by the erosive action of glacial meltwaters that periodically flooded the valley over 9,000 years ago. Sweeping curves along the valley are defined by limestone bluffs of Mississippian age, a term of geologic time recognized worldwide and named for this segment of the Mississippi Valley between Keokuk and Burlington.
These exposures of resistant
bedrock are assigned to the Mississippian Subsystem, one of the basic divisions
of geologic time recognized throughout the world. In fact, Mississippian-age
strata were so-named in the geologic literature of the late 1800s for this Iowa
segment of the Mississippi River valley. To geologists, these rock outcrops are
the starting point for the definition of strata of this age.
The lime-rich sedimentary
rocks actually had their origins 345 million years ago in shallow tropical seas
that submerged the continent’s interior, including Iowa. The rocks bear fossil
remains of abundant marine organisms that inhabited these warm seas, an
environment resembling the clear waters of the Florida Keys today. Lime
sediments were precipitated from seawater and also were produced biologically as
animals such as corals, crinoids, brachiopods, and snails built their skeletal
The Mississippi River itself
appeared much later in time, likely originating with south-bound glaciers that
periodically gripped the midcontinent in sub-Arctic cold between 2.5 million and
10,000 years ago. Bedrock promontories, such as here at Keokuk, were left
standing as the river gradually deepened its valley, especially during the later
chapters of the state’s Ice Age history. Episodes of glacial melting,
especially between about 100,000 and 9,000 years ago, sent meltwater floods
surging down the Mississippi, scouring the valley floor still deeper, and then
partially back-filling the gorge with thick deposits of sand and gravel.
The most significant event in the river’s modern history has been construction and maintenance of a nine-foot navigation channel and its accompanying lock and dam system built to facilitate commerce throughout the region. Lock and Dam #19 is situated right outside the Keokuk Water Works, and its operation and river traffic add considerable interest to visitors.
But the story is not complete.
In 1877, citizen William Stripe’s concern for safe drinking water led him to
found a private company known as the Keokuk Waterworks. Its purpose was to
gather and treat Mississippi River waters for distribution to citizens and
industries of the prospering city. In 1938 the company was purchased by the City
of Keokuk, and a new treatment building was constructed, with further
improvements in subsequent decades. Then, in 1995, on the same piece of ground,
an entirely new treatment plant was begun as a phased replacement of the older
The first important issue was
location. The site on the Mississippi River still looked good over 100 years
later because it already had functioning river-intake pipes and underground
settling tanks that could be converted to clean water storage. In order to
construct the new building on top of the 1979 settling tanks, engineers designed
a pillar anchored into underlying bedrock to support the weight of a new 1.3
million-pound re-carbonation unit.
Construction at the existing site also entailed excavating into the steep bedrock bluff to create more space. Contractors removed about 200,000 cubic feet of rock and cut a stair-step into the bluff so that massive cone-shaped water clarifiers and softeners would have a solid foundation. Phase I of the new facility is complete, and includes a 14 million-gallon-per-day treatment process. With an eye to future expansion, the pipes and space are in place to add new filters and more clarifying and softening cones as needed.
|Layers of geode-bearing clayey dolomite, examined by geologist Robert McKay (left), add great visual and educational interest to the Keokuk Water Works, both inside and outside the plant. The geodes, Iowa’s official “State Rock,” are lined with quartz (chalcedony, lower left) and calcite on quartz (below).|
Excavation into the valley
bluff exposed two of the local Mississippian bedrock formations, the upper part
of the Keokuk Limestone and the lower portion of the overlying Warsaw Formation,
a more shaley interval. Both rock units, particularly the Warsaw, are widely
known for the spectacular crystal-lined geodes that are embedded in the clayey
Though geodes are known from many localities around the world, one of the most famous collecting regions occurs within a 35-mile radius of Keokuk. Rock collectors commonly refer to these specimens as “Keokuk geodes,” and in 1967 the Iowa General Assembly declared the geode as the official “State Rock.” These roughly spherical masses of silica-rich minerals separate easily from their host rock. The most prized specimens have hollow interiors lined with pointed crystals of quartz and other minerals. They can range in size from that of a walnut to a basketball.
The geode-laden rock bluff was so interesting that it was decided to incorporate a portion of the bluff into the water plant’s long west wall. About 9 feet of geode-bearing dolomite is well exposed along the driveway outside the new plant (see photo above), and the rock wall then continues some 300 feet along the entire length of the building’s interior where the wall stands 12 feet high (see photo below). Smaller, less accessible exposures of Warsaw Formation are present behind the building.
|Mississippi River water is pumped through a network of pipes into massive cone-shaped clarifiers where sediment and dissolved minerals are removed from the swirling water. Keokuk’s expanded, state-of-the-art water plant includes a 12-ft-high portion of the 345 million-year-old Mississippian limestone bluff along one entire interior wall of the 300-ft-long building.|
Today’s Keokuk Water Works
is situated against a bluff of limestone significant in the history of the
country’s geological investigations, a bluff of sedimentary rock detailing a
345-million-year-old history of ancient seas and their inhabitants. It overlooks
a younger valley that was host to torrents of glacial meltwater floods several
thousand years ago, and whose modern descendant, the Mississippi River, flows
past the front door, providing Keokuk and its residents with their drinking
The Keokuk Municipal Water
Works recognizes its unique geography and geology. They have incorporated the
bedrock, the bluffs, the valley, the river, and the lock and dam into their
plant design. Inside the plant, the ancient geode-laced Mississippian strata
form a striking visual counterpoint to the modern array of piping, tanks,
valves, massive cones, catwalks, and computerized operations that organize and
treat the river’s water into a municipality’s water supply.
Keokuk Water Works encourages
public tours of their plant. They know these visits can increase the public’s
understanding not only of their local drinking water supply and its quality but
awareness of broader regional and state water supply issues as well. In this
scenic setting, that awareness includes a unique reminder that we depend on and
interact daily with geological resources and geological processes past and
* Lewis, Henry. 1967 (originally published in German in 1854). The Valley of the Mississippi Illustrated. Minnesota Historical Society, p. 219. Quote from J.N. Nicollet’s 1843 Report Intended to Illustrate a Map of the Hydrographical Basin of the Upper Mississippi River. 26th Congress, 2nd session. Senate Documents, no. 237 – serial 380, p. 24, 26.