Devonian Fossil Gorge of Iowa


The Iowa River is one of several southeastern flowing rivers that drain the eastern half of Iowa to join the Mississippi River along the state’s eastern boundary. Record floods that swept the nation in the early 1930’s prompted Congress to establish the Flood Control Act of 1938. In an attempt to reduce flooding on the Mississippi River, the U.S. Army Corps of Engineers was authorized to construct several dams on tributary rivers, including the Iowa River. In addition to moderating flows on the Iowa River, Coralville Lake’s less evident, but equally important, role is part of the comprehensive flood control system for the Mississippi River. Construction of the dam started in 1949, but was delayed by the Korean Conflict.

The focus of this story is on the effects of two flooding episodes in 1993 and again in 2008. Both times water breeched the concrete spillway and revealed a layer of bedrock from the ancient Devonian seafloor which existed in this region 375 million years ago.

The area in the lower right corner of the previous image is labeled with the dam and spillway for the lake. Zooming into the Google Earth map to the dam gives the image below. Some key points are labeled A thru F where I have photographs or video for illustration. At point A, there is a tower to control the release gates to regulate the flow out of the lake. Three gates can be opened independently.

During drought, the minimum flow rate of 150 cubic feet per second provides a sufficient flow to meet downstream domestic and industrial water needs. During time of flood, the outflow can be increased to about 20,000 cfs via a 23 foot diameter flow pipe through the dam. Inflow to the lake can exceed 40,000 cfs during rainy flood episodes. The lake level rises and potentially goes over the spillway to the right of point B.

This is the gate control tower situated at point A. Scanning to the left is the lake and dam. The spillway is out of sight at the far end of the dam to the left.

Part way down on the gate control tower is a blue sign designating the high water marks for the 1993 and 2008 floods.

While standing at point A, I turned around and looked below the dam to the outflow pipe release area. It is 23′ in diameter and emerges below the concrete structure in the foreground. During low water time in 2011 of this picture, the Corps was repairing some parts of the release apron. The flow was diverted to the right and back into the river past the grassy area.

The footage below is from June 9, 2008 and shows the water release at near maximum flow. Normally, there are people standing nearby to watch the powerful surging water. The danger caused the park officials to close this section. A few days later, that viewing spot and the entire grassy area were under several feet of water.

From vantage point B, there is a clear view of the spillway at the west end of the dam. A truck provides a sense of scale. There is a blue marker designating the flood levels when the water topped the spillway in ’93 and ’08.

From January to September 1993, the Iowa River basin received the most rainfall in 121 yrs of record keeping. May = 6.51″, June = 12.31″, July = 10.83″, and August = 15.16″. Water passed over the spillway for 28 consecutive days.

The first six months of 2008 were the wettest on record, surpassing the 1993 amounts. By design, if the release gates can’t handle the flow, the spillway carries the excess water. Here is footage of the overflow on June 13, 2008, four days after the video above. The flow increased in volume and intensity over the next two days and crested on June 15.

The Flood of 1993 was the costliest, most devastating flood in U.S. history according to the U.S. Geological Survey. Floodwaters covered as many as 23 million acres of agricultural and urban lands in the Upper Midwest for weeks. The unusual duration and magnitude of this event was triggered by a wet-weather pattern that had persisted since early in the year, followed by a series of intense rainstorms in late June and July. Iowa found itself in the center of the catastrophic flooding that resulted.

Among the effects of the 1993 summer floods in Iowa was the overflow of the emergency spillway at the U.S. Army Corps of Engineers’ Coralville Lake, an event which lasted 28 days. The floodwaters ripped out a road and campground, and scoured away as much as 15 feet of glacial-age sediments to expose a remarkable expanse of 375-million-year old Devonian Era fossil bedrock. Thousands of visitors followed in the flood’s wake. The broad horizontal surfaces of limestone have provided the public with an opportunity to walk across an ancient sea floor and to see a clearly visible picture of bygone life forms that thrived in the tropical waters that once covered interior regions of North America.

Today, there is a visitor center to provide information to families and park visitors. Some photographs at point C in the image near the top of the diary shows the scoured out gorge through the bedrock. A walkway takes you from the visitor center to the bedrock exposed by the flowing water.

The fossil shells and skeletons of sea-dwelling animals are seen in abundance in the broad expanses of limestone. The closer one looks, the more one sees. These fossils are the remains of a multitude of creatures that inhabited a warm tropical seaway covering Iowa about 375 million years ago, during the Devonian period. Most of the fossils seen are from animals that inhabited the sea bottom and filtered small food particles from the water. Brachiopod (clam-like) shells are among the most conspicuous, and a great variety of species can be seen. Some limestone beds are crowded with their shells. Crinoids are also present. Known as “sea lilies,” and plant-like in their form, crinoids are actually animals related to starfish. Most were fragmented into pieces by scavengers and bottom currents before fossilization, and their stem segments are abundant. However, some exceptional specimens at the site were buried intact, displaying their stem, head, and arms as they would have appeared in life.

Partway down the gorge I turned to look back toward the spillway and the expanse of bedrock.

I turned and walked farther down the slope of bedrock in the gorge to point F. There, it rounded a curve and led back into the Iowa River. All along the way were the abundant fossil remnants of life millions of years ago.


6 thoughts on “Devonian Fossil Gorge of Iowa

  1. Thanks for this introduction to one more fascinating place I’ve never heard of. Too bad I didn’t know about it when we spent a few days in Iowa some years ago for a wedding, or I’d gladly have detoured to see and photograph the fossils. I did photograph the wedding couple, who were anything but fossilized.

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