What you see above is the Coralville Reservoir on the Iowa River in east central Iowa. It is basically in my backyard. I enjoy opportunities to be near it and walk its boundaries. It is a recreation area and flood control lake built by the Army Corps of Engineers.
The Iowa River is one of several southeastern flowing streams that drain the eastern one-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 stream flows on the Iowa River, Coralville Lake's less evident, but equally important, role is in the comprehensive flood control system for the Mississippi River. Construction of the dam began in 1949, was delayed by the Korean Conflict and was finally completed in 1958.
The focus of this diary is on the effects of two flooding episodes in 1993 and again in 2008 which caused water to flow over the concrete spillway and reveal a 375 million year old layer of bedrock from the ancient Devonian seafloor which existed in this region.
Join me in the fossil gorge below the squiggle. ⬇
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. 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.
Part way down on the gate control tower is a blue sign designating the high water marks for the 1993 and 2008 floods.
Turn and look below the dam to the outflow pipe release area. It is 23' in diameter and emerges below the concrete structure in the foreground. During this low water time, the Corps is doing some repairs to the release apron. The flow is diverted to the right and back into the river.
This footage from June 9, 2008 shows the water release at near maximum. Normally, there are people standing nearby to watch. But, the danger caused the park officials to close this section. A few days later, that grassy area was under several feet of water.
Moving to point B gives 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. In 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 is used. Here is some footage of the overflow on June 13, 2008, five days after the video above. The flow increased in volume and intensity over the next days.
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) fossiliferous 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 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 setting in the scoured out gorge through the bedrock. A walkway takes you to the bedrock.
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.
Water is essential to our existence. It bathes us, gives us drink, soothes us, strikes fear, offers fun and offers peace. Water is soft and trickles through the fingers. Water can cut and plow through eons of time. We are mostly water.