Water levels at Coralville Lake have approached the spillway lip a number of times over the years but it wasn't until 1993 that it finally overtopped due to heavy and continuous rains. During this time, maximum lake inflows reached 41,000 cubic-feet of water per second. For 28 days, as much as 17,000 cubic-feet of water per second flowed down the spillway, obliterating the road and the campground. Fifteen feet of unconsolidated river-bottom silt and sand were rapidly eroded and washed away, exposing the Middle Devonian Cedar Valley Group limestones below. Up to five feet of limestone was then eroded near the end of the spillway and stone slabs weighing as much as two or three tons were carried hundreds of feet downstream.
When the flood abated, the eroded gorge surface revealed a succession of 375 million year-old bedding planes with diverse and abundant fossils commonly standing out in relief. A mysterious mound of glaciated soil was somehow protected and left behind.
The Devonian Fossil Gorge soon emerged as the name for the attraction. When first admitted to the gorge site, over the Labor Day weekend of 1993, 16,000 visitors were counted entering from just one of the two available entrances. Since then, there have been several million visitors.
Recognition of the regional public interest in the educational, recreational and tourist appeal of the Gorge motivated formation of a local committee, working closely with the U.S. Army Corps of Engineers, to develop a plan for development of the site. Donations from local industry and merchant community, especially limestone producers, as well as a private individuals and state and local government bodies totaled nearly $500,000.
An entry plaza, finished and dedicated in 2001, was constructed in a hexagonal shape, reflecting the symmetry of the abundant colonial coral Hexagonaria. It is surrounded by six stone monoliths of 425-million-year-old Silurian Anamosa dolomite. These pillars are six feet wide, eighteen inches thick, and up to fifteen feet high. Interpretive exhibits adorn the monoliths. A handicap-accessible walkway leads from the Entry Plaza south to an adjacent overlook plaza.
The Biostrome walkway, that lead down into the gorge, was lined by a succession of 14 massive Cedar Valley limestone boulders, taken from adjacent quarries, and arranged from youngest to oldest. Construction of a pathway within the Gorge was avoided, but 20 “discovery points” were marked by numbered hexagonal brass plates: maps and explanatory brochures were printed and made available.
One result of the flood waters of 2008 was the sidewalk north then down to the Biostrome plaza near the gorge floor was badly damaged. Approximately half of the lower sections were entirely washed away, and above that part, the sidewalk was completely undercut. Several of the lowest/oldest limestone boulders were also washed away entirely.
And all, except for one, of the numbered brass plates were washed away. Then that one too disappeared when someone could not resist a unique souvenir and the lone #7 marker is also gone now. And then the mysterious mound of glacial soil that was left behind in 1993 had vanished completely when the waters receded in 2008.
Fortunately, the entry plaza was spared, and the stone monoliths with their story panels remain. Most of the damage has been repaired and markers have been replaced at new items of interest.
The gorge itself was not significantly changed, the layer of Devonian age rock is almost 200 feet thick, but the changes that did occur are dramatic. The greatest transformation is at the southern-most end of the gorge, where the flood waters rejoined the Iowa River channel. This narrow part of the gorge had become quite overgrown and rugged and it is now widened and scoured down to the bare bedrock once again.