Glacial Age Floods
by E. Arthur Bettis III and Deborah J. Quade
Iowa's valley landscapes were formed through the action of flowing water. Iowans experienced this geologic process firsthand during the soggy summer of 1993 when they witnessed some of the worst flooding on record. Although the Flood of '93 was unparalleled in its impact on Iowa's people and economy, its geologic and hydrologic impacts pale in comparison to those associated with glacial meltwater flooding that occurred between 13,500 and 12,000 years ago in valleys draining the Des Moines Lobe of north-central Iowa.
The Des Moines Lobe glacier advanced rapidly into Iowa about 15,000 years ago during a period of climatic warming. The initial advance of the glacier was followed by ice stagnation, then by at least three more rapid advances and longer stagnation phases. While the Des Moines Lobe was in Iowa, voluminous amounts of meltwater were released from on top of, within, and under the glacier into outwash streams that flowed from the ice margins. In many instances, large meltwater floods filled the valleys and had dramatic impacts on the landscape. Meltwater floods developed in three different ways: normal seasonal melting of glacial ice; episodes of extreme melting triggered by unusually warm or rainy periods; and sudden bursts of temporary glacial lakes. These events introduced pulses of meltwater that established the major valleys in the upper part of the present-day Des Moines, Boone, Iowa, Little Sioux, Big Sioux, Raccoon, Skunk, and Winnebago river basins. Outside the margin of the Des Moines Lobe where pre-existing valleys weren't covered by the glacier, meltwater floods eroded valley walls, deepened some valleys, and filled others with sand and gravel.
What is the physical evidence of glacial meltwater floods in Iowa's valleys? Affected river valleys display several features that can be traced back to their heritage of glacial floods. These include multiple terrace levels (former floodplains) capped with sand and coarse gravel deposits, many short steep tributaries and few long branching ones, and relatively deep gorges with many sandy terrace levels. Such features formed rapidly during major glacial flooding, much like the Devonian Fossil Gorge at Coralville Lake and the Saylorville Gorge in the emergency spillway of Saylorville Dam formed during overflow of their respective reservoirs in 1993.
Multiple terrace levels in Iowa's valleys record many episodes of powerful glacial-flood-induced downcutting that carved deep valleys. In Boone County for example, the Des Moines River valley was cut about 220 feet deep during floods of the late glacial period, forming the picturesque gorge characteristic of this part of the valley. Another of Iowa's scenic valleys, the deep and narrow Little Sioux River valley between Gillett Grove and Peterson, formed when the Des Moines Lobe ice front blocked the eastern flow of rivers such as the Ocheyedan, forming Glacial Lake Spencer which then overflowed and drained, swiftly cutting a new course to the west.
The general absence of long, branching tributaries in many glacial-flood-influenced valleys indicates that the valleys formed quickly, with little time for development of an extensive tributary network. The aerial photograph shown here, of a portion of the broad Mississippi Valley, shows numerous steep, short, tributary valleys (dark-green branching patterns) along the upland margins in Illinois (on the right). Also seen, east of the river channel, are remnants of a terrace built by meltwater floods and marked by the dimpled patterns of sand dunes and, west of the channel, sweeping braided patterns marking somewhat younger flow paths. (Note barge in middle of channel for scale.) These features formed as glacial meltwater and lake-burst floods forced the river to different levels and positions during glacial retreat in Minnesota, Wisconsin, and Illinois.
The layering of sand and gravel in terrace deposits within Iowa's valleys speaks to the great variations in discharge during sediment accumulation. Attempts to estimate the flow conditions that accompanied deposition of the sand and gravel are based on the layering and grain size of the deposits. Estimates of ancient hydrologic conditions indicate that many glacial flood events far exceeded the largest floods ever measured in the recorded history of these streams.
Further information about glacial-age floods is found in comparing the thickness and properties of sand and gravel in river valleys that were close to the former ice front with deposits more distant from the glacier. Sand and gravel in valleys near the glacial sources tends to be relatively thin -- on the order of 10 to 20 feet, relatively coarse, and deposited in channels that shifted rapidly across the late-glacial floodplain. On the other hand, in valleys distant from the glacier front, such as the Mississippi in Iowa, the late-glacial sand and gravel is much thicker, finer grained, and deposited in braided channels that looked somewhat like the Platte River channel in Nebraska today.
In contrast to most modern floods, whose impacts are often damaging, the impacts of Iowa's glacial floods have been valuable to us. Most sand and gravel aggregate, one of Iowa's largest mineral industries, is obtained from deposits laid down during glacial floods. The size, composition, and thickness of these deposits, which are important to the economics of production, varies along valleys primarily because of variations in flow conditions during glacial meltwater floods. These deposits are also important sources of groundwater for wells, serving many of Iowa's urban centers which were built along river valleys. Finally, the scenic valleys that were carved by these great floods are an integral part of Iowa's landscape diversity.
Adapted from Iowa Geology 1994, No. 19, Iowa Department of Natural Resources