Linked
Depressions on the Des Moines Lobe
by E. Arthur Bettis III and Deborah J. Quade
Network of linked depressions: This high-altitude color-infrared photo shows a dark-toned webbed pattern across Kossuth County near the town of Fenton (upper right) that reveals routes taken by glacial meltwater through a disintegrating ice sheet about 12,000 years ago. These linkages often contain sand and gravel, which can serve as pathways for contaminants to enter groundwater and surface-water resources.
Iowa's Des Moines Lobe (see Landform Regions and Surface Topography of Iowa map) forms the southernmost extent of the Prairie Pothole Region of central North America. Prior to agricultural drainage, this region contained abundant wetlands, many associated with "prairie potholes" or "kettles." Recent geologic studies of the Des Moines Lobe have changed our ideas concerning the origin and hydrology of these wetlands and their relationship to other aspects of the landscape. These new interpretations have valuable application to assessing the potential for contaminants to reach water resources in the region.
Geologists previously thought that Iowa's potholes and kettles formed when chunks of buried glacial ice melted to create isolated, bowl-shaped depressions on the freshly exposed land surface between 14,000 and 11,500 years ago. These depressions were viewed as being "closed," that is having no drainage outlets. Since then, detailed examination of aerial photographs and subsurface earth materials reveal that many of the Des Moines Lobe's depressions are only partially closed, and they actually join with neighboring depressions to form linked systems.
These are subtle features when looked at on the ground, but when viewed from the air, the linked depression systems stand out as dark web-like patterns (see high- and low-altitude photos). The links outline the routes of former meltwater channels, and some of these pathways actually connect drainageways that today lie in two separate surface drainage basins. The darker tones of the linked depression systems indicate greater soil moisture and the presence of groundwater near the surface. The contrasting lighter tones are better-drained, slightly higher portions of the land surface.
Chain of prairie potholes:
This low-altitude photo shows a linked depression system
consisting of several shallow basins joined by low
saddle-like areas. Darker vegetation and more soil
moisture mark the low outlets between the basins.
Doolittle Prairie State Preserve, Story County.
Photo by Gary Hightshoe, Iowa State University.
The linked-depressions originated as part of a glacial karst system that developed in a stagnant glacier loaded with sediment (see model, below). As the glacier's surface melts, water enters cracks in the ice and begins to widen and deepen them by melting. These eventually form drainage tunnels within the non-moving glacier that join with other drainageways near the base of the ice. As water flows through the system, sediment within the ice also enters the tunnels. In time, fine-grained silt and clay are flushed from the tunnels, but more coarse sand and gravel settle along their routes. When ice melting is complete, the former branching passages, with their permeable sand and gravel deposits, are preserved as linked systems set into and interfingering with other surrounding glacial materials.
Meltwater tunnels
in a stagnant glacier:
Water drains into cracks in immobile glacial ice laden with
soil material and enlarges them into a series of
interconnected tunnels. As meltwater moves sediment along
these routes, sand and gravel are concentrated. Today
these materials link many shallow depressions on the land
surface.
Two characteristics are essential to development of glacial karst systems in modern glaciers, and so also must have characterized the Des Moines Lobe glacier: 1) the ice sheet must be stagnant - no longer moving, for the system of sinkholes and tunnels to form and remain open; and 2) the surface of the glacier must be covered by enough sediment to prevent the ice from melting too quickly. Such debris can be carried up to the ice surface by compression and shearing within the glacier or carried onto the surface by wind.
An important ramification of the pattern of glaciation of the Des Moines Lobe is the influence of the resulting linked depression system on groundwater movement and vulnerability in the region today. The few field studies that have been undertaken so far suggest the sand and gravel deposits that accumulated in the former tunnels form a connected network that hydrologically links the semi-closed depressions with existing drainageways. This means that rather than the sluggish shallow groundwater system previously envisioned for large parts of the Des Moines Lobe, the linked depressions actually act as an extensive system of "natural drainage tiles" joining poorly drained upland areas with surface waters. This linkage provides a previously unrecognized pathway for dissolved contaminants, such as crop nutrients, to enter the region's streams, rivers, and lakes. More research is needed to further document the extent of the linked-depression sand and gravels, as well as to determine the direction, speed, and seasonal variability of water flow through these features. This information will aid the development of scientifically sound agricultural management practices designed to protect Iowa's groundwater and surface-water resources in the future.