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Why Watersheds are Important

Wetlands: Their Geological Connection

by Carol A. Thompson


Marsh, Dallas County

Photo by Ron Johnson

Think of a wetland, and one pictures a watery area with cattails, rushes, and waterfowl. Now think of the scientists who study wetlands, and botanists or biologists probably come to mind. Geology is seldom mentioned in the context of wetland studies, yet geology plays a critical role in understanding wetland dynamics. Three characteristics make wetlands unique -- vegetation, soils, and hydrology. The vegetation is dominated by plants adapted to wet conditions; the soils are developed in water-saturated materials; and sites are either saturated, periodically flooded, or contain permanently standing water.

Hydrology may be the single most important factor in the establishment and maintenance of specific wetland types. For example, wetlands receive water from various sources: precipitation, surface water runoff, and groundwater. Each source is characterized by a certain water chemistry, which in turn affects the type of vegetation and diversity of species. The permanence of a water source determines the type of soil that develops, which also influences the type of vegetation present. Understanding the hydrology of a wetland is important to decisions involving its future and to evaluating trade-offs involved in protection, development, and mitigation. Wetlands are often valued in functional terms; for instance, does the wetland reduce flooding, does it recharge groundwater, or does it improve water quality? To address these questions and provide adequate wetland evaluations requires an understanding of why wetlands occur in a particular place and where the water comes from. These are fundamentally geologic questions.



Forested wetland, Louisa County

Photo by Bill Ohde


Iowa's wetlands can be defined on the basis of their hydrology and landscape position. Forested wetlands are commonly associated with backwater sloughs or oxbow ponds along river bottomlands throughout the state. These riparian wetlands are a result of natural stream meandering processes and are linked to flooding cycles. As such, they are one of the more dynamic wetland systems and are dominated by periodic surface-water flows, with smaller contributions from groundwater sources.



Marsh, Palo Alto County

Photo by Carol Thompson


Marshes in Iowa usually occur in basins along floodplains or in upland areas, particularly in the recently glaciated landscapes of north-central Iowa. Depending on a basin's depth and the nature of its surrounding sediments, these wetlands can be fed by surface water and rainwater, which results in ephemeral wetlands, or they can receive significant groundwater inflow, which is typical of more permanent wetlands. The hydrologic relationships can change even on a seasonal basis. In some wetlands, groundwater enters one portion, flows through the wetland, and exits along the other side. During wet periods, these basins act as discharge points, with groundwater flowing to the wetland. During drier periods, however, a few wetlands will change flow direction, recharging water to the ground until they dry out. All these interactions take place through the wetland sediments, which exert a profound influence on the rate of groundwater movement. Iowa's wetland restoration efforts have been directed primarily at marsh lands and generally have been successful.


Meadow   Wet meadow, Emmet County

Photo by Carol Thompson


Wet meadows and wet prairies typically do not have standing water most of the year, but they are characterized by waterlogged sediments. The hydrology of these sites is among the least understood of any wetland class. Many sites are located on low-gradient slopes and are affected by surface water flooding as well as groundwater seepage. This class of wetlands may be the most threatened in Iowa, and restoration efforts generally have been unsuccessful.


Wetland   Fen, Dickinson County

Photo by Carol Thompson


Fens are Iowa's most unique wetland type. They are found primarily along the margins of the freshly glaciated landscapes in north-central Iowa and scattered throughout northeastern Iowa. These wetlands are sustained by groundwater flow and include saturated peat deposits, often in mounded positions along hillslopes and stream terraces. The water is highly mineralized compared to most wetlands, and, as a result, fens contain numerous state-listed rare and endangered species. Because of their unique hydrology, fens are unlikely candidates for restoration projects, and the few that still remain need to be protected. With each passing year, more people are realizing the value associated with the preservation of natural wetland systems. These sites are recognized not only for their recreational and wildlife benefits, but increasingly for their importance as part of the natural hydrologic cycle. The management and restoration of Iowa's wetlands needs to be a cooperative venture among all segments of the state's scientific community.


Adapted from Iowa Geology 1993, No. 18, Iowa Department of Natural Resources