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The Planet

October 1997, Volume 4, number 8

The Science of the Swamps

A swamp full of standing water is a wetland. So is a saltwater marsh that floods regularly with the ebb and flow of the tide. So is an oxygen-depleted, dead-body-preserving peat bog. But whether it's an isolated prairie pothole or an estuary that connects to the open ocean, a wetland acts as nature's sponge to control floods, and as a pollution filter to keep our water clean. The hydrologic process (drawing, below) describes how water enters and leaves a wetland. Wetlands are responsible for the following functions:

  • They absorb and store excess water by releasing water more slowly than they gain it, reducing costly flood damage from storms, snowmelt and runoff.

  • They recharge aquifers, which is especially important if cities and farms place a heavy strain on the groundwater.

  • They modulate temperature and weather by remaining cooler in the summer and warmer in the winter.

  • They trap sulfur and heavy metals, which combine and precipitate as insoluble compounds.

  • Microorganisms in wetlands break down organic pollutants.

  • The leaves and roots of wetland plants stabilize the soil and slow wave action to reduce shoreline erosion.

Water levels in wetlands can vary widely over the course of a year, going from nearly dry to totally flooded. These periodic shifts in water levels mean that wetlands can act as two different ecosystems, accounting for the large numbers of species that rely on them. (Forty-three percent of endangered and threatened animals and plants in the United States depend on wetlands in some way.) For example, bottomland hardwood wetlands -- characterized by hardwoods like cypress, hickory and elm trees -- are only flooded during part of the year. When the forest floods, fish like bass and herring move in, feed on the accumulated vegetation on the forest floor and spawn. Fish and crawfish, in turn, provide food for mammals like mink, otters and raccoons. When the area is relatively dry, the fish stay in the channels and mammals and birds move throughout the forest.

The complexity, variety and importance of wetlands make it particularly difficult to restore or recreate them. The creation of a new marsh in one area does nothing to reduce flooding downstream from a wetland that was filled in another. [Wetland Diagram]

The amount of water entering and leaving through these different processes varies widely from wetland to wetland -- precipitation could account for all of the water flowing into a wetland, or the wetland could be fed mainly by groundwater.

The amount of water a wetland is storing can vary throughout the year; if the amount of water going out is greater than the amount going in (not an uncommon occurrence), then the wetland has an available storage capacity and can store runoff effectively and help prevent flooding.

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