Wetlands have been termed "nature s kidneys" because by filtering and purifying our streams and rivers, they play a role that is very similar to real kidneys. Wetlands help purify our drinking water by naturally filtering polluted runoff from city streets, buildings and agricultural lands. They trap sediments, accumulate nutrients, transform a variety of toxic substances into less harmful materials, and can remove potentially dangerous microorganisms from surface waters.

Both economists and ecologists agree that water purification is one of the most valuable services that wetlands provide. The protection of the source of New York City s water in the Catskills is a classic example of how much value society places on the role of wetlands in providing clean water. The city has spent more than $1 billion to purchase land surrounding their upstate reservoir, and to conserve and restore the nearby forests and wetlands. This has saved the city anywhere from $6 billion to 8 billion, which is what a new water treatment facility would have cost!

So how exactly does water purification work? First of all, wetlands act to slow down moving water, which swirls around plants such as cattails, bulrushes and arrowleaf. This allows sediments suspended in the water to gradually settle to the ground. These sediments contain nutrients from fertilizer application, manure, leaking septic tanks, and municipal sewage that can cause a massive overgrowth of algae in streams and rivers. This algal growth causes the depletion of oxygen in the water, which leads to the loss of many types of fish species like trout, bass, and bluegill.

But when a wetland exists between the source of nutrients and a stream, plants and microorganisms can absorb excess nutrients. In fact, a forested streamside wetland can filter up to 80 percent of the phosphorous and 90 percent of the nitrogen out of the incoming water! This natural filtering ability translates into enormous financial savings for society. According to a study by the Natural Resources Defense Council, if half of all the existing wetlands were destroyed, it would cost $62 billion per year to upgrade sewage treatment plants to handle all the extra pollution.

Other pollutants, like lead, mercury and pesticides, are trapped by soil particles and are gradually broken down by microbes. Some types of wetlands are so good at this filtration function that environmental managers construct similar artificial wetlands to treat storm water and wastewater near urban centers. These constructed wetlands are not only much less expensive than sewage treatment plants; they often do a better job of filtering out harmful microorganisms.

Wetland plants aren t the only ones working to clean our waters. Ranchers and farmers in the west are realizing that nature s busiest engineer, the beaver, is a great neighbor to have. The  lakes that form upstream of beaver dams can be extremely useful to reduce nutrient pollution in agricultural watersheds. These beaver ponds have been known to retain up to 1,000 times more nitrogen than streams without beaver dams! An insect called the caddisfly also helps clean polluted wetlands by constructing nets that filter particles out of moving water. Like little fishermen, they continuously clean their nets, eating any nutrient-rich snacks that accumulate.

Constructed wetlands also add value to an ecosystem by providing habitat for wildlife and increasing opportunities for recreational activities. In North America, more than 600 artificial wetlands have been constructed for municipal sewage treatment. Constructed wetlands are also increasingly being used in treating contaminated groundwater, high-intensity livestock operations, acid mine drainage, industrial wastewater, and agricultural and storm water runoff. While natural wetlands are also able to function this way, many sensitive species of wildlife and plants can be degraded or destroyed in the process. So it is better to construct new wetlands as an alternative to costly water treatment facilities, leaving natural wetlands healthy and intact.