Wetlands are a distinct ecosystem that consist of low-lying, flooded lands with lots of still, standing water. They can be either naturally occurring or man-made. One important aspect of wetlands is the protection and preservation of rivers and coastlines. The water produces lots of plants which help keep the water still. These wetlands then help prevent runoff and erosion of rivers and coastlines. There are also numerous organisms that inhabit these wetlands to break down and digest any waste products before they contaminate water systems. If the water is drained from a wetland area, that area is then permanently altered and the plant life will die. The animal life will either die or relocate themselves to a more suitable area.
I just saw a school science project the other day that involved testing the efficiency of a man-made wetland area that helps protect a local river from agricultural runoff of nearby pastures. The fecal bacteria count was very high in the pasture (not surprising) but very low below the wetland because the wetland helped to remove the dangerous bacteria before it could reach the river. If the wetland were drained, the organisms that break down the fecal bacteria would die off and the river could become contaminated.