Wetlands 101: What Iowans Need to Know

Iowa spent a century draining its wetlands to make way for row crops, and we were extremely good at it. An estimated 97.5 percent of the wetlands that covered this state before settlement are gone. Today we are building some of them back — not out of nostalgia, but because a wetland carries a remarkable return on investment when it comes to two major water challenges in Iowa: cleaning water, slowing it down, and storing it for future use. 

What makes a wetland a wetland 

A wetland is not a pond. To be classified as a wetland, water must sit at or near the soil surface long enough during the growing season that saturation changes the soil chemically, and the plants can tolerate having their roots underwater. Unlike a pond, a wetland does not require standing water year-round, and its vegetation grows up through the water rather than around the edge. When soil stays saturated, water fills the pore spaces that normally hold air. The oxygen is used by bacteria and other microorganisms and cannot be replaced quickly enough. What remains is called an "anaerobic," or oxygen-free, zone — and it is where the nitrate removal happens. 

Why wetlands were drained in Iowa, and what changed 

Glacial activity that ended roughly 12,000 years ago left much of north central Iowa flat, poorly drained, and pocked with shallow depressions. As water fell on the land, it naturally moved across the surface and through the ground to fill low-lying regions. Because that water had nowhere to go, early European settlers found the middle and northwestern regions of the state difficult to farm. In the late 1800s, clay drainage tiles were installed in these areas to move water to subsurface pipes where it could flow downstream. These tubes, which today are made from plastic instead of clay, are perforated so water can seep in. Once inside the tube, the water flows away from the field and into larger tiles or nearby creeks or waterways. Without tile drainage, Iowa could not farm all the acres it does today, but the practice has consequences. 

By artificially lowering the water table, the level below which the ground is saturated, tile drainage eliminated wetland after wetland and changed the soil chemistry. Today, roughly 85 percent of Iowa's land is in agricultural use, and much of it is underlain by extensive tile drainage. As time went on and synthetic fertilizers and manure application became more common, this artificial drainage system also began to serve as a conveyor of agricultural runoff, including excess nutrients that are harmful to human and ecosystem health. Nitrate, a negatively charged molecule made of one nitrogen atom and three oxygen atoms (NO3-), being the most signficant.  

How a wetland removes nitrate 

Nitrate dissolves in water similarly to how sugar dissolves in tea, which means no filter is fine enough to strain it out — a distinction worth holding onto, because most people assume a wetland works like a sieve. It does not. As nitrate enters a wetland, bacteria living in that oxygen-free soil pull the oxygen atoms off the nitrate molecule and use them to "breathe." The process is called denitrification, and what it releases is nitrogen gas — the same inert gas that makes up about 78 percent of the air in our atmosphere. The nitrate isn't captured, stored, or moved somewhere else. It is broken down and returned to the atmosphere through a series of reactions.  

Three factors control how much denitrification happens: (1) the nitrate concentration of the incoming water, (2) the temperature of the water, and (3) the length of time the water stays in the wetland. Only the last is something engineers can design for, which is why a wetland's placement and its size relative to the drainage area are critical details. 

Two decades of wetland monitoring by researchers at Iowa State University show removal of roughly 40 to 90 percent of the nitrate arriving in tile drainage, which equates to about 1,340 pounds of nitrate-nitrogen per acre of wetland per year. The wetlands in that study had pools between 0.25 and 3.5 percent of the watershed draining into them. Well-sited wetlands do not require taking meaningful acreage out of production because often less than one percent of the land can treat the water coming from the other 99 percent. 

The same feature handles water quantity 

These water quality improvements depend on water slowing down and staying a while, which is also how a wetland reduces flooding. Water held upstream during a storm reduces the volume sent downstream all at once. Some of that stored water evaporates, some is taken up by plants, and some recharges groundwater instead of running off. In a state that spent a century engineering water to leave fields and ditches as fast as possible, a wetland does something the rest of the engineered landscape no longer can: it makes water wait. 

What arrives without being invited 

Build a wetland in a landscape that has lost nearly all of them and it fills, quickly and largely on its own. Jim O'Connell, a farmer in Linn County, never stocked his wetland. There are fish in it anyway, having moved up the connection from the creek. Herons, ducks, red-winged blackbirds, frogs, turtles, and dragonflies find these places within a season or two. For a state that eliminated nearly all of this habitat type, each restored wetland returns something Iowa once had in abundance. 

There is also a growing body of research on what these places do for the people near them. Studies across 18 countries have linked recreational visits to freshwater "blue space" with improved wellbeing and reduced anxiety and depression, and wetland-specific research has found measurable mood improvement from as little as 10 minutes spent in one — on top of the health benefits of having a reason to walk, fish, paddle, and watch birds. 

The bigger picture 

The Iowa Nutrient Reduction Strategy attributes roughly 93 percent of the nitrogen entering the state's waterways to nonpoint sources, and agriculture is far and away the largest contributor. That is not a controversial finding — it is the consistent conclusion of Iowa's own monitoring. Iowans pay for this twice: once at the farm gate in lost fertilizer, and again at the treatment plant when utilities remove nitrate from drinking water. Wetlands intercept it in between, at a fraction of the cost of either. The science is settled and the practices work. What's missing is the scale.