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Since Europeans began settling in Minnesota, about half of the state's wetlands have disappeared. But in recent years, the state has stopped the loss and actually gained a few acres, according to data released last month.

Wetland quality is another matter.

"From a strict acreage standpoint, Minnesota is holding steady and maybe even gaining small amounts of wetlands, but there's some concern with the type changes," said Steve Kloiber, wetland monitoring coordinator at the Minnesota Department of Natural Resources (DNR). "Not all wetlands are the same, and they don't have the same functions."

Between 2006 and 2014, Minnesota gained just barely more wetland acreage than it lost, according to the most recent data, published in September by the DNR.

Perhaps the most visible function of wetlands is providing habitat for all sorts of animals, including birds, amphibians, insects, fish and more. Wetlands also refresh groundwater supplies, cleaning the water that flows through them.

If wetlands aren't drained, filled or paved over, they can even act like giant sponges scattered across the land, sucking up extra water during heavy rains and reducing flood peaks.

One factor that may have exacerbated flooding in Houston during Hurricane Harvey was the amount of impervious surfaces—things like concrete, asphalt and roofs that don't absorb rainfall. The impervious area of metro Houston increased by about one-third from 2001 to 2011, the latest available data, according to a Bloomberg report. During that same time, the Twin Cities' impervious area grew by 15 percent in the seven-county metro.

Now, any city would have trouble with the 50 or so inches of rain Harvey brought to Houston, but limiting both wetland loss and growth of impervious area could help.

Minnesota started monitoring wetlands in 2006 to study the effect of 1991 legislation meant to stanch the loss of wetlands, looking at both the quantity and quality of wetlands.

In 1991, Minnesota's Wetland Conservation Act declared a "no-net-loss" policy of wetland management. Developers or landowners wanting to drain or fill wetlands are required to either develop replacement wetlands on their own or purchase offsetting credits from wetland banks, privately developed wetlands certified by the state.

Minnesota now has more than 400 of these wetland banks, the most of any state in the nation, according to a May 2017 report from the Minnesota Board of Water and Soil Resources.

But much of the new wetland acreage in Minnesota in recent years has been of a type that is often less valuable.

There are three notable types of wetlands tracked by the DNR.

Emergent wetlands are what most people probably picture — a classic marsh. There's standing water for at least part of the year, but the water isn't all that deep, and various grasses and non-woody vegetation will grow above the surface of the water.

Cultivated wetlands might not even be recognized as wetlands by most people. These are typically low, wet spots in a farm field that, although technical wetlands, are plowed and lack wetland vegetation most of the year.

There are also unconsolidated bottom wetlands, in which the soil and water depth aren't friendly to native vegetation—think of a typical pond. Kloiber, of the DNR, said this category is growing the most and that many of these new wetlands are man-made. Man-made wetlands in Minnesota usually have fewer native plants and are less habitable to native species.

Most of the state’s 16,500 square miles of wetlands are in good shape, according to the latest available data from the Minnesota Pollution Control Agency (MPCA).

But the good news is concentrated in the northern part of the state.

Most of northern Minnesota, except the western edge, falls into an ecoregion known as "mixed wood shield" (colored in blue on the map above). That region contains about 75 percent of the remaining wetlands in the state. Wetlands that still exist in the other ecoregions are in significantly worse condition and are likely to be overrun with invasive plant species like aggressive cattails and reed canary grass.

Mike Bourdaghs, research scientist at the MPCA, said even though wetlands refresh groundwater and clean the water that flows through them, sending too much water to wetlands isn't good, either.

Human changes to hydrology, like running ditches and stormwater into natural wetlands as is commonly done in the metro area, can be overwhelming.

"When wetlands are used by people for these various functions, if they're used to the maximum extent they can be, the wetlands tend to have degraded quality," Bourdaghs said.