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.

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, monitoring coordinator at the Minnesota Department of Natural Resources. "Not all wetlands are the same, and they don't have the same functions."

From 2006 to 2014, Minnesota gained just barely more wetland acreage than it lost, according to data the DNR published in September.

Perhaps the most visible function of wetlands is providing habitat for birds, amphibians, insects, fish and other animals. Wetlands also refresh groundwater supplies, cleaning the water that flows through them. They can even act as giant sponges, 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 — 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 impervious area grew by 15 percent in the Twin Cities' seven-county metro.

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

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

Minnesota's Wetland Conservation Act declared a "no-net-loss" policy. Developers or landowners wanting to drain or fill wetlands are required to either develop replacement wetlands 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, according to a May report from the Minnesota Board of Water and Soil Resources.

But much of the new wetland acreage in Minnesota has been of a type that is often less valuable: unconsolidated bottom wetlands, in which the soil and water depth aren't friendly to native vegetation — think of a typical pond. Kloiber said this category is growing the most. Many are man-made, which 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. Wetlands in other ecoregions are in significantly worse condition and are likely to be overrun with invasive plant species like aggressive cattails and reed canary grass, the DNR data show.

Micah Emmel-Duke is on freelance assignment for the Star Tribune.