Why rivers flood

by Trevor Russell

Rivers naturally flood, but three main factors have contributed to an alarming increase: land use, drainage and climate change. (Photo by MPCA: Zumbro2, CC BY-NC 2.0)

Spring approaches, and with it community water supplies at risk. Sediment and pollutants washing downstream. Catastrophic floods right around the corner.

A recent Star Tribune article reminds us of the growing threat high flows and erosion pose to the Minnesota River — and the toll that takes on our waters and communities.

River flows: always changing, but not always natural

We all know that the amount and intensity of rain and snow impact river flows, which naturally vary seasonally and from year to year. Some years are wet, others dry. Some floods are modest, others massive. 

However, our annual floods are, increasingly, not just a natural phenomenon but a human-made one. As human activities alter the natural water cycle, we are changing how water interacts with the landscape. 

As we noted in our 2016 State of the River Report, flows in the Mississippi River are increasing, driven largely by major increases in its tributary the Minnesota River – where average flows have more than doubled in the past decade.

Meanwhile, sediment pollution in the troubled Minnesota River is 10 times natural levels, and cropland fertilizers like nitrates are washing downstream at an increasing rate. Combined, high flows and the pollution and flooding they create are a growing threat to river communities.

What’s going on, and what can we do to slow the trend?

Three factors of increased flow

Three main factors are behind this alarming increase in Minnesota River flow: land use, drainage, and climate change.

  • Land use: Prairies and wetlands absorb and evaporate water effectively, but we have replaced much of Minnesota's natural landscape with crops that only consume water during the late spring and summer. This allows more precipitation to run off the landscape and enter surface waters. Although cities and towns are a much smaller share of the problem (because they cover so few acres), hard surfaces such as roads, rooftops and driveways also contribute runoff that can increase flows in nearby water bodies.
  • Drainage: Artificial agricultural drainage – which includes ditching, subsurface tiling and wetland drainage – also impacts river hydrology. Water that would have naturally ponded on the surface and evaporated is instead drained into surface waters. Same goes for urban stormwater pipes – though again to a lesser degree.
  • Climate: We’re seeing more precipitation in the Upper Midwest, and it's coming more frequently in large events that supercharge river flows. Climatological changes may be responsible for as much as 40% of the recent increases in river flow.

Taken together, these factors have fundamentally altered the ability of the land to store and evaporate water. As a result, our rivers are carrying more water and are more erosive, less stable, and more susceptible to runoff pollution.

And we’re seeing this in the Minnesota River we know today.

Erosion costs more than just water quality

High flows and erosion take a toll on our homes and communities. They can create a striking financial cost as well.

As the Star Tribune article noted: “In just the past 10 years, the Minnesota [River] has washed away more than 50 feet of land in front of a city of Mankato drinking well. The well, which supplies about a third of the city’s drinking water, is now teetering less than 8 feet from the river’s edge. City officials, worried that they were one strong rainstorm or snowmelt from losing it entirely…could spend up to $7 million fighting erosion in the area.”

Up and down the Mississippi, flood damage last summer came with a $2 billion price tag, and we can expect more of the same in the future.

Making a change

While cropland practices are the major driver, it's essential to remember that this problem is not farmers' fault – farmers respond to the market conditions and incentives we’ve set out for them. Nonetheless, heavily drained croplands with bare soils nine months of the year simply can’t hold back enough water to keep our homes and communities safe.

As riverside communities throughout the Midwest stare down another potentially catastrophic flood season, we can no longer look the other way. It’s time to change the conversation on how to hold back more water on the landscape.

We need to change the incentives, markets and cropping systems that dominate our land use throughout the Minnesota River basin and the Midwest. 

Agriculture 2.0: clean-water crops 

Luckily, there are solutions on the horizon. Perhaps the largest prize: clean-water crops.

Perennial crops (like Kernza, perennial wheat) live on the land year-round. They have very deep root systems (16 feet+) that absorb water and hold soil in place, and can be harvested and sold for a premium on the market.

New cash cover crops (like camelina and pennycress) keep vegetation on the landscape during spring and fall (before and after row crop establishment and harvest). They can be harvested and sold before the next summer's crop takes over.

Both approaches improve soil health, make croplands more resilient to extreme weather, and increase productivity (and profitability) of those lands. Best of all, these next-generation clean-water crops help restore a more natural water cycle.

Leading the way

Such changes will require a coordinated statewide effort — one FMR is proud to lead alongside researchers, environmental NGOs, farm groups and agribusinesses from across the state. Towards this end, two of FMR's main priorities continue to be the Working Lands program and the Forever Green Initiative. 

Working Lands aims to use the power of markets to establish new cropping systems that restore our waters, while maintaining overall farm profitability. FMR and our allies first established the Working Lands program concept in 2015 and have been building upon it since.

The University of Minnesota Forever Green Initiative is a crop development program essential to the success of the Working Lands approach. Through it, Minnesota can accelerate our adoption of economically viable crops that farmers can plant to reduce runoff pollution and habitat loss.

We are working with partners from across the state to build a network to support key projects, build political momentum, and ultimately realize our vision of achieving continuous living cover on Minnesota’s agricultural landscape. This network will launch in early 2020, so stay tuned for more news on this important development.

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