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What lies beneath doesn't lie
Spring flood predictions are in the air, and you can bet that within the next several weeks a whole lot of that wayward water will be taking Grade A Midwestern topsoil along for the ride.
To a certain point, such catastrophic erosion events are inevitable when rivers like the Minnesota give the land a good hydraulic hammering. But in the big scheme of things, more troubling is the runoff that occurs on a routine basis in our agricultural watersheds. A few tons off that acre and a few tons off this acre adds up when spread across millions of those acres.
In fact, the amount of sediment the Minnesota River is dumping into Lake Pepin has increased ten-fold during the past 150 years. You get one guess as to what land use change has occurred in the river basin during the past century and a half.
In a sense, the ancient history of the Minnesota River valley makes it a prime candidate for dumping huge amounts of sediment into Lake Pepin. After all, around 12,000 years ago floods from glacial meltwater lowered the valley bottom by almost 230 feet in places. That means the tributaries feeding into the river have had to carve out sharp valleys to reach the river -- producing a lot of sediment, and a lot of sediment-scouring hydraulic power, in the process. The bottom line is that today the Minnesota River is responsible for 25 percent of the flow into Lake Pepin, but 85 to 90 percent of the sediment load.
This geological accident has prompted some to argue that almost all of the sedimentation seen today in the Minnesota River is inevitable and completely natural, and has little to do with the fact that over 90 percent of the basin's land is planted to annual row crops.
Unfortunately for the all-soil-erosion-is-natural crowd, we have Lake Pepin, which serves as a perfect collection basin for all that gets dumped into the upper reaches of the Mississippi. Eroded soil preserved in the cold muck of a lake bottom doesn't lie. During the past several years, scientists have been taking core samples from the lake's bed, unearthing a connection between human use of land and water quality deterioration that can't be dismissed.
The results of this research were summarized nicely by a special "Mississippi River" package of eight papers published in the May 2009 issue of the Journal of Paleolimnology. The studies show that yes indeed, the Minnesota River's geologic history has made the basin a "natural" source of soil erosion-something not likely to change until the next round of glaciers grind through.
But the Journal of Paleolimnology also paints a pretty incriminating picture of how even though humans' role in erosion history is relatively brief, it's been inordinately powerful -- core samples show a dramatic increase in sediment accumulation beginning with European settlement in 1830.
Just as the Minnesota River contributes more than its share of sediment to Lake Pepin, certain tributaries of that river are overachievers in their own right. Research shows that although the Blue Earth and Le Sueur watersheds only account for one-fifth of the Minnesota's drainage area, these heavily farmed regions contribute as much as half of the basin's sediment erosion.
And with that sediment has come other pollutants such as phosphorus, an important crop nutrient that causes major ecological problems once it finds its way to water. Phosphorus accumulation in the lake's sediment has increased 15-fold since 1830, increasing from 60 to 900 metric tons annually. Overall, current sediment-phosphorus concentrations are approximately twice those of pre-settlement times.
Roughly 17 percent of the lake's volume has been replaced by sediment since 1830. At current rates, in 340 years the lake will be just one big, flat field of mud. Midwestern rice paddies, anyone?
As the Journal of Paleolimnology papers show, the increase in sedimentation and phosphorus loading correlates almost perfectly with the plowing of the prairie and the replacement of perennial plants and wetlands with row crops. Not only do these row crops leave the soil vulnerable to erosion, but they don't hang onto rainfalls and meltwater as efficiently as deep-rooted grasses, trees and other perennials. In addition, all those high-yielding corn and soybean fields wouldn't be possible without tile drainage, which sends water rocketing into our waterways, short-circuiting natural percolation systems.
The result? More water than ever is moving more quickly than ever through the watershed, making it difficult for soil, and whatever is attached to that soil, to stay in place. And fast moving water is a powerful scouring agent, exacerbating any "natural" erosion already taking place in the basin.
To be fair, all the fingers of blame can't be pointed at row crop agriculture. For example, total phosphorus accumulations in Lake Pepin's sediment also correlate pretty directly with increased discharges from wastewater treatment plants. It's Lake Pepin's bad luck that it sits below the mouth of the Minnesota River as well as downstream of all those flushing toilets in the Twin Cities. Now there's a farm-city connection.
Speaking of the farm-city link and ecological destruction, one of the most vexing Journal of Paleolimnology papers is one by Laura D. Triplett, Daniel Engstrom and Mark Edlund, which describes the history of sedimentation and phosphorus loading in the St. Croix River. It turns out that basin also has a perfect place -- Lake St. Croix -- to read land use history. Core samples there show that by the period between 1950 and 1960, sedimentation was eight times what it had been a century earlier.
Perhaps thanks in large part to the waterway being designated a National Scenic Riverway in 1968, sedimentation levels have dropped significantly in the past 50 years or so. That's the good news. But phosphorus pollution in the St. Croix, which jumped sharply after 1940, is still roughly four times what it was before European settlement.
Intense logging and moldboard plowing may no longer be a threat to the St. Croix, but poorly planned development is. In fact, in 2008 both Minnesota and Wisconsin declared Lake St. Croix to be an "impaired water" under the Federal Clean Water Act because of excess phosphorus. And just a year ago, American Rivers named the Lower St. Croix one of the 10 most endangered rivers in the U.S. because of uncontrolled urbanization.
As Daniel Engstrom points out in a Journal of Paleolimnology editorial, so much for the St. Croix's reputation as a pristine waterway.
The St. Croix example shows that severely reducing agriculture's presence in the Minnesota River watershed is not the way to save Lake Pepin (and the Mississippi in general). For one thing, the soil in this region is much too rich to not be put to work producing food. And if we don't farm there, it's likely that an unsustainable land use such as urban sprawl will just move in to take the place of row crop agriculture.
No, the answer is to diversify the farming systems in the basin, to make that land once again able to retain runoff efficiently. As research conducted in the Minnesota River watershed by the Multiple Benefits of Agriculture Project has shown, farming and good water quality can go hand-in-hand. But it will require the kind of agricultural systems that rely on diverse crop rotations and perennial plant systems such as pasture.
"There had been three inches of rain in a couple hours time. Martin charged into the house and says, 'You gotta come see this!' He took me to the beginning of the drainage ditch a half-mile down the road. The heavy rain flowed across the neighboring row-cropped field and carried with it its load of topsoil. The water gushing out of the tile outlet was brown and frothy. We proceeded 200 yards farther along the ditch, and found the second tile outlet that drained the adjacent field, spewing its load into the already brown water.
"There was the beginning of a river of thick chocolate-colored paste. The deterioration in water quality in just 200 yards was startling. I wondered just how many of these tiles were emptying into this ditch system between here and the Minnesota River, 25 miles away -- how many more along the banks of the Minnesota as it flows into the Mississippi where how much more sediment with its load of fertilizers and toxic chemicals was pouring into the river?
"Marty and I then crossed the road to check the tile exiting our pasture. Perennial vegetation was holding the soil in place and soil organisms had built a living soil structure. That meant the pasture was slurping up the rainfall and holding it in place for the plants to use later."
And the St. Croix? Sustainable agriculture holds the key there as well. If communities could start seeing the value of keeping acres in sustainably-managed farmland, rather than subdivisions and malls, then runoff of all kinds -- including phosphorus -- could be reduced. In order for people to place such a value on farms, efforts such as the St. Croix River Valley Buy Fresh Buy Local initiative must succeed.
In this age when we seem -- for now -- to be able to use technology to make even the dirtiest water potable, it seems that telling people they must support sustainable agriculture because it will keep Lake Pepin from filling up three centuries hence probably won't fly. They've also got to see a connection to something even more immediate and close to home: good food on their supper table.