By Dr. Robert Thorson

Some will think I’m a heartless scientist. But I know a good opportunity for addressing a lingering environmental issue when I see one.

When I saw my first images of the I-35W bridge collapse between the Twin Cities of Minneapolis and St. Paul, my thoughts went immediately to the human tragedy — the grief and agony of lives lost and a great American city embarrassed and disabled. But within a few seconds, I became distracted by the turbid gray-green color of the river. Something similar happened to my thoughts when I was reading stories about rescue divers in the water. I switched from the human drama to the scientific cause of the murkiness, which sometimes restricted visibility to less than a foot.

The answer is water pollution. Thirty-five years after passage of the Clean Water Act, our nation’s most important river is greatly improved with respect to toxic chemicals and fecal pathogens. But with respect to runoff-generated suspended solids and nutrients, it is light-years away from what was witnessed by the Rev. Louis Hennepin, who reached the site in 1680. Since then, the voyageurs killed off the beaver; loggers rode roughshod over countless lakes and streams; land clearing for farms increased surface runoff; chemicals for agriculture tainted the groundwater; our appetite for easy meat has increased feedlot runoff; and millions of humans still dump municipal effluent into nearby rivers.

Minnesota likes to think of itself as the “land of sky blue waters,” a phrase seared into my brain from the Hamm’s Beer commercials I heard growing up. This remains true for remote headwaters far from human influence. But downstream, rivers become off-color, especially the Mississippi River within and below the Twin Cities.

This problem begins at Lake Itasca, the Mississippi’s fountainhead. “Transparent” was the word used by the 19th-century polymath Henry Rowe Schoolcraft, brought there in 1832 by his Chippewa guide, OzaWindib. But when I saw the lake in 2006, its visibility was restricted to just a few feet because of algae. This contrasts with the 12-foot or greater visibility for the smaller, leaner lakes draining to it via groundwater. Satellite data published by the Minnesota Department of Natural Resources validate this comparison.

The Mississippi River traces out a great question mark on the northern Minnesota landscape, heading northwest from Itasca, then northeast to Bemidji, then southeast to Jacobson, then southwest to near Brainerd before straightening out and heading south. With every mile, the growing stream becomes richer from the wastewater effluent of village and city water treatment plants, the steady leakage from tens of thousands of lakeside septic tanks and the shallow groundwater from every fertilized lawn, garden and field. Green algal growth is the result. Meanwhile, the runoff from roofs, streets and parking lots erodes channels and washes in dust from around the world, smokestack emissions from western power plants and aerosols from a nation of cars. Gray suspended solids are the result.

Curious about the status of water flowing beneath the collapsed I-35W bridge, I consulted the Upper Mississippi River Water Quality Assessment Report, published in 2002 by technical staff from the U.S. Environmental Protection Agency. They report on 11 separate water quality parameters for a hydrologic reach called HUC 07010206, which represents the headwaters of the Mississippi north of its junction with the Minnesota River.

The key to the gray-green murkiness is indicated by elevated levels (greater than 30 micrograms per liter) of the plant pigment chlorophyll a, an index of living algae, especially phytoplankton. Algae concentrations from 1980 to 1999 have been stubbornly resistant to remediation, largely because phosphorous values have remained unnaturally high.

To update this conclusion, I compared the 20-year EPA data set with a 10-year overlapping data set analyzed by Metropolitan Council Environmental Services for the Twin Cities. The year 2005 was a worse year for soluble phosphorous in the upper Mississippi than the previous 10-year average.

When the I-35W bridge is replaced and traffic resumes, I hope that those who cross it will look down and — after reflecting on the tragedy — make an equally serious commitment to bring nutrient levels down. Those of us whose bridges still work can look down and make that commitment today for the rivers we care about most.