Ask a 'sotan: Why are the rivers different colors?

Updated: Jan. 27, 2020, 10:30 a.m.

Ask a 'sotan is an occasional series exploring questions from curious Minnesotans about our state. Have a question about life in Minnesota? Ask it here.

Have you ever driven past or hiked along a Minnesota river and wondered what gives it that particular color?

One MPR News listener, Patricia in Red Wing, did. She asked, "Why is the Mississippi River so muddy when compared to other rivers, like the St. Croix? Has it always been this way?"

For help answering her question, we reached out to Trevor Russell, water programs director of the nonprofit advocacy group Friends of the Mississippi River.

Does the Mississippi River really look muddier than other rivers?

Russell said the answer to that question depends on where you are. From its headwaters at Lake Itasca and down through northern and central Minnesota, the Mississippi River is relatively clean and clear.

But the river’s color — and water quality — changes rather dramatically as it flows south through farmland and urban areas. In southern Minnesota, the Mississippi looks much murkier than it does up north.

As the river meanders past St. Cloud and the Twin Cities — and eventually flows down to the Gulf of Mexico, it often appears yellowish-brown and is sometimes called the Muddy Mississippi.

What causes that muddy color?

Sediment — fine particles of sand, silt, clay and other soil materials suspended in the water — is the main cause of the river’s brownish hue.

Near Fort Snelling State Park in St. Paul, the murky Minnesota River flows into the relatively clear Mississippi. There’s a distinct line in the water where the two rivers meet.

The Minnesota River carries a lot of sediment. It runs off farm fields and or washes into the river from eroding stream banks. Researchers estimate that about three-fourths of the sediment in the Mississippi River in the southern part of the state originated in the Minnesota River.

Adding to that are the extensive farm drainage systems in the Minnesota River valley. The systems are designed to prevent crops from drowning during a wet spring or fall, Russell said. But they also bring with them a lot of runoff.

“When we drain that water off the field, it all goes downstream, pushing more water through those streams and making the Minnesota River much more erosive than it would otherwise be,” he said.

South of the confluence with the Minnesota, the Mississippi looks a lot muddier. It's on the state's impaired waters list, thanks to high levels of sediment that don’t meet state standards.

Do the sand and sediment simply turn the river a different hue, or are they a more serious water quality concern?

The color of a river doesn't necessarily indicate whether it's polluted, Russell said. Many of the pollutants that worry scientists — salt, nitrogen, phosphorus, bacteria — can’t be seen with the naked eye, so just because a river is clear doesn't mean it's clean.

However, when sediment creates cloudiness in the water, organic matter and pollutants such as phosphorus and nitrogen can attach to the fine particles and get carried downstream. So if a river is murky, it probably means there's more in the water than just sand or soil.

Every year, excess nitrogen and other nutrients from the Mississippi River form a dead zone roughly the size of New Jersey in the Gulf of Mexico where oxygen levels are too low to support marine life.

How does the Mississippi River's color compare to other Minnesota rivers, such as the St. Croix?

The St. Croix is a remarkably clear river. A key reason: The St. Croix flows through a lot of forest, and not as much farmland as the Mississippi. Much of the St. Croix has been protected since 1968 as a national scenic riverway, so the land on either side hasn’t been developed.

All that natural vegetation helps absorb water before it flows into the river, so there's not as much sediment getting into the St. Croix.

And even though it's relatively clear, the St. Croix does still have some pollution issues. The lower stretch of river, from Taylors Falls to Stillwater, was recently added to the state's impaired waters list because of too much phosphorus.

Along the North Shore of Lake Superior, some rivers and streams are a dark brown color. Does that mean they’re polluted?

Probably not. Sometimes a river's color is caused by geological conditions, or the type of rock or minerals it flows through. That's why some rivers look bright blue or orange.

Those brown rivers and streams on the North Shore often contain something called tannins — organic matter in roots, decaying leaves and bark. They can give the water an amber tint, so it looks like tea or root beer. But despite their rich hue, northeastern Minnesota streams are generally pretty healthy.

There are better ways to judge the health of a river than by the water’s appearance, Russell said.

“In general, it is best not to look purely at the color of the water, but how the water is responding to changes in the landscape upstream,” he said.

To learn more about the health of a certain river, the Minnesota Pollution Control Agency has detailed information on all waterways in the state and which ones don't meet water quality standards.

Correction (Jan. 27, 2020): An earlier version of this article misidentified Trevor Russell's position with the Friends of the Mississippi River.

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