Lake Superior’s chocolate color

I’m guessing it’s all the mud from the Wisconsin rivers that is causing our lake to turn a deep shade of brown. I’ve watched it slowly ooze its way across the lake all day. All that sediment can’t be good for anything. I wonder how common an occurrence this is to have this much water coming down rivers this time of year.



about 13 years ago

Also from Lester River and all of the Duluth streams.


about 13 years ago

I always heard that the brown tint in all our local Duluth streams is from the high iron content.  Can anyone verify this?

Wes Scott

about 13 years ago

Red clay more so then iron. It makes the water cloudy and hard for fish to feed near shore. They go out farther for the clear water. I'm not sure if there is any negative effect of this.


about 13 years ago

The Nemadji River is a source of a lot of the sediment - some would be transported in the river naturally, but land use changes cause more of the clay to erode. Pollutants can bind to the sediment particles, so it can be bad for the big lake (and the river itself). Good info on the City of Superior website:


about 13 years ago

I've always enjoyed the fact that I seem to live near a large lake of diarrhea.


about 13 years ago

Brown color (aka root beer color) of north shore streams is related to tannin in the water (from plants) vs iron.

It is quite common after a big rain to see the stripe of brown in the lake at various river mouths, especially from south shore streams with more clay soils in their river beds.


about 13 years ago

I am also been concerned about the algae on the rocks along the shore. In the seven years this is the worst I've seen. Due to higher temps? Higher contamination? Does anyone know?


about 13 years ago

Barb and I were invited for a cruise on the Blue Heron (UMD research vessel) yesterday and we learned that the brown color does in fact come primarily from the Nemadji and that UMD has a research project going about the effect of sulfates on wild rice.


about 13 years ago

I just drove by Lake Michigan's north shore. Love the UP!

I noticed that up between Manistique and St. Ignace, there's definitely more algae in the shallow water by the shore than there was a few years back. The water isn't noticeably warm, but I might not be able to notice the difference it takes to change something like algae growth.

Wes Scott

about 13 years ago

Lake Michigan has more algae than in the past. I drove along the shore and all the rocks are just coated. I am guessing it's the warmer, shallow waters and perhaps climate change. We are much warmer during the past few summers. Lake water temps are higher than average. None of it is good for the Great Lakes.


about 13 years ago

The lakes were filled by melting glaciers.  It should be expected that they would warm over time, especially considering it would take Lake Superior billions of years to refill from its tributaries.


about 13 years ago

@Bob, I'm assuming that you're grossly exaggerating intentionally ... at least spend a few minutes reading the wiki site or other sources about flow rates and residence time ... climate change is real.


about 13 years ago

I was flying over the lake yesterday and noticed it was more chocolatey than I'd  ever seen it. Small boats were leaving little wakes of clear water, so the sediment must stay toward the top of the water...which seems a bit odd.

Barrett Chase

about 13 years ago

I'd like to see some actual expert opinion on the algae matter. Though an absence of algae is "prettier," my understanding is that it sometimes indicates higher levels of pollution (since it's harder for things to live in polluted water). 

But I admittedly know very little on the subject.


about 13 years ago

From Wikipedia:

"Lake Superior may have warmed faster than its surrounding climate.[13] Summer surface temperatures in the lake appeared to have increased by about 4.5 °F (2.5 °C) since 1979, compared with an approximately 2.7 °F (1.5 °C) increase in the surrounding average air temperature. The increase in the lake's surface temperature may be related to the decreasing ice cover. Less winter ice cover allows more solar radiation to penetrate the lake and warm the water. If trends continue Lake Superior, which freezes over completely once every 20 years, could routinely be ice-free by 2040.[14] These warmer temperatures can actually lead to more snow in the lake effect snow belts along the shores of the lake, especially in the Upper Peninsula of Michigan."

"Because of its size Superior has a retention time of 191 years."


about 13 years ago

The 'chocolate' color in Lake Superior is due to red clay (and silt or 'mud') sediment from the Nemadji River.  The clay is red from iron that is bound up in the clay.  The reason there's so much clay in the lake right now is the torrential rain that washed out several bridges in the Nemadji river basin last Tuesday, contributing a greater sediment load to the lake than normal.  The Nemadji flows through mostly clay, so strong rain events contribute more clay to the river, and thus the lake. Clay stays in suspension much longer than silt, sand, or gravel, so it'll take awhile to settle out. This is a natural process that will continue essentially forever (or until the next ice age, or the Nemadji basin elevation erodes to the same elevation as the lake).  The St. Louis River does not flow through nearly as much clay so it contributes much less sediment to the lake.


about 13 years ago

The increase in Algae could be a result of a multitude of things, higher temperature, increase in phosphates (lawn fertilizer).  The algae will grow at pretty much any temperature above freezing, but it will often grow slower than the lake grinds it off the rocks, I would think that a noticeable increase would be a combination of warmer water and phosphate pollution.

Did you know that if you don't fertilize your lawn you don't have to mow as often.  It's true!


about 13 years ago

There has been an increase in algae attached to the rocks in Lake Michigan and the other great lakes, largely due to the zebra mussels, which filter so much out of the water that it becomes clearer; this clarity allows more light to penetrate, leading to the growth of this algae attached to the rocks on the bottom.
I don't know of any studies showing greater amounts of algae in Lake Superior, though studies have shown greater amounts of algae near stream mouths; no surprise, as the streams bring nutrients into the lake.  Here in Duluth we have the mighty St. Louis, not to mention all the lovely fertilized lawns, sewer overflows, and doggie "treats" left on the ground, all of which are great sources of even more nutrients to help that algae grow.

As to the red color, seems folks have answered that; the Nemadji is a big source of clay; part natural, part due to land cover changes, part still due to destabilized banks created when the entire area was clearcut 100 years or so ago.


about 13 years ago

I talked to a scientist guy who studies algae, and he said invasive species such as zebra mussels are making the water in Lake Michigan clearer, and that makes more sunlight reach the algae beneath the surface, and that makes the algae grow. It washes up on beaches and smells awful. The Wisconsin DNR says the same thing     here.

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