Readers Want to Know: Sometimes Lake Superior seems reddish-brown. Why?
The next time you are asked to draw water, donít be so quick to grab the blue or even green marker ...
Answer: Lake Superior, at least the western arm of it, can turn color when heavy rains or strong winds launch red clay particles and other sediments into the water column.
The southwestern shoreline of Lake Superior has large deposits of glacial-lacustrine red clay. The clay bluffs are especially prone to erosion, and the clay particles can float in Lake Superior for days to weeks. The fine red clay can be found from Duluth to the Apostle Islands.
In the Duluth-Superior area, the Nemadji River is known for its clay-laden flow. Nearly all the sediment in the river is due to bluff erosion and slumping. Seventy-four percent of it ultimately ends up in Lake Superior. The Nemadji River alone deposits about 100,000 tons of silt and clay into Lake Superior a year ó about 17 dump truck loads.
Lake Superior can also turn a muddy red or brown when high winds create enough wave action to stir up bottom sediments. The St. Louis River and smaller tributaries to Lake Superior deliver many other brownish or tan soil particles that end up in the Harbor and the lake. Even though the apparent color of natural water spans the color spectrum, most children pick the bluest blue crayon to illustrate it. This proclivity is borne of the fact that more red, orange, and yellow wavelengths are absorbed by water than are blue ones. When sunlight enters the water, it is mostly blue light that is left for us to see. The deeper the water, the stronger the effect. For instance, tap water in a drinking glass looks clear but the same tap water filling a swimming pool looks turquoise. Covering even larger areas, like Lake Superior, water can look exceptionally blue because the surface is also reflecting the color of the sky.
But once in awhile, looking at Lake Superior will have you seeing red. Other times ... it's apparently brown. Still others, it seems green. Which begs the question: why?
"Apparent color" is different from "true color" in the language of water. "Apparent color," the color we see on any given day, comes from a combination of dissolved minerals and compounds, plus particles, both living and nonliving, that are suspended in the water. Green algae can give water a distinct apparent blue-green color. Some bodies of water in the mountains shine like turquoise because of "glacial flour," finely ground bits of rock liberated from melting glaciers. The "true" color of water is the color perceived after it has been filtered or centrifuged to remove suspended solids, leaving only the dissolved substances such as iron, manganese, and copper, which can produce striking blue-green colors. The brown tint to otherwise clear water, especially in northern Minnesota, is part of the water's "true color." It comes from tannic acid. This natural compound found in plants is useful in tanning hides and making dry wines. Tannins leach from decaying plant material into water as seasons pass, "staining" water to look like tea or root beer. In the context of Lake Superior, the St. Louis River is especially known for its tannin content. As the St. Louis River flows out to Lake Superior, sunlight breaks down the tannins, causing the water to become clearer and clearer until the tea-colored water becomes crystalline blue.
Oh yes ... and don't forget the sky ... the color of the sky can reflect off the water's surface and make a lake like Superior look dramatically different from one hour to the next: blue, green, gray, golden and even purple.
The next time you are asked to draw water, don't be so quick to grab the blue, or even green marker. Consider the particles and dissolved materials that wash in from the watershed as well as the life within the water column. Think of the sky overhead and the rocks underneath. Ask yourself, "why this color?" And, continue to do your part in keeping water the color of ... amazing.
For more information on the color of stream and lake water in Northern Minnesota visit:
- Lake Superior Streams
- Water on the Web
- "Old Carbon in Lake Superior" (audio)
- "The Color of Lake Superior" (audio)
By Sea Grant Staff