As fall hikers and leaf-peepers watch Minnesota’s rivers carve along cliffs, spill down cascades, and rush across rocks, those waters often sport a dark amber glow. Sometimes there might even be a good foam.
While some visitors might worry brown water means it’s dirty, it’s just nature at work. You can thank the tannins in autumn leaves and decomposing plants for these river hues.
Yellowish or brownish tannins are bitter compounds found in plant tissues from seeds to bark. The rivers pick up colors much like a steaming glass of water turns brown when you drop in loose tea leaves.
North Shore state parks such as Gooseberry Falls, Cascade River and Tettegouche often display these root-beer rivers and hummocky foam at the base of waterfalls. They pick up high levels of tannins while flowing through wetlands and swamps with decaying organic matter. Falling leaves and needles from trees such as oaks, maples and tamarack this time of year also contribute to tannin levels in rivers.
Naturally occurring tannins help trees protect themselves from threats such as bacteria, fungus and predators.
In plants, bitter-tasting tannins prevent animals (and humans) from eating unripe fruit, such as wild raspberries and blueberries, until their seeds are ready to be eaten and distributed. Tannins absorbed from skins, stems and seeds of grapes, as well as the oak aging barrels, provide body to red wines.
If a river or lake looks like a milky, muddy brown, rather than clear, it could be due to heavy, windy autumn and spring rainfalls. High-flowing waters and choppy currents stir up silt and clay. That can be seen along Lake Superior, as well, but sediment settles with calmer weather and allows clarity to return.
Lisa Meyers McClintick has freelanced for the Minnesota Star Tribune since 2001 and volunteers as a Minnesota Master Naturalist.
