Autumn’s arrival is tinting Minnesota’s rivers with a rich, root beer-like color. The brownish hue comes from tannins, natural compounds released as fallen leaves and decaying vegetation break down. 

As fall hikers and tourists watch Minnesota’s rivers, 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.

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.

This seasonal transformation is especially noticeable in parks where the water often takes on a tea-like appearance. Trees such as oaks, maples, and tamaracks shed leaves and needles that contribute to the tannin levels, infusing the rivers with earthy tones. 

The effect is similar to steeping loose tea leaves in hot water, as the rivers absorb the pigments and organic matter from the forest floor.