One of the springs in the CRWA.
One of the springs in the CRWA.

The Brown Springs In The Cannon River Wilderness Area

Comments and photos from Dr. Daniel Jones, University of Minnesota, 2015

Small rust-colored springs can be perennially found in at least three locations in the park. These springs emerge at or near the contact between the Prairie du Chien Group and the St. Peter Sandstone, and are likely formed by artesian flow from the Prairie du Chien.

The emergent water from the springs is anoxic (contains no oxygen), has near-neutral pH, and contains dissolved ferrous iron. As the springs are exposed to oxygen in the air, the dissolved iron is rapidly oxidized, forming iron oxide minerals that give the springs their characteristic rusty color. All of the iron springs in the park seem to have a similar chemical composition, which suggests a common source.

Iron oxide filaments in the spring water (Leatherman tool for scale)
Iron oxide filaments in the spring water (Leatherman tool for scale).

The source of the iron is not known with certainty, but it may originate from pyrite or other iron-bearing minerals in the Prairie du Chien. When pyrite oxidizes, it releases sulfuric acid and ferrous iron, but in this case the surrounding carbonate rocks of the Prairie du Chien would neutralize any acid produced.

The “brown slimes” that gives the springs their characteristic color are iron oxide minerals produced by bacteria that make a living off the dissolved ferrous iron in the spring. Much in the same way that we humans eat sugar, breathe oxygen, and produce carbon dioxide as a waste product, these iron-oxidizing microorganisms get their energy by essentially “eating” iron, “breathing” oxygen, and producing iron oxide as a “waste product.” In other words, the bacteria in the spring are “rusting” the iron in the water. (These bacteria do not cause any diseases, and are simply minding their own business.) See the attached slides for more information on the iron-oxidizing bacteria and their metabolism.

A Carleton College geology class discusses one of the springs.
A Carleton College geology class led by Dr. Clint Cowan discusses one of the springs.

Two types of iron-oxidizing bacteria, Gallionella and Leptothrix, are especially abundant in the springs. These organisms can be readily observed under the microscope because they construct unusual and beautiful iron oxide sheaths and twisted stalk-like structures, all as a way to deal with their iron oxide waste.

The document linked below contains some images and analytical information about the springs and their microbial inhabitants.

Click here for more analysis and photomicrographs of the iron springs in the CRWA.