Spring Bay Fen is the only remaining example of an Illinois River cold water tall shrub fen in the state. The fen is a "bog-like" community dominated by several shrub species: red-osier dogwood, nannyberry, poison sumac and pussy willow. This community is similar to a bog in the structure of the peaty soil and springs, however, the pH of the spring water is different. The calcareous water flowing through the soil creates an alkaline pH to the soil-water, resulting in a fen community rather than an acidic bog. The plant communities have rare plants such as black ash, spotted phlox and highbush cranberry. More typical plants include skunk cabbage, blue iris, spreading goldenrod and spotted touch-me-not. Wood duck, heron, egret, bullfrog, garter snake and beaver are just a few common animal inhabitants. Record floods occurring in both 1983 and 1984 seemed to have had little adverse impact on the significant features.
A stream runs through the fen, so clear you can see to the bottom.