Järnsjön is located just west of Järnforsen and is flooded by Emån. The lake is part of Järnforsens FVO. It is a shallow little lake and its surroundings are dominated by wetlands and farmland. The vegetation in the lake is dominated by sap and leaf reed. 

The Iron Lake is an important bird lake with valuable wetlands south of the lake. It has an interesting history as a polluted lake where environmental toxins were stored at the bottom from upstream activities. To restore the lake and get a better water quality to Emån, the lake was dredged in 1994, which has been positive for both the lake and the entire river downstream.