Geology of Iowa Fens

Fens are peatlands which are dependent on groundwater discharge to provide nutrient enrichment. Fens are found in a variety of landscape positions and in most Iowa landform regions. This paper presents a classification system for Iowa fens based on landscape position, stratigraphy, and hydrologic factors. Iowa fens can be separated into six categories: 1) fens along valley wall slopes; the groundwater source for these fens is sand and gravel buried between glacial tills (inter-till); 2) fens in hummocky topography on the northwestern margin of the Des Moines Lobe landform region; the water source is sand and gravel buried within glacial till (intra-till) which exhibits artesian flow; 3) fens on edges of exhumed sand and gravel on the Iowan Surface; the water source is the exposed sand and gravel; 4) fens on benched alluvial terraces or glacial outwash settings; the water source is sand and gravel exposed at the surface; 5) fens recharged by bedrock aquifers; and 6) fens in abandoned channel areas whose water source is either sand and gravel exposed at the surface or bedrock aquifers. The lithology, weathering characteristics and thickness of the deposits in the fens groundwater basin influence infiltration rate, and thus recharge to the fens. The constancy of the water source and the rate of flow to the fen are important both in the long-term maintenance of the fens and in controlling their vulnerability to hydrologic and chemical disturbance