Abstract

Over the past few million years, thick Tertiary coal beds in the Powder River Basin of northeast Wyoming and southeast Montana have burned naturally. The coal fires have collapsed and baked the overlying strata to produce clinker-a suite of hard, reddish to black, highly fractured, thermally metamorphosed rocks. This clinker, as thick as 200 feet, shapes and colors much of the landscape of the basin. Being more erosion resistant than unbaked strata, clinker persists during downcutting to form caprock on hills and buttes. The broken topography of clinker hills and escarpments has created a unique set of ecologic niches that protect plants and animals from the weather extremes of the open plains. Well-drained soils and water at depth encourage species such as ponderosa pine and skunkbush sumac to colonize areas of clinker. High permeability and infiltration rates enable the 1600 square miles of clinker in the basin to store large amounts of water from rainfall and snowmelt, and protect it from evaporation. These unconfined clinker aquifers have very high transmissivity and storativity values. Peak flows in clinker-dominated drainages are attenuated, and water is discharged gradually to streams during periods of lower flow. Springs which emerge from the base of clinker form the headwaters of several perennial streams and provide wetland habitat for many species. In the Rochelle Hills of Wyoming, groundwater from clinker recharges coal, overburden, and spoil aquifers downdip to the west. Some mines encounter inflow from large saturated clinker bodies updip, where water is dammed against the face of the less permeable coal. Groundwater quality in clinker varies widely; quality appears better in well-drained areas where soluble ash at the base of the clinker has dissolved away, and poorer where water has ponded along a coal contact.