| The potential adverse geochemical impacts of discharging coalbed methane (CBM) product water into stream drainages are well recognized and reasonably well studied. However, not well recognized or understood are the impacts of heat commonly conveyed with CBM product water (pumped from underground coal beds) entering the Powder River and its tributary streams. This study shows that heat transported with CBM product water has an annual visible impact on the thermal balance of the Powder River during winter. However, the long-term effects on the river and its ecology are unclear. The study, conducted over two winters (2009-2010 and 2010-2011), entailed detailed surveys at two representative sites where CBM water was discharged into the river. Besides adding to river's flow, the most visible influence of CBM water discharge was the frequent formation of lengthy open-water leads extending along a channel bank typically for several kilometers along the river. The observed leads, which persisted throughout the two winters, were three to seven meters in width. An analysis shows that, for constant values of air temperature and CBM water temperature discharged, the surface area of the open-water leads scales with the discharge rate of CBM water. The leads comprised a form of buoyancy current flowing in the river, cooling and eventually dissipating when exposed to frigid air. Lead presence altered flow distribution, concentrating flow along the lead, causing modest scour of the bed and, at some locations, accelerating bank erosion. Because the bed at one site scoured down to expose rock, it presently is unclear whether deeper bed scour would have occurred there. The magnitudes of the measured channel changes were determined to be less than those typically caused by spring ice cover breakup and the larger spring flows conveyed by the river. Possible ecological aspects of lead formation are recommended as a topic of further research. The report additionally provides suggestions on how to manage lead formation, should further research on ecological influences indicate that lead extent should be minimized. Lead size can be reduced by several actions that decrease inflow water temperature and promote greater transverse mixing across the river. In addition, the study provides insights into winter fluvial processes in Wyoming streams. |
