Characterization and mapping of buoyant river plumes in southern Lake Michigan.

摘要

Data from field surveys and stream gages were analyzed from southern Lake Michigan in order to characterize the physical, biological, and chemical environments associated with river plumes. Five Lake Michigan tributaries were examined: Burns Ditch (IN); Trail Creek (IN); St. Joseph River (MI); Grand River (MI); and Muskegon River (MI). The field surveys indicate that specific conductance is a robust parameter for distinguishing river water from lake water for most of these southern watersheds. For most of the year, the plumes were found to be buoyant, with the exception of a brief period in late fall when air (and hence river) temperatures typically fall below the receiving lake water temperature. Field surveys indicate that the lift-off point for stronger buoyant plumes is consistently close to the river mouth, which agrees reasonably well with analytical predictions. For tributaries with very small flowrates, the plume lift-off point is upstream of the river outlet, causing an estuary-like environment in the river with buoyant river water atop colder lake water. The vertical extent of the surface plumes is found to be modest, less than 2 m thick at most in-lake locations. The horizontal extent of the plumes is also found to be relatively small, ranging from 0.13 to 8.5 km2 in area, suggesting in-plume residence times of 0.2 to 21.8 hours. Application of recently-developed plume classification charts for wind-affected buoyant plumes indicates that most buoyant plumes are expected to be shore attached, offshore spreading, or diffuse offshore spreading.