The Niagara River discharges into Lake Ontario at a typical rate of 6500 m3/s, and is the lake's principal source of suspended and dissolved materials. The discharge plume is frequently warmer and thus more buoyant than the surrounding lake surface water. Using thermal satellite imagery collected over a 4-year period, it was found that the plume executes an anticyclonic turn after exiting the river mouth in 53% of the images in which the plume is visible, while in 26% of the images the plume moves offshore. The cyclonic plume was the least common trajectory occurring in only 9% of the images. Temperature and velocity data are used to examine the buoyant anticyclonic plume. Estimates of the bulk shear Froude number suggest that the plume is unstable at the offshore edges of the turning region and stable in the interior. The stability of the interior is enhanced by the presence of a strong seasonal thermocline directly beneath the plume. The momentum balance within the plume is examined using a local coordinate system. Near the river mouth the plume is vertically mixed to the bottom and the large initial flow deceleration is balanced by bottom stress. Away from the river mouth the principal balance is between Coriolis acceleration, relative acceleration (inertial turning), and the cross-stream baroclinic pressure gradient. ¿ American Geophysical Union 1992