Evidence for the inherent unsteadiness of a river plume: Satellite observations of the Niagara River discharge

We ; present satellite data from the Niagara River plume showing that a large eddy, or bulge, forms and grows in the plume near the mouth of the river. The plume consists of a semicircular bulge region immediately offshore of the mouth and a narrow current that propagates east along the shoreline. During the low-wind period from 27 to 29 May 1999, the width of the bulge more than doubled and the current width increased only slightly. Approximately one-third of the Niagara River water accumulated in the bulge near the mouth during this period, implying that the transport rate in the shore-parallel current was reduced to two-thirds of the river discharge rate. The effective radius and the growth rate of the bulge computed from the satellite images was in good agreement with previous laboratory results and a numerical model of the Niagara River plume. This is the first field evidence of the unsteadiness of the bulge. When buoyant water is discharged from a large-scale river into an otherwise quiescent basin, it is turned to the right (in the Northern Hemisphere) by the Coriolis force and eventually forms a current along the right-hand coast. This is observed in numerous river plumes (Masse and Murthy 1992; Hickey et al. 1998) and in large ocean inflows (Lanoix 1974). Traditionally, models have assumed that the coastal current transports all of the river water away from the mouth and, therefore, that the plume is in steady state. Idealized numerical, laboratory, and analytical studies, however, predict that a growing anticyclonic eddy, or bulge, forms at the river mouth (Nof and Pichevin 2001; Fong and Geyer 2002; Horner-Devine et al. 2006), contradicting the steady-state assumption. In these models, the freshwater transport in the coastal current is lower than the river discharge, implying that some of the river water returns to the bulge instead of flowing down the coast. Thus, the alongshore transport and retention of the plume water near the mouth differ significantly in an unsteady plume in comparison with a steady-state plume. The reduced flow in the coastal current may have important consequences for transport of sediments, nutrients, and contaminants typically carried by river water since the coastal current is the primary means of removing river water from the vicinity of the river mouth. Although field evidence of a bulge has been documented in some river plumes, no study has confirmed the growth, and hence, unsteadiness, of the bulge in nature. Determining whether or not the bulge is indeed unsteady is difficult with in situ data since synoptic measurements of a feature of this size are challenging. Additionally, variability due to winds and tides may obscure the relatively smaller change in size associated with bulge unsteadiness. In this paper we use satellite data to present evidence of a growing bulge in the Niagara River plume in Lake Ontario and compare the observed bulge growth with idealized laboratory and numerical modeling experiments. The Niagara plume is a convenient choice for testing the unsteady behavior observed in idealized models since tides are minimal in Lake Ontario. In addition, we select a time period when the winds are relatively low.