Exchange of Water Between the Keweenaw Waterway and Lake Superior: Characteristics and Forcing Mechanisms

The Keweenaw Waterway is a navigation passage with openings to Lake Superior at either end. Analysis of wind data and moored thermistor measurements indicates that movement of water into and out of the waterway is strongly influenced by surface winds through at least two mechanisms. One is direct forcing of waterway flows by the surface wind stress. The other is flow driven by pressure gradients arising from wind-driven changes in coastal lake level. Not surprisingly, the relative importance of direct wind forcing in driving near-entry flows appears to diminish with increasing depth. Also indicated is a tendency for winds to generate counterbalancing flows at the two entries, with wind-driven inflow at one entry matched by wind-driven outflow at the other entry. On a number of occasions, temperature data indicate flow out of the waterway entering a coastal water column modified by upwelling. This is presumably because upwelling favorable winds tend to depress coastal lake level, and create a pressure gradient driving flow out of the waterway. INDEX WORDS: Large lake—small lake exchange, inlet dynamics. *Corresponding author. E-mail: [email protected] INTRODUCTION Embayments connected to large lakes are important components of the large-scale systems in which they are embedded. For example, large embayments serve as nursery areas for various fish species (Boehlert and Mundy 1988) and are areas of seasonal recruitment for coastal zooplankton that hatch from resting stages (Kerfoot et al. 1999). Embayments also can be conduits for the passage of materials of terrestrial origin that make their way to the large lake environment. In the study reported here we examine the movement of water into and out of the Keweenaw Waterway, a navigational passage that cuts across Lake Superior’s Keweenaw Peninsula. The data employed consist of records from thermistors deployed at various locations throughout the waterway and measurements of wind velocity from meteorological stations located on Lake Superior. Although this data set is relatively sparse, especially by modern standards, it is the first set of measurements to allow quantitative investigation of water mass exchange at the waterway’s entries. In the sections to follow, we offer a description of the frequency and hydrographic character of Lake Superior water intrusions into the waterway, and investigate mechanisms by which surface wind may affect the movement of water into and out of the waterway. We first offer a brief description of the study area and measurement techniques.