estimate due to obvious field sampling difficulties and variability of 

 discharge rates and storms with time of season). Along the north shore of 

 the lake the estimated average annual yield is 1.1 ton/km 2 , a relatively 

 low figure due to geology (basically igneous and Precambrian strata), soil 

 types, vegetation and land use. Along the south shore, where Pleistocene 

 lake development sequences left a thick, exposed layer of easily erodible 

 red clay, estimates range in excess of 6.8 ton/km 2 (Great Lakes Basin 

 Commission, 1975). Thus, the subject of turbidity is basically one of red 

 clay erosion along the Lake Superior south shore. 



Of particular concern from the viewpoint of sediment erosion control 

 has been the question of ultimate source of such lake turbidity: tribu- 

 tary inflow, lake shore erosion by high water levels, storm activity, and 

 resuspension of previously deposited clay. The latter source is of signi- 

 ficant debate considering the circulation patterns of Lake Superior. 

 While the inflow and outflow rate of this lake is small in comparison to 

 the water mass, the lake water is not standing still. It is kept in con- 

 stant motion principally by the wind, which not only generates the visible 

 surface waves (in excess of 4.9 meters), but stirs and mixes the water 

 throughout the lake. Both water movements and rates of mixing are in- 

 fluenced by the formation of temperature (and associated density) thermo- 

 clines. In the summer, Lake Superior becomes divided into an upper layer 

 of warm readily circulating water, the epilimnion, and the lower layer of 

 cold, relatively undisturbed water, the hypolimnion. The contact between 

 these two zones where rapid temperature changes takes place is termed the 

 thermocline. When the lake is stratified, the hypolimnion is essentially 

 physically and chemically isolated from the remaining water. In Lake 

 Superior, nearly 95 percent of the lake's volume is in the hypolimnion 

 (Federal Water Pollution Control Administration, 1969). The summer strati- 

 fication begins to develop in mid-July, with the epilimnion reaching its 

 maximum temperature of approximately 21 °C in August. In the winter 

 months, the lake can be considered, for all practical purposes, to be iso- 

 thermal . 



Because currents in the lake are motivated principally by wind, and 

 the winds are variable, the horizontal movements of lake waters exhibit 

 infinite variety, and frequent changes in both direction and speed. The 

 net circulation is counter clockwise, with the possibility of large cy- 

 clonic eddies occurring in the western arm (Great Lakes Basin Commission, 

 1975). Upwelling occurs in the lake when winds cause horizontal surface 

 movement of water away from the shore and the surface waters are replaced 

 by colder, deeper water (Upper Lakes Reference Group, International Joint 

 Commission, 1977). 



Considering such currents, storms, and precipitation cycles, it has 

 recently been estimated that lake turbidity is primarily due to shoreline 

 erosion of lacustrine clay by storm currents. The most recent available 

 data (Sydor, 19/5) indicates rates for the three causes to be: 



1. Tributary erosion into Lake 



Superior: >bU0,0UU metric tons per year 



137 



