SURFACE CURRENTS IN LAKE MICHIGAN. 

 1954 AND 1955 



INTRODUCTION 



Before 1954 the only publication describing the 

 major surf ace -current circulation in Lake Michigan 

 was by Harrington (1895). Subsequent investigations 

 were either local or of limited scope (Crohurst and Vel - 

 dee 1927; Deason 1932) oiwere discussions of Harring- 

 ton's conclusions (Townsend 1916) and statements 

 as to the inadvisabiUty of studying the lake currents 

 as a whole (Judson 1909). The recent analysis of syn- 

 optic surveys conducted in Lake Michigan in 1955 

 not only contributed to knowledge of the lake cur- 

 rents (Ayers et al. 1958) but also inttoduced oceano- 

 graphic procedures and interpretations into the study 

 of the circulation of the lake. 



The present study of the circulation of Lake Mich - 

 igan was part of the 1954-1955 fishery -Umnological 

 survey conducted by the M/V Cisco of the U. S. Bureau 

 of Commercial Fisheries, Fish and WildUfe Service. 

 The 1954 work was confined to the central and south- 

 em part of the lake and the 1955 survey was conducted 

 in the central and northern area (fig. 1). Smith(1957) 

 who commuted briefly concerning the use of drift 

 units and the determination of lake currents stated from 

 preliminary analyses of the datathat the major surface- 

 current struaureofLake Michigan can change over a 

 3 -week period. This repOTt completes the analysis of 

 returns of all drift units released in Lake Michigan in 

 1954 and 1955. 



John Ayen of the Great Lakes Research Institute 

 made suggestions in the jweparation of the manuscript; 

 Anthony PerUck and Robert Wetzel drafted the figures; 

 and the U. S. Coast Guard made wind records available. 



GENERAL FEATURES OF LAKE MICHIGAN 



Lake Michigan, the third largest of the Great Lakes 

 (22. 400 square miles) lies wholly within the United 

 States. The maximum depth is 924 feet, the mean is 

 276 feet, and the channel depth of the Straits of Mack- 

 inac is about 100 feet (Emery 1951). The long axis 

 of the lake, approximately 325 miles. Ues in a UOTth- 

 northeast to south -southwest direction. Excluding bays 

 and Inlets, the maximum east -west width of 85 miles 

 is about 43-00 • north latitude approximately on a line 

 from Grand Haven. Michigan, to Milwaukee, Wisc<msin 



CURRENT -GENERATING AGENTS AND 

 MODIFYING FACTORS 



The circulation of Lake Michigan is an inte- 

 gration of the effects of generating agents (wind and 

 density distribution) modified by morphology of the 

 lake, the Cotiolis force, and hydrauUc currents. 



Wind 



The wind creates currents in two ways. First, 

 the most direct and obvious effect is the formation of 

 shallow wind drift; second, the transport of ir«ex by 

 the wind stress alters the density distributi<Hi and, c«mi- 

 sequently, the currents. Movements of water in Lake 

 Michigan in the Calumet district of Illinois and Indiana 

 are considered to be wind drifts (Crohurst and Veldee 

 1927). Similarly, the prevailing westerly winds over 

 Lake Michigan have been considered the cause of the 

 general west-to-east drift of bottles across the lake 

 (Harrington 1895; Deason 1932). For the particular 

 years and area in which these investigators worked, it 

 is possible that westerly winds did prevail. Recent evi- 

 dence indicates, however, that in southern Lake Mich- 

 igan from May through August winds tend to be variable 

 and not from any one prevailing direction (Cooperman, 

 et^. 1959), 



Density 



Ayers et ^. (1956, 1958) have demonstrated that 

 density currents are common in the Great Lakes. They 

 have been careful to point out that extreme caution 

 must be used in determining circulati<m from the distri- 

 bution of density because of the ease with which the 

 cfrculatlon can be changed by wind stress. At times it 

 is impossible to teU whether the distribution of density 

 causes the cnnents, or the currents cause the distribution 

 of density (Sverdrup, Johnscm, and Fleming 1942). 



Morphology 



A most important but little nderstood factor in 

 the circulation of Lake Michigan is morphology. The 

 Great Lakes* basins acquired their major topographic 

 features in pre -Pleistocene time by subaerial erosion 

 and stream development actingon a variety of bedrock 

 formations (Hough 1953, 1958). The level of Lake Mich- 



