/ 



igan has varied since the appearance of the first pre- 

 decessor lake 24, 000 years ago (Hough 1953, 1958) 

 from 640 feet above sea level in the Glenwood stage 

 13, 000 years ago to 230 feet above sea level in the 

 Chippewa stage 5, 000 years ago (Zumberge and 

 Potz[g]erl956). For the last 90 years the mean level 

 has been 580. 6 feet. 



The bottom topography formed by these glacial 

 events has been described in detail by Emery (1951). 

 The lake can, in general, be divided into a southern 

 basin of gentle bottom gradients and a northern basin 

 with considerable fjord -like top)ography. 



AccOTding to Sverdrup, Johnson, and Fleming 

 (1942), Ekman has examined the influence of bottom 

 topography on ocean currents. He concluded that in 

 middle and high latitudes currents tend to follow bot- 

 tom contours. He found also that a current is deflect- 

 ed cum sole when entering shallow water and contra 

 solem when entering deeper water. If Lake Michigan 

 behaves as an ocean, the bottom topography would 

 effect major modification of the circulation, espe- 

 cially in view of the extremely irregular topography 

 in the northern basin. 



Near coast lines, secondary effects of the wind 

 become important through alteration of density which 

 in turn cause littoral currents (Sverdrup, Johnson, and 

 Fleming 1942). In Lake Michigan where the ratio of 

 length to width is approximately 5:1, the littcxal cur- 

 rents become increasingly important in the general 

 circulation of the lake. 



Another effect of the confining nature of the coast 

 line is the actual piling up of water in some areas 

 and the removal of water in others when a strong wind 

 blows for a prolonged period from one direction. Ap- 

 preciable slopes ofthe water surface, which must be ad- 

 justed eventually, can be inferred in Lake Michigan 

 from water-level reccwds. 



Cofiolis Force 



The effects of the Corlolis force upon currents 

 were described in detail by Sverdrup. Johnson, and 

 Fleming (1942). If the water surface is unlimited and 

 the water is very deep in comparison with the depth 

 of frictional resistance (i. e. , the depth at which mix - 

 ing due to wind is negligible) a deflection of 45° cum 



sole of the surface current from the generating wind 

 direction is theoretically possible according to Ekman. 

 Deflections of this magnitude were actually observed 

 by Forch(1909), Galle (1910), and others. Ekman 

 proposed that in shallow water the deflection would be 

 less, and it has been found so by Wilting (1909) in 

 depths of 9 to 65 meters and by Mandelbaum (1955) 

 in less than 20 meters of water. 



Studies of the Coriolls fcsce in water movements 

 of the Great Lakes are lacking. Because depths far 

 greater than the depth of frictional resistance exist in 

 Lake Michigan, and because of the relatively large 

 surface area, it is suspected that Coriolis deflection 

 exists as has been empirically demonstrated by Ayers 

 et aL (1958). 



Hydraulic Currents 



Hydraulic currents are caused by the inflow and 

 outflow of water from a basin. Maximum inflow into 

 Lake Michigan from rivers and streams occurs gener- 

 ally in the spring and is minimum in the fall. Outflow 

 is limited to the Straits of Mackinac (55, 000 cfs. -- 

 Powers and Ayersl' ) and the Chicago drainage system 

 (approximately 3, 000 cfs. --U. S. Corps of Engineers 

 1957). It appears in general that effects of the hydraulic 

 currents in Lake Michigan are mostly local, occurring 

 at the mouths of rivers and in the vicinity ofthe outlets. 



TYPES OF DRIFT UNITS 



Three types of units were used in 1954: drift bottles 

 with metal drags (fig. 2A); plastic tubes with metal 

 drags (fig. 2B); drift envelopes similar to those de- 

 scribed by Olson (1951) (fig. 2C). Two types were 

 released in 1955--drift bottles with metal dra^ or 

 with sand ballast (fig. 2D). 



Drift Bottles 



The bottles were 4-ounce, Boston-round (4 1/2 

 inches long, 1 9/10 -inch diameter) stoppered with 

 corks, sealed with beeswax, and capped. Metal drags 

 caused the bottles to sink until only about 1/2 inch was 

 exposed. These bottles and drags were similar to those 

 used in Lake Huron and Saginaw Bay (Johnson 1958). 



1/ Powers, C.F. , and J. C. Ayers. Water transport in 

 the Straits of Mackinac region of Lake Huron. Limnol. 

 and Oceanogr. Accepted for publication in 1960. 



