eastern shore of LaJce Huron. All recoveries 

 from 200 releases at stations 9-13 were from 

 the eastern shore of Lake Huron. Apparently 

 the surface current on Lake Huron during 

 the summer and fall of 1956 had a net circu- 

 lation from west to east. 



Although the direction and intensity 

 of local winds were important in explaining 

 surface drift in Saginaw Bay, they appear 

 less significant in Lake Huron proper (fig. 

 42 D) . On August 3, 20 releases were made, 

 10 at each of the indicated stations, within 

 2 1/2 miles of one another. The large dif- 

 ference in direction of drift from the two 

 stations of bottles that were out approxi- 

 mately the same length of time and released 

 at nearly the same time indicates forces 

 other than wind at work in the formation of 

 currents. 



Certainly, wind conditions play a 

 prominent role in formations of surface 

 currents in Lake Huron. However, the rela- 

 tionship between wind and currents is not 



STATUTE MILES 



Figure 16. — Location of 22 recoveries from 

 40 drift-bottle releases at a Lake Huron 

 station. Triangle marks release point; 

 X's mark recovery points. 



nearly so obvious as in Saginaw Bay. 

 According to Millar (1952) the energy input 

 into a lake from a day's wind may not be 

 completely dissipated until 12 days later. 

 If this relation holds in Lake Huron, the 

 prevailing winds assume a prominent role in 

 formation of the general surface current 

 pattern in the lake. 



As was true in Saginaw. Bay, location 

 of Lake Huron returns from a particular 

 station can vary widely throughout the 

 season. The wide scatter of the 45 returns 

 from 80 bottles released off Harbor Beach 

 (10 bottles each at 8 different times from 

 June through October 1956) is strong evi- 

 dence of the instability of the lake cur- 

 rents (fig. 15). The drift throughout the 

 investigation from some stations, however, 

 could be much more stable (fig. 16). 



Dynamic heights 



The use of the dynamic-height method 

 of determining current flow depends upon 

 the availability of a subsurface reference 

 plane at which currents are absent. Ayers 

 et al . (1956) has indicated that Lake Huron 

 has certain characteristics concerning 

 circulation that are psuedo-oceanic. In 

 calculating the relationship between wind 

 and depth of mixing for oceans, Sverdrup, 

 Johnson, and Fleming (1942) derived the 



formula D = 7.6 



where D is 



^~Sl 



the depth in meters, W the wind velocity in 

 meters per second, and v the latitude for 

 which the calculation is made. If 44° (the 

 approximate average latitude for southern 

 Lake Huron) is substituted for <f and 6.7 

 meters per second (a common wind velocity 

 over Lake Huron during the summer) is sub- 

 stituted for W, D, or the depth of the layer 

 that is stirred up by wind becomes approxi- 

 mately 60 meters. Mortimer (1954) believed 

 that mixing (water movement or currents) 

 may occur in some Icikes to depths three 

 times that of the thermocline. The average 

 depth of the thermocline in south-central 

 Lake Huron during the summer of 1956 was 

 50 feet. In order to lessen the probability 

 of currents below the base, a depth of 60 

 meters (somewhat more than 3 times the 50- 

 foot level of the thermocline) was taken as 

 a reference plane for dynamic-height calcu- 

 lations. 



18 



