Use of this method to determine 

 currents was limited because records were 

 available from only one transect for any 

 one day. For best results a large number 

 of stations over an area should be avail- 

 able so that dynamic-height contours can 

 be drawn with the Kinimura of uncertainty. 

 The dynamic heights at points along the 

 transect gave only an approximation of the 

 initial direction of flow of current. For 

 instance, if the dynamic heights indicated 

 a northerly component, current flow could 

 actually be northeast, northwest, or even a 

 mere fraction of a degree north of east or 

 west. The use of this method in conjunction 

 with drift bottles was of some value in 

 determining possible surface-current flow 

 in the lake (figs. 17 A-D, see page 20). 

 This analysis indicates that surface-current 

 patterns may be much more complicated than 

 has been realized. 



The dynamic-height method in surface- 

 current calculations might not be applicable 

 in and around the littoral zone. On two 

 occasions off Canadian shores, August 3, 

 1956 (fig. 17 B), and October 27, 1956 (fig. 

 17 D) , calculations indicated currents near 

 shore to be to the south but bottles re- 

 leased in the area on these dates drifted 

 to the north. The possibility exists, how- 

 ever, that the bottles at first drifted to 

 the south but later their direction of drift 

 was reversed. 



topography of the free surface but may show 

 a number of features which, instead of 

 being associated with the general distribu- 

 tion of mass, are brought about by the 

 presence of internal waves. In view of 

 this circumstance which, so far, has not 

 received great attention, conclusions as to 

 general currents based on charts of geopo- 

 tential topography should be used with even 

 more reservation than has been previously 

 emphasized." However, in support of the 

 dynamic-height method of determining cur- 

 rents they remjirk that, "So mamy reserva- 

 tions have been made that it may appear as 

 if the computed currents have little or no 

 relation to the actual currents. Fortu- 

 nately, however, most of the assumptions 

 made lead only to minor errors, and currents 

 can be correctly represented in the first 

 approximation by means of the slopes of a 

 series of isobaric surfaces relative to one 

 reference surface." 



Appraisal of the general pattern 



Although no one characteristic current 

 system is indicated for Lake Huron, the 

 following general remarks concerning surface 

 circulation in Lake Huron in 1956 seem per- 

 tinent: There is a general but highly 

 variable west-to-east drift; the most highly 

 developed west-to-east drift occurs during 

 August and September; there is some inflow 



The rate of change in the 

 dynamic heights is still a mat- 

 ter of conjecture. Prominent 

 changes over a period of appro- 

 ximately 5 weeks resulted in 

 new current patterns as shown 

 by figure 17 (A-D). Variation 

 from one day to the next was 

 so small that no significant 

 change in current pattern re- 

 sulted (table 6). Over a 

 period of days, however, the 

 total of these small differ- 

 ences produced the prominent 

 changes noted between cruises. 



What effect internal 

 waves in the Great Lakes have 

 upon the geopotential topo- 

 graphy should be resolved. 

 According to Sverdrup, Johnson, 



and Fleming (1942), " charts 



of geopotential topography may 

 not represent the average 



Table 6. — Dynamic heights In meters (reference level 60 meters) 

 for stations on the Harbor Beach, Mlchigan-Goderich, Ontario, 

 transect on successive days during three months of 1956 



19 



