July 30. As this measurement was made with a cur- 

 rent meter, the value is probably excessive. General 

 westerly winds had prevailed during the 3 preceding 

 days. 



Since the subsurface current observations shown 

 in table 8 were made with a current meter suspended 

 from a boat, the tabulated values are too high (see 

 under "Methods"). One can only state, then, that 

 subsurface currents were witnessed at stations 15 and 

 17, and that the velocities were less, probably by as 

 much as 50 percent, than the tabulated values. That 

 the bottom waters are not physically stagnant is also 

 apparent from the fact that the coldest bottom tem- 

 peratures were sometimes found to the eastward and 

 sometimes to the westward of station 15, the deepest 

 part of the lake. 



From the observed data and information collected 

 verbally during the survey, it may be concluded that 

 current velocities exceeding 1 m.p.h. are extreme- 

 ly rare in Lake Erie. 



Subsurface Movements of the Deep Hole Waters 



The movement of the bottom water of the Deep 

 Hole was first witnessed during the 1928 survey when 

 the eastern limit of 5° C. water advanced 20 miles 

 from July 27 to August 15 and then returned, 2 weeks 

 later, to the west. This eastward movement was 

 greater than was observed during the 1929 survey and 

 is all the more remarkable because of the compar- 

 atively small area of cold water remaining in 

 August. 



Table 9 gives the area included within the 6° 

 C. isotherm on each of the 1929 cruises when the 

 limits were located, and also shows the relative po- 

 sition of the cold-water mass. The 6° isotherm was 

 arbitrarily chosen as the most suitable temperature 

 to show these movements graphically. The 7° and 

 8° isotherms would have the same general shape and 

 would include but slightly greater areas during the 

 summer months. Isotherms of less than 6° do not in- 

 clude a sufficient area, especially in the 1929 season, 

 when the bottom temperatures were slightly higher 

 than in 1928. Additional 1929 stations were there- 

 fore selected to permit a more accurate development 

 of the 6° curve. 



There is an area of 300 square miles in which 

 the bottom temperature on all of the 1929 cruises 



Table 9. --Area and position of 6° C. 

 bottom water in 1929 



remained below 6°, regardless of whether the cold- 

 water mass extended to the eastward or to the west- 

 ward. The boundary of this area of "perpetual" cold 

 water is approximately the 40 -meter depth curve, and 

 its center is shown by the (+) in figure 16. When the 

 6° water is more or less evenly distributed (horizon- 

 tally) about this center, its position is herein called 

 "normal, " and when the distribution is eccentric, the 

 position is termed eastward or westward. Due to sum- 

 mer warming, the area of 6° bottom water in Septem- 

 ber was only one -third as great as in the latter part of 

 May (table 9). Therefore the movement of the cold 

 water as shown by the eastward and westward Umits 

 on the various cruises is subject to correction accord- 

 ing to this seasonal shrinkage. Cruises 4 and 5, 1929, 

 were only 2 weeks apart and, as the area did not 

 change greatly during that interval, the 6° isotherms 

 of these cruises were chosen as best depicting the 

 actual movement and are shown in figure 16, laid 

 down over the depth curves. The curve of cruise 2 is 

 also shown as an example of the normal position of the 

 cold-water mass, although the area on cruise 2 was 80 

 percent greater than that on cruise 5. When the cold 

 water was eastward (cruise 5) the southwest portion 

 of the isotherm lay approximately on the 40 -meter 

 curve but on cruise 4, when the cold water was west- 

 ward, the northeast portion lay along the 40-meter 

 curve. Similarly, the aforementioned area of "per- 

 petual" cold water was not crossed by the 6° isotherms 

 on any of the cruises. 



The isotherms of figure 16 show a horizontal 

 movement of 16 miles in 2 weeks on the west side 

 and 5 miles on the east side. This was the maximum 

 movement measured in 1929. The 1928 survey did 

 not include the western limits of the cold water, but 

 the eastward limit on August 15, 1928, is shown in 



59 



