21 meters. Strong winds (maximum velocity, 54 

 m. p. h. ) during the first half of September caused seas 

 of sufficient size to break down the thermocline com - 

 pletely and place the Great Plain in a homothermous 

 condition again, as shown by temperatures at this sta- 

 tion September 16, when the water from top to bot- 

 tom was about 19. 5°. 



The life of the thermocline in the Great Plains 

 was certainly less than 3 months and probably not over 

 2 months. It is probable that the thermocline does not 

 form in this area in years of frequent and severe sum- 

 mer storms. When this occurs, it is not likely that 

 bottom temperatures under 17° C. will be encounter- 

 ed during the summer. In such years, cold water in 

 summer would be limited to about 10 percent of the 

 area of the lake, whereas in favorable years it may re- 

 main in 50 percent of the bottom area. It may be 

 stated, then, that the frequency and intensity of sum- 

 mer storms control the extent of cold bottom water 

 which, in turn, may limit the habitat of cold-water 

 communities of the lake. 



WATER MOVEMENTS 



Currents 



During the period of the survey, the average ef- 

 fluent of the lake was, in round numbers, 240,000 

 cubic feet per second, computed as in Bulletin 37, 

 U. S. Lake Survey. This amount was above normal, 

 due to the prevailing high lake level. If the flow, in 

 passing down the lake, were confined to the upper 4 

 meters, it would have an average velocity of 0. 097 

 m. p. h. at the restricted cross section between Erie 

 and Long Point. However, the depth affected is prob- 

 ably greater than 4 meters, and although the velocity 

 at the surface in the middle of the "stream" would 

 be greater than the average velocity of the section, 

 it is nof likely that this natural eastward flow ever 

 greatly exceeds 0.1 m. p. h. , except near the inlets 

 and outlets. 



Along the Atlantic Coast local winds set up sur- 

 face currents having velocities of about 1 1/2 percent 

 of the wind velocity.!' Lake Erie is a large body of 

 water and should have somewhat similar wind ef- 

 fects. If so, a 67 -mile wind would set up a current 



1/ Current Tables, Atlantic Coast, U. S. Coast 

 and Geodetic Survey, Washington, 1930. 



of 1. m.p.h. The average summer wind velocity, 

 as obtained from continuous readings, is about 11 

 m. p. h. which would theoretically set up a cunent of 

 0. 16 m. p. h. As the prevailing wind direction is 

 southwest, the resulting current would be approximate- 

 ly east by north (deflectional effect of earth's rotation 

 on currents is to the right in the northern hemisphere). 

 This is also the direction of the general flow of the 

 lake. Consequently, there is a prevailing easterly 

 movement of the surface waters, the velocity of which 

 may reach 1. m.p. h. during severe protracted storms. 

 Not aU of the average wind is from the southwest, how- 

 ever, and therefore only a portion of the average veloc- 

 ity of the 0. 16 m.p.h. is in an easterly direction. Fur- 

 thermore, the water of the lake, being confined, does 

 not have a uniform velocity in any given wind. A 

 wind blowing along the longitudinal axes of the lake 

 would of course cause stronger currents in the restrict- 

 ed Erie- -Long Point section than in the more open 

 portions of the lake. 



Temperatures in the Great Plain indicate that the 

 natural eastward movement is mainly confined to the 

 middle one -third of the plain. Temperatures were 

 fractionally higher in this longitudinal belt than in the 

 adjacent waters on either side, and it is believed that 

 the origin of the slightly warmer "stream" is the warm, 

 shoaler waters of the western section. 



The general eastward movement of the surface 

 waters is not, of course, continuous and is even re- 

 versed during easterly blows, so the net prevailing ve- 

 locity is probably not more than 0. 1 m.p.h. It would, 

 at this rate, require 3 months for the water particles 

 to pass from the Detroit River to the Niagara River. This 

 velocity is not easily measured under ordinary field con- 

 ditions. As only part of the easterly flow passes down the 

 Niagara, there is a prevailing westerly return movement 

 of the waters below the frictional depths. 



The Shearwater anchored for current observations on 

 25 occasions during the summer of 1929. Six of these 

 observations were made at station 15 (Deep Hole) and 

 9 at station 17 (Long Point) where, with the exception 

 of the inter -island channels of the western section, 

 the strongest currents of the lake are encountered. 



When the lake level of the eastern portion is al- 

 tered, either by winds or by barometric -pressure dif- 

 ferentials, water is forced to pass around Long Point 

 in the adjustment of the level of Long Point Bay. 



56 



