the lake, disturbing the hydrostatic equilibrium and necessitating an almost 

 continuous record to approximate the true mean lake level. The effect of the 

 disturbing agents is felt long after they have ceased to operate, so that, 

 even in their absence for a time, the level at any one point continues to 

 fluctuate. 



Atmospheric pressure is the agent responsible for most of the distur- 

 bances. '(rtJhen the atmospheric pressure is different on different parts of the 

 lake, water is forced from the area of high pressure to the area of low pres- 

 sure. According to Hayford (1922), this direct effect of pressure, while by 

 09 means negligible, is less important than its indirect effect in producing 

 viinds. Waves are the most obvious disturbances of the surface resulting from 

 wind action. Wind is also a powerful agent in setting up currents to leeward, 

 where the water piles up against the shore. Seiches prolong the time during 

 which disturbances resulting from differential atmospheric pressure and winds 

 affect the lake, and currents from any cause have a tendency to disturb the 

 normal hydrostatic equilibrium although the effect is probably not great for 

 a majority of currents. Tide;, also have the same tendency, but Hayford 

 regarded them as of minor importance. In the more detailed discussion of 

 transient fluctuations which follov;s, dif Te^rential atmospheric pressure and 

 winds are treated in the sections on seiches and currents. To discuss them 

 separately would result in undue repetition. 



Waves 



Up to the present, the effects of transient disturbances on determina- 

 tions of the mean lake level alone have been mentioned. Because of the con- 

 struction of the automatic level gauges, waves do not affect such determina- 

 tions, but they have other effects of importance. The extreme shallowness 

 in the western part of the lake tends to make large waves break and mix the 

 water to considerable depths. Since currents also result in mixing, it is 

 impossible to determine how much is due to waves, but it seems probable that 

 in Western Lake Erie this factor alone would be sufficient to explain occa- 

 sional complete mixing from top to bottom. The mixing results in the usual 

 homothermous condition of the water, permits free interchange of gases, and 

 prevents long-continued stratification of the completely passive plankters. 

 The violent action of waves on shores and reefs tends to break up the colon- 

 ial algae and undoubtedly causes the death of many delicate organisms. Waves 

 also add to the turbidity of the water, especially in the very shallow areas. 



There have been no exact measirrements of waves on Lake Erie; in fact 

 the only accurate data available for the Great Lakes are those of Gaillard 

 (190ii, p. 81) for Lake Superior at Duluth. The largest waves observed by 

 him in the ship canal had a height of 23 feet (7 meters) and a length of 275 

 feet (8U meters) . From the accounts of navigators, Gaillard estimated that 

 at rate intervals in the deep water of Lake Superior there are waves 2C to 

 25 feet (6.1 to 7.6 meters) in height and 275 to 325 feet (8U to 99 meters) 



37 



