energy inparted to the system is dissapated by friction, mostly with the 

 solid earth. Persistence o£ the motions is a function of scale — motions 

 on a scale of inches may die off in a period of minutes, and those on a 

 length scale measured in feet may die off within an hour, but motions 

 whose scale is measured in miles may persist for days after their generat- 

 ing cause has been terminated. 



Currents are modified by the basin shape and the density stratifi- 

 cation of the water. In most freshwater lakes, the major factor con- 

 trolling water density is water temperature. For freshwater, maximum 

 density occurs at a temperature near 39°F (4°C). When the temperature 

 is warmer than 39°F, it is common that the temperature increases upward 

 through the water column and there is little mixing between water layers 

 of different temperatures. For most purposes, it can be assumed that 

 water cannot flow through the boundaries of the basin. Also, water 

 tends to flow along surfaces of constant density and when flow exists in 

 one direction a compensating flow in the opposite direction must occur 

 somewhere within the basin. Thus near basin boundaries, the current 

 direction is necessarily very nearly parallel to the boijndaries, regard- 

 less of the direction of the generating force. Near the bottom, flow 

 may be forced parallel to the boundaries of underwater valleys because 

 flow in other directions would tend to ipset the density stratification 

 of the water. IVhen surface water is forced to flow away from a shore, 

 continuity of the ivater mass is maintained by the ipwelling of cold 

 water. This process is the primary cause for the cold surface current 

 near the west coasts of both the American and African Continents . 



2. Examples of Wind- Generated Currents . 



An example which shows that wind generates currents in directions 

 other than the wind direction can be visualized by considering a long 

 narrow basin filled with water of constant density. If a uniform steady 

 wind blows across the basin in a direction which makes an acute angle 

 with the sides of the basin, friction between air and water will tend to 

 generate a current in the direction of the wind. However, this current 

 will be constrained by the sides of the basin and thus the actual surface 

 current will be parallel to the boundaries of the basin in the general 

 direction of the wind. A return flow at some greater depth will be 

 generated in order to maintain continuity for the mass of water in the 

 basin. Therefore, the wind generates two currents in opposite directions, 

 neither of which is parallel to the wind direction. 



The example described provides a qualitative understanding of the 

 seiches in Lake Erie. Winds blowing from a generally west- southwesterly 

 direction lead to low water levels in the western part of the lake and 

 high water levels in the eastern part of the lake. Based on more detailed 

 analyses, it is possible to obtain quantitatively accurate predictions of 

 the flow. Studies pertaining to Lake Erie have been made by Henry (1903) , 

 Keulegan (1953), Harris (1954), Platzman (1963), Irish and Platzman (1962), 

 Harris and Angelo (1963), and Richardson and Pore (1969). 



