MOVEMENTS OF THE LAKE WATERS 493 



Waves. — The same wind that generates a surface drift that becomes 

 a longshore current where it impinges on the shore also develops waves. 

 At a few points the water of the wave may roll up on the beach at the 

 shoreline and run directly back again, but only at those points where 

 the shoreline is parallel to the wave front. On the Great lakes, where 

 the shorelines sweep in great open curves, with chords often from 4 to 8 

 miles in extent, the wave front very frequently advances at such an angle 

 to the shoreline that the wave rolls up the beach obliquely. Where this 

 happens the water never returns by the same path that it came, but 

 runs off obliquely; so that material on the beach tends to travel along 

 the shore by a zigzag path, the angle at which the wave strikes the shore 

 determining the angle between any two limbs of this path of travel.^ 



Interrelations, waves, and currents. — Along a stretch of lake shore 

 M^here the waves are impinging obliquely and where a number are always 

 breaking at once, the tendency will be for the waves themselves to gen- 

 erate a longshore current flowing in the direction in which the bisector of 

 tlie acute angle between the wave front and the shoreline points. This 

 wave-generated current is always accordant with the longshore drift cur- 

 rent caused by the same wind, and as they operate together in shore 

 transportation they may be referred to simply as the longshore current. 



After the wind which has started the waves and currents has ceased 

 to blow, the swells still continue for some time, and even after the swells 

 have ceased to be perceptible the longshore wind current remains quite 

 strong, the momentum which the water acquired dviring the period of 

 storm not being expended for some tune after the storm has ceased. It 

 not infrequently happens that a new wind from a different quarter may 

 spring up and start to generate a current in an opposite direction. This 

 affects the surface waters first, while the lower water still retains the 

 motion in the original direction; also the water immediately along the 

 shore is affected to the bottom some time before the deeper water offshore 

 has had its momentum destroyed and "its direction of flow changed. 



TRANSPORTATION DVRINa STORMS BY WAVE-O-ENERATED LONGSHORE 



CURRENTS 



In the shifting of materials along the shore the only effective agents of 

 transportation are the wave-generated longshore currents and the waves 

 associated with them. Transportation of all but the finest materials 

 ceases as soon as the swells disappear, and is at its maximum at the time 

 the waves are largest. The longshore currents are not of themselves 



° See discussion by Sir Sanford Fleming : "Toronto harbor, its formation and preserva- 

 tion." Canadian Journal, vol. ii, 1853-1854, pp. 105-107 and 223-230. 



