SAND-WAVES IN TIDAL CURRENTS 339 



of waves with definite length, and the ridges are 

 separated by a distance which is often one hundred 

 times as great as their height. It is of interest, 

 however, to note the height of these ridges in 

 relation to the speed of the current. Taking an 

 average of the values found for the wide and 

 narrow parts of the river respectively, we find that 

 a current of 4- 6 5 feet per second, with an average 

 depth of 36 feet, maintained ridges of a height 

 of 4 feet 6 inches. A velocity of 2- 93 feet per 

 second observed in the River Dovey gave a height 

 of sand-wave 97 inches, so that, reckoning the 

 height as proportional to the square of the velocity, 

 a current of 4-65 feet per second should give a 

 height of only 2 feet. The greater height of the 

 sand-reefs in the Mississippi is, I think, mainly 

 due to their not being true waves but ridges not 

 at right angles to the current. Thus they are main- 

 tained partly by lateral scour, and are not wholly 

 dependent for their upkeep upon the pumping 

 action of an eddy with horizontal axis. 



