SAND- WAVES IN TIDAL CURRENTS 335 



form, the whole profile may gradually move down- 

 stream. . . ."I 



The average dimensions of the sand-reefs are 

 as follows : — 



At Bullerton (Wide River). At Fulton (Narrow River). 



4-4 feet 47 feet. 



300 „ 600 „ 



13 n 22 



10 to 30 „ 45 to 60 „ 



3'5 feet per second 5*8 feet per second 



Height of reefs 

 Distance apart 

 Daily motion 

 Depth of water 

 Velocity of river 



On the nearly flat tops of these reefs series of 

 regular sand-waves of about 1 5 feet length are 

 often observed, and these are at right angles to 

 the current. 



Although the travelling reefs, wholly or partly 

 on account of the windings and the irregular cross- 

 section of the river, are not true groups of travelling 

 waves definite in length and permanent in form, 

 yet the detailed examination to which they have 

 been subjected throws light upon some problems 

 of sand-waves. The engineers report that it seems 

 probable that the reefs are forms which freshly 

 deposited sediment tends to assume under the action 

 of a current of sufficient velocity. The heavier 

 portions of the eroded material taken from their 

 weather slopes are deposited on the lee of the 

 ' Report of the Mississippi River Commission, 1882, p. 108, 



13 



