156 



exceeds the duration of the flood by more than 2)^ hours. A study of 

 the figures shows tliat tlie differences in the durations of the flood and 

 ebb currents is to be ascribed principally to the larger areas of the 

 cross sections of the river during the flood; because of which the tidal 

 prism is filled in a shorter period. The fresh-water discharge evi- 

 dently is insufficient to have any large effect upon the tidal flow 

 except in the upper reaches of the river. 



314. Other characteristics of the flow in the estuary, such as the 

 relative timing of the tides and currents, the average and maximum 

 discharges and currents, are quantitatively and defmitely brought 

 out by the diagrams at the successive stations. The total volumes 

 of the inflow and outflow at the stations during the tidal cycle are 

 readily derived by measuring the areas between the instantaneous 

 discharges and the hne of fresh-water discharge. 



315. Conclusion. — The cubature of a tidal channel affords complete 

 and rehable data on the cUscharge and mean velocities at successive 

 stations along the channel. It is perhaps the only means by which 

 a satisfactory determination of the discharge in the wide sections in 

 the lower part of an estuary may be secured. On the other hand the 

 cubature of a long tidal channel is a costly undertaking. It affords 

 no information on the distribution of the velocities in a cross section 

 of the channel, or of the distribution of the flow through the channels 

 on either side of islands and through other secondary channels. 

 Direct measurements of the current velocities in the sliip channel of 

 an estuary are of far greater value to navigators than most refined 

 computations of the mean velocities throughout the entire cross 

 section, and are more readily made. The proper design of training 

 works also may depend principally upon the distribution of the 

 velocities in the cross section. For these reasons, extensive cubature s 

 of tidal channels have not often been made. Nevertheless the com- 

 plete and convincing data afforded by a detailed cubature of the tides 

 in a channel is of such value in the planning of works dependent upon 

 the discharges and velocities that its cost is fully justified when major 

 works of this character are under consideration. Thus the applica- 

 tion of the piinciples of cubature to the estuaries of the Sacramento 

 and San Joaquin Rivers in California afforded information essential to 

 the study of a proposal to construct, at great cost, a barrier dam to 

 prevent the intrusion of salt water mto the lower reaches of these 

 rivers. The cubature of the Delaware River has corrected misconcep- 

 tions of the inffuence of fresh-water discharge upon its tides and 

 currents, and has contributed to the measures by which the large 

 expense of maintaining the ship chamiel in the river has been greatly 

 reduced. 



In summary, a cubature of an estuary affords much desirable infor- 

 mation, but is not warranted imless the information is worth its cost. 



