235 



450. A local increase in the strength of the current in a section of an 

 estuary therefore decreases the tidal range upstream, and retards the 

 progress of the tide up the estuary. A local decrease in the strength 

 of the current tends to increase the tidal range, and speeds up the 

 progress of the tide. The decrease in the range as the tide passes 

 through the contracted sections usually results in a less range in the 

 wider and deeper sections upstream, while because of the tendency 

 toward an increase in the wide and deep sections, the range of the 

 tide normally increases as it approaches a contracted section. The 

 larger tidal ranges are therefore found ordinarily at the contractions, 

 and the smaller ranges in the wide and deep sections of an estuary. 

 The advance of the tide up the estuary is more rapid where the tidal 

 range is increasing than it is when the range is decreasing. 



451. Dee]) channels of estuary jorm. — Submerged valleys, unfilled by 

 alluvial deposits, afford some long closed tidal channels, flaring toward 

 the entrance like an estuary, but so deep that the frictional resistance to 

 flow is very small. The convergence of the shores of the ideal estual 

 becomes less as the depth increases and the frictional resistance to 

 flow decreases. In a channel so deep that the flow is essentially 

 frictionless, cj> is so close to 90° that cot is practically zero. The 

 width of the ideal estuary then closely approximates, from equation 

 (316): 



The ideal estuary becomes an endless channel of uniform width and 

 depth. It follows, therefore, that in a closed channel of finite length 

 the tides maintain a constant range, and the currents a constant 

 strength, only when the channel is so shallow that the frictional 

 resistance to flow is material. When its depth is so great that the 

 frictional resistance is negligible, the tides and currents take the 

 general characteristics of those produced by frictionless flow in a 

 closed canal, discussed in chapter VII. The wave lengths of the 

 principal tidal components become so long in a deep channel (par. 326) 

 that the length of nearly all natural channels is but a fraction of these 

 wave lengths, and the range of the tide characteristically increases from 

 the entrance to the head. If the effective length of the channel approxi- 

 mates one-quarter of the component wave lengths corresponding to its 

 depth, the range of the tide at the head of the channel may be very 

 large. In timing, the tides approach the condition of a stationary 

 wave, which rises and falls simultaneously. 



452. Tides and currents in the Bay of Fundy. — The Bay of Fundy, 

 on the Atlantic coast of Canada, just north of the State of Maine, 

 affords the outstanding example of the heights to which the tide may 

 rise at the head of a fairly deep natural channel. An interesting 



192750—40 16 



