240 



As pointed out in paragraph 155, these deformations of the tide 

 curves are reproduced by overtides and compound tides whose periods 

 are multiples or sums or differences of the periods of the principal 

 tidal components. The corresponding deformation of the velocity 

 curve at a tidal station is the accumulated effect of the tidal distortions 

 at the stations upstream, upon the rate of tidal storage and release of 

 water, and is consequently greater than the deformation of the tide 

 at the station. A typical shape of the velocity curve at Philadelphia 

 was shown on figure 50, page 155. 



461. It should be noted that the deformation of the tides and cur- 

 rents as they travel up an estuary is due primarily to the difference 

 between the depth in the channel at high and at low tide, and depends 

 therefore on the ratio of the tidal range to the mean depth. Although 

 the deformation ma}^ be increased because of the stronger ebb and 

 weaker flood currents resulting from fresh-water discharge down the 

 estuary, the latter is not the essential reason for these deformations. 



462. Slope of mean river level. — Since in each section of an estuary 

 the ebb current runs out at the lower tidal stages and the flood current 

 runs in at the higher stages, the frictional resistance to the flow of the 

 ebb is greater than that to the flow of the flood current. As a con- 

 sequence the mean river level in an estuary has an upward slope from 

 the sea, even though the fresh- water flow is negligible. In a channel 

 deep enough to be navigable by ocean shipping at low tide, this slope 

 is very small. In a shallow estuary it may be considerable. 



463. The tidal bore. — The successive instantaneous profiles in a 

 tidal estuary show the water surface advancing up the channel as a 

 long wave, outwardly resembling, in a general way, the advance of a 

 wind wave toward the shore. In nearly all estuaries the slope of the 

 front of the advancing wave is very small. This slope steepens as the 

 depth of the channel decreases, and as the currents increase with the 

 rate of rise of the tide. The rate of rise of the tide rarely is sufficient 

 to create an excessive slope on the front of the wave even when the 

 estuary is so shallow that much of its bed runs bare at low tide; but 

 if the range of the tide is so large that its rise is exceptionally rapid, 

 and if the fast rising tide encounters a strong outflowing current, the 

 advancing wave may trip and break, like a wind wave breaking oil the 

 shore. The incoming tide then rushes up the shallows in a breaking 

 wave, generally called the tidal bore, but otherwise kno-wn as the 

 "Aegre" or "Hygre" in England, the "Mascaret" in France, and the 

 "Proroca" in South America. A bore is formed in one of the shallow 

 tidal branches at the head of the Bay of Fundy; it forms also in the 

 mouth of the Colorado, at the head of the Gulf of California; and in 

 the shallow waters at the head of Cook Inlet, Alaska; but does not 

 appear to form in any other estuaries on the North American Conti- 

 nent. Because of the large tidal range at many localities on the 



