237 



In contrast, the Lynn Canal, in southeastern Alaska, is a narrow 

 fiord, 80 miles long, with a prevaiHng depth of 1,000 feet or more. Its 

 length is but one-twentieth of the wave lengths of the principal semi- 

 diurnal components at the depth. The mean tidal range increases 

 from 12.6 feet at Barlow Cove, near the entrance, to but 14.6 feet at 

 Skagway, at the head of the fiord. The tide is so nearly a stationary 

 wave that high water at the head occurs but 5 minutes after high water 

 at the entrance. 



454. Ejffed qfjresh water discharge. — The fresh-water discharge of a 

 river increases the ebb currents in its tidal reaches and decreases the 

 flood currents. In the wide part of a typical estuary, nearer its 

 junction with the sea, the tidal discharge from the storage in nearly 

 the whole of its tidal prism may be so much greater than the fresh 

 water discharge that the latter has but little effect upon the currents 



O 50000 lOOOOOcfs 



Discharge of Delaware River above Trenton 



Figure 80.— Relation of high and low water to fiuvialdischarge at Trenton, N. J. 



The discharge from tidal storage steadily diminishes upstream, the 

 ebb currents increase, and the flood currents decrease until a point 

 is reached at which the flood current disappears. Above this pomt, 

 the current fluctuates in velocity, but does not change direction. 

 The mean tide elevation in the river slopes upward from the sea at 

 an increasing rate as the ebb currents become the stronger. If the 

 river has so ample a cross section that the slope is small, the fluctua- 

 tions of the tide may extend far up the stream, diminishing as the 

 backwater from a dam diminishes, until at some point the tides 

 disappear, and with them the tidal storage, and the last traces of 

 tidal fluctuation in the river current. 



Quite obviously, the range of tides in the upper reaches of a tidal 

 estuary diminishes when the fresh-water discharge increases, and may 

 disappear when the river is in flood; as the backwater from a dam 

 diminishes and eventually disappears A\-ith the increasing river dis- 

 charge. The observed heights of liigh water and of low water at a 

 tidal station may be plotted against the upland discharge to afford a 



