AN OCEANOGRAPHIC MODEL OF PUGET SOUND 687 



The entire system lies within an area of about 40 by 90 nautical miles. 

 It may be subdivided into four general sections which are partly isolated 

 by vertical or lateral constrictions. The main basin extends from a 

 40-fathom threshold sill at the confluence of Admiralty Inlet with the 

 Strait of Juan de Fuca, south to a 26-fathom sill at the Tacoma Nar- 

 rows. The section south of the Narrows consists of a 100-fathom pri- 

 mary basin with many branching channels and inlets. Hood Canal, 

 averaging about 2 miles wide and having a depth of 100 fathoms, ex- 

 tends about 50 miles southwest from Admiralty Inlet and is partly 

 separated from it by a 30-fathom sill. The fourth section extends with 

 diminishing depth from Possession Sound, about 25 miles from the 

 entrance to Admiralty Inlet, north through Skagit Bay to Decention 

 Pass. This pass is a very restricted channel about 150 yards wide and 

 16 fathoms in depth connecting with the Strait of Tuan de Fuca. Pu- 

 get Sound is relatively deep in comparison with other inshore waters 

 of the United States with a maximum depth of 155 fathoms and with 

 50 per cent of its total volume lying below 25 fathoms. 



The tides of Puget Sound are of the mixed type characterised by 

 marked differences in the successive heights of low waters. Consider- 

 able changes in the tide with respect to character, range, and time 

 occur within the area. Periodically, the tide near the mouth at Port 

 Townsend loses its mixed characteristics and for several days each month 

 becomes virtually diurnal. This effect does not extend into the sys- 

 tem for any great distance and the tides are semidiurnal over the rest 

 of the system at all times. The tidal range generally becomes progres- 

 sively greater landward from the entrance. Port Townsend has a diur- 

 nal range of 8 feet while the inlets at the southern extremities have 

 15 feet. The rate of progression of the tidal wave into the Sound is 

 altered by the topographic characteristics, particularly at the Tacoma 

 Narrows, a narrow constriction between two of the larger basins, and 

 changes with the character of the mixed tides. 



The tidal currents also are subject to wide local variations and 

 are dependent upon the tide range. The maximum velocities occur in 

 constricted channels and may reach about 7 knots. The tidal prism in 

 Puget Sound amounts to about 5 per cent of the volume below mean 

 lower low w'ater. 



Numerous rivers and small streams feed into Puget Sound from a 

 drainage basin of about 11,300 square miles within which precipitation 

 varies locally from 20 to over 100 inches per year. The eleven largest 

 rivers, having a combined mean yearly runoff of about 40,000 second- 

 feet, account for about 80 per cent of the total fresh water influx, the 

 Skagit alone contributing about one third. The peaking characteristics 



