in almost exact agreement vith the semi-diurnal astronomical tide period. 

 The peak tidal oscillation at mileage 65 (Figure 8lc ) is immediately 

 explained. That it declines beyond this, is the result of the leak effect 

 to the higher river reaches. The low tide crest height, found at mileage 

 30, effectively defines a quasi-node. 



We return to a consideration of the fluctuations of height of the two 

 leading waves of the tsunami. Figure 8la shows the tide wave takes 3.6 

 hours to propagate from the river mouth to mileage 65. Also, the third 

 mode of oscillation for the river, as a closed-end open-mouth canal of 

 parabolic bed (cf. Wilson, 1966), is 



T3 = 0.259 T-L = 3.6h hours {ko) 



It follows then that a tidal induced oscillation of this period could have 

 positive antinodes at the mouth and at mileage 65 (Figure 8lb), and a 

 negative antinode at about mileage 33. Just 1.8 hours later the signs of 

 the antinodes could reverse. Since the leading waves are about 1.8 hours 

 apart in time, they would become enhanced or reduced in height by this 

 canal oscillation, accordingly as they rode the positive or the negative 

 antinode. The nodes of the canal's third mode of oscillation are thus 

 revealed by the crossing points of the crest height lines for the two 

 leading waves in Figure 8lb. The usual requirement that a node for an 

 open-mouth basin oscillation be located at the mouth is waived in this 

 case since the leakage effect to the upper river reaches beyond mileage 

 65. 



A similar effect may actually be present with the tsunami waves 

 riding the second tide crest but the effects are obviously severely 

 damped. There is some justification for believing that the 3.6-hour 

 Columbia River oscillation is a direct subharmonic effect of the tsunami 

 itself. 



South of the Columbia Eiver along the Oregon coast no well-established 

 pattern of wave arrivals can be determined from the data of Schatz et al 

 (196H), shown in Figure 79. The approximate tsunami front and tide crest 

 positions and times show that the leading waves of the tsunami would have 

 occurred everywhere along this shore on the top of the high spring tide. 

 Runup 10 to 15 feet above the high tide line (Figure 79) was confirmed in 

 August 1966 by one of the authors (Wilson), who observed long, debris lines 

 on this entire stretch of coast at about this level. 



Other data in Figure 79 were obtained from the Corps of Engineers 

 Office, Seattle (Hogan et al, I96I+ ; Whipple and Lundy, I96U ) . 



The north-central portion of the California coastline is shown in 

 Figure 82. The data here are from the extensive survey of the Corps of 

 Engineers, San Francisco (Magoon, I965), and indicate that runup along 

 this part (south of Crescent City) was generally much less than farther 

 north. Magoon' s data are all referred to mean lower low water (MLLW) 

 datum. To make them camparative with data given in Figures 77 to 79, 



126 



