TIDAL DATUM PLANES 111 
Jan. Mar. May July Sept. Dec. 
6.5 Seattle 
Ketchikan 
8.0 
8.5 
Fic. 52.—Monthly lower low water below monthly sea level. 
Yearly Lower Low Water 
The annual variation balances out within a year, and therefore yearly values of 
lower low water may be expected to show much smaller differences than monthly values. 
This is borne out by Figure 53 which is a plotting of the yearly heights of lower low 
water at three Pacific coast stations. Generally, lower low water from one year to the 
next differs by several hundredths of a foot, though at times it may be as much as 
2 or 3 tenths of a foot. Within the 25-year period represented in Figure 53 the dif- 
ference between the highest and lowest yearly values of lower low water was 0.6 foot 
at Los Angeles, 1.0 foot at Seattle and 0.5 foot at Ketchikan. 
A comparison of the diagrams of Figure 53 with the corresponding diagrams of 
yearly sea level in Figures 32 and 33 shows some resemblance between them indicating 
that, in part, the variation of yearly lower low water is due to the variation in sea level. 
To determine the nature of any other elements in the variation of yearly lower low 
water, the variation due to change in sea level may be eliminated by subtracting each 
