and diffusion in plume dispersion. In contrast to 

 flow conditions in 1963, southward transport by 

 the geostrophic component of motion in July 1964 

 is indicated to be larger (fig. 11) and to predom- 

 inate over transport by the much diminished 

 Ekman component of June and July 1964 (fig. 12). 



The plumes of 1962 and 1963 woidd be expected 

 to be similar on the basis of the similarity of 

 June-July wind-induced transports (fig. 12) and of 

 June discharge rates from the Columbia River. 

 They differed markedly, however, in salinity. 

 Although the 1962 plume was nearly as broad as 

 that of 1963, it displayed larger gradients and 

 smaller values of salinity; these differences imply 

 reduced importance of diffusion processes during 

 1962. Because wind effects were about equal in 

 the 2 years, differences in the geostrophic com- 

 ponent of flow must have caused this difference in 

 the relative importance of diffusion, and (together 

 with variation in river discharge volume) the 

 differences in salinity characteristics of the plume. 

 Comparison of dynamic height patterns of the 2 

 years (fig. 11) reveals two significant differences in 

 the nature of geostrophic flow: greater current 

 speeds and a pronounced northward component in 

 the outer plume province and offshore province 

 in 1962. The apparent reduction of diffusion effects 

 relative to advective effects in 1962 was accom- 

 plished by faster geostrophic transport of the 

 plume. Further, the breadth of the 1962 plume 

 and the lateral disposition of pockets of less 

 saline water could have been the consequence of 

 transport of the plume from southwest to north- 

 east, indicated by geostrophic flow in the offshore 

 part of the study area. 



Geostrophic flow in 1961 was similar to that of 

 1962 m current speeds and in the presence of the 

 offshore northward component; probably the 

 plumes of the two years were produced by the 

 same balance of forces. The principal difference 



between the two plumes was in salinity — the 1961 

 plume was less saline than that of 1962. This 

 difference appears to have resulted from high run- 

 off in 1961 : average discharge rate of the Columbia 

 River for June 1961 was about 1.7 x 10'' m.' sec."' 

 or 1.5 times greater than the discharge for June 

 1962 (estimated from Budinger et al., 1964, fig. 34, 

 p. 51). 



Differences between years in near-surface salin- 

 ity of the offshore province were insignificant. 



Heating 



The general effect of the fresh-water plume on 

 temperatiu-e was described above. Briefly, heat is 

 constrained to a smaller volume in the plume than 

 beyond so that, by July, plume temperatures 

 exceed offshore temperatures. Because the degree 

 of plume development varied widely from summer 

 to summer during 1961-64, one may reasonably 

 expect to see variations among the respective 

 temperature distributions. 



Temperature of the mixed layer in July at any 

 location in the study area may be considered to be 

 the net expression of the following factors: tem- 

 perature at the start of the heating season (about 

 March); heat gain, principally across the sea 

 surface, between March and July; heat loss by 

 advection and diffusion; and depth over which 

 heat changes are distributed (thickness of the 

 mixed layer and thermocline). If it is assumed for 

 present purposes that advective heat change is 

 important only in the near.shore regime (as upwell- 

 ing), and that the meridional gradient of heat- 

 exchange across the sea surface is constant while 

 the zonal gradient is zero over the study area, then 

 one must only consider variation of initial tem- 

 perature, lateral and vertical diffusion, and free- 

 mLxing depth to explain the mixed-layer tempera- 

 ture patterns of figure 3. 



July temperature near the top of the permanent 



Table 2. — Maximum temperature changes and salinity gradients in the plume province and 

 average discharge rates of the Columbia River, 1981-64 



[Ranks are provided to facilitate comparisons! 



520 



U.S. FISH AND WILDLIFE SERVICE 



