The next four figures present longitudinal profiles showing 
the evolution over time of critical water quality parameters in 
the segment of the River between Chain Bridge and the U.S. High¬ 
way 301 Bridge (Morgantown). All data were collected during 
August and as nearly as possible at comparable discharges. In 
Figure 2, the earlier profiles show very low dissolved oxygen 
immediately downstream from the Washington, D.C. area. More 
recent profiles all show improved conditions. That is the pri¬ 
mary result of the work that's gone on in the D.C. area sewage 
treatment plants in the last 20 years. The dissolved ammonia 
profiles in Figure 3 show a great improvement in the ammonia 
picture following bringing on line the nitrification facility at 
the Blue Plains sewage treatment plant between 1980 and 1981. 
Figure 4 shows total phosphorous as "P." The 1968 profile shows 
P values much greater than any of the others and the gradual 
improvement has continued since 1977. 
Figure 5, the final longitudinal profile shows chlorophyll- 
a. The observation was made earlier that it is much more diffi¬ 
cult to illustrate the cause-and-effect relationships governing 
these longitudinal profiles. The reason being, that there is no 
significant nutrient limitation on chlorophyll-a in the tidal 
river, at any rate, until chlorophyll-a concentrations of bloom 
proportions are reached. 
We have developed a phytoplankton growth index, which in¬ 
corporates all of the independent variables that were discussed 
in the previous presentation. In addition, we included a mea¬ 
sure of spring-time inflow which improves the predictive capa¬ 
bility of the index. Figure 6 gives a plot of index values and 
descriptions of the independent variables. 
In the tidal river the answer to the question of better or 
worse is, with respect to dissolved oxygen, much better. With 
regard to the occurrence of alga blooms, it depends on hydro- 
logic factors, on sunlight, on temperature, and on spring-time 
inflow rather than what we've been able to do with regard to 
reducing nutrients. We haven't reached a point where there is 
any significant nutrient limitation at least until after bloom 
conditions are well established. Here also, the answer to the 
question better or worse is better because modern-day blooms 
haven't been accompanied by the periods of anoxic degradation 
that were common in the 1960s. 
There has been considerable discussion of nutrients here. 
Before I proceed to the estuary, I would like to briefly address 
nutrient budgets, where the nutrients come from, and where they 
go in the Potomac River. The first column in Figure 7 shows the 
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