Bay-water salinities were relatively low in 

 July 1965 (average, 11.0 p.p.t.) because of 

 large discharge from the Trinity River in 

 May. Salinity increased thereafter and reached 

 a seasonal high in the fall (21.5 p.p.t.). Large 

 winter flows from the Trinity River again 

 lowered salinity throughout the estuary in 

 January and February 1966, but levels were 

 higher in March and April. 



A major flood of the Trinity River in late 

 April and May 1966 then reduced salinity 

 throughout the estuary to the lowest level in 

 5 years. Salinity dropped to less thain 2 p.p.t. in 



bays, but its effect was evident only about a 

 month in the lower bays (fig. 22). During May 

 and early June, however, salinity in the lower 

 bays increased as salinity declined in the 

 upper bays. River flow did not reach the lower 

 bays until late June, aind the maixinnum lower- 

 ing did not occur until early July. Recovery 

 was rapid in the lower bays. 



The effect of the constriction between the 

 upper and lower bays of the Galveston estuary 

 is to delay the time it takes fresh water from 

 the Trinity River to reachthe Gulf. Conversely, 

 even though Gulf salinities reached a high in 



30 



20 



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 to 



O 

 CD 



cr 



LOWER BAY 



RIVER DISCHARGE 



W=WINTER 

 SP = SPRING 

 S = SUMMER 

 F =FALL 



W I SP I S 



1963 



W I SP I s 

 1964 



F I W I SP I Si F I W I SP 

 1965 



Figure 20. —Influence of seasonal trends In Trinity River discharge on salinity in the upper (Trinity) and lower (lower 



Galveston) bays of the Galveston estuary. 



Trinity Bay; less than 6 p.p.t. in upper Galves- 

 ton Bay; and less than 10 p.p.t. in lower 

 Galveston Bay where the salinity is usually 

 highest (20 to 30 p.p.t.). Reduced river flow 

 in June 1966 produced a rapid increase in 

 salinity in the lower bays of the systenn, but 

 the salinity in the upper bays remained 

 relatively low. The seasonal influence of 

 Trinity River on bay salinities is depicted in 

 figure 20. 



An excellent opportunity to study how Trinity 

 River influences bay salinities arose in May 

 1963 when almost 600 million cubic meters of 

 fresh water was discharged. Before May, 

 river flow was small and bay salinities were 

 increasing. River flow was again small after 

 May for the rest of the year (fig. 21). Salinity 

 was depressed considerably in the upper bays, 

 but the effect was small in the lower bays 

 and tidal pass. The constricting effect of Smith 

 Point, Eagle Point, and Redfish Bar in mid- 

 Galveston Bay was strikingly evident during 

 the summer when prevailing southerly winds 

 tend to prevent the flow of waters Gulfward 

 from the upper bays. In ternns of time, the 

 influence of 600 million cubic meters of fresh 

 water in May lasted until August in the upper 



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 a. 

 u. 

 o 



z 

 I- 



o 



z 



Q. (/> 



< 



o 



CD 



a. z 



uj o 

 i>- 



Oio 



-I 

 < 



o 



Figure 21.— Effect of May 1963 Trinity River flow 

 (5.8 X 10^ m.^) on salinity of Galveston estuary. 



22 



