We do not know at what level the concentration 

 of these wastes becomes detrimental to the 

 primary producers. 



Fresh-Water Discharge and Salinity 



Such natural factors as wind, rainfall, evap- 

 oration, tidal intrusion, fresh-water discharge, 

 and basin size and configuration influence the 

 salinity regimen of an estuarine system. Of 

 these factors, fresh-water discharge is per- 

 haps the most significant because it meas- 

 urably dilutes the salt waters of the sea to 

 form the estuarine "buffer zone" or ecotone 

 and transports large amounts of nutrients 

 derived from land drainage to the estuary. 



The Galveston estuary possesses a typical 

 salinity gradient; salinity is lowest in the 

 upper bays and increases Gulfward. Because 

 Texas is characterized by periods of drought 

 and excessive rainfall, tributary discharge 

 into this estuary is frequently erratic. This 

 variability is reflected in the salinity gradient 

 and hydr ological zonation of the estuary. During 

 periods of high river flow, salinities over 

 much of the estuary are depressed, whereas 

 during drought, salinity becomes high. 



Many estuary-dependent species require 

 or prefer different hydrological conditions. 

 Fortunately, even during wet and dry cycles, 

 the salinity gradient in Galveston estuary is 

 characterized by a broad range of conditions 

 that accommodates many species. During 

 drought, however, the high salinity zones may 

 enlarge, so that species that prefer high 

 salinity may occupy more of the estuary. 

 The reverse is also true. Tributary dis- 

 charge controls, to a large extent, the amount 

 of area comprising a particular salinity zone. 

 Thus, it becomes imperative to describe the 

 type of gradient zonation most suited to the 

 complex of estuarine organisms and to deter- 

 mine the amount of fresh water required to 

 perpetuate the zonation. We must also deter- 

 mine how departure from optimum conditions 

 will affect these resources. 



For the Galveston estuary, we based our 

 first efforts to relate river discharge and 

 salinity on records from October 1960 through 

 December 1964 of discharge from the Trinity 

 River, the major source of fresh water. 

 Seasonal and annual variations in river dis- 

 charge were considerable. Generally, 1961 and 



1962 were years of high tributary discharge, 

 whereas river flow was considerably reduced 

 in 1963 and 1964. During the wet year of 1961, 

 salinity for the entire estuary averaged about 

 6.5 p.p.t., whereas during the dryer year of 



1963 it averaged 18.5 p.p.t. During the 4-yr, 

 period the average salinity of Trinity Bay, 

 which receives directly the Trinity River 

 discharge, was about 10.0 p.p.t. Salinity of 

 lower Galveston Bay during this period aver- 



aged more than 22.0 p.p.t. Each of the sub- 

 bays of the estuary is influenced differently 

 by tributary fresh water (fig. 28). 



Seasonal differences in salinity are also 

 evident. River flow is usually highest during 

 winter and spring. River water moving through 

 the estuary toward the Gulf in winter is aided 

 by the prevailing northerly winds and seasonally 

 low tides. Under these conditions, fresh waters 

 from the Trinity River can pass through the 

 estuary in less than 1 mo. During the summer 

 and fall, when river discharge is usually 

 lowest, dispersion of fresh water is usually 

 hindered by prevailing southerly winds and 

 higher tides. Thus, 1-1/2 to 2 mo. may 

 elapse before fresh water from Trinity River 

 is completely dispersed throughout the estuary. 



A much more sophisticated analysis of these 

 data will be required to develop suitable 

 mathematical models that will enable us to 

 predict how specific volumes of tributary 

 fresh water will affect the gradient and zona- 

 tion of salinity in the estuary. 



TRINITY RIVER DISCHARGE 



(CUBIC FEET PER SECOND ) 



o 



Ul 



1 r 



lX 



20-1 



10- 



TRINITY BAY 



■II 



20-n 



10- 







^20. 



< 



°= 0- 

 I.I '-' 



UPPER SflLVESTON BAY 



LlJllJ 



EAST BAY 



I I I I 



30-1 



LOWER GALVESTON BAY 



■I I I II 



^ 1961 1962 1963 ' 1964 4-YEAR 



8-Morvth Avtroga, Jon -Aug. 



Figure 28. — Relation between average annual Trinity 

 River discharge and salinity in the subbays of Galveston 

 etsuary, 1961-64. 



33 



