For the remainder of the year, the salinities are generally near 10 ppt in much of Trinity Bay, 10 ppt 
in upper Galveston Bay to 25 ppt near Bolivar Roads, less than 20 to 25 ppt west to east in West Bay, 
and 10 to 25 ppt east to west in East Bay. 
Nutrient gradients in the Galveston Bay System reflect the richer nutrient composition of the 
contributory freshwater streams and the nutrient-poor saline waters of the Gulf of Mexico (17). In 
addition, nutrients are generated and contributed by biochemical cycling in bayhead deltas as well 
as by marshes and nonpoint sources from agriculture. Magnitudes of freshwater inflows, winds, 
currents and biological activity complicate understanding the effects of nutrient processes at any one 
time. 
Measurements of water quality in the Trinity River upstream of the delta indicate that mean 
monthly organic nitrogen varies from 0.39 mg/L to 0.79 mg/L(16). Concentrations in the upper part 
of the Houston Ship Channel/Buffalo Bayou area, in contrast, ranged from 1.0 mg/L to greater than 
2.0 mg/L. Maps displaying average organic nitrogen from 1968 to 1987 show a gradient of concen¬ 
tration from greater than 0.5 mg/L in the upper reaches of the Houston Ship Channel to 0.5 mg/L 
to 0.2 mg/L down-channel and along the northwestern shore of Trinity Bay. Concentrations continue 
declining gulfward by several orders of magnitude, and there is a plume of 0.2 mg/L to 0.1 mg/L 
organic nitrogen flowing through Bolivar Roads. Both West Bay and East Bay have negligible organic 
nitrogen concentrations of less than 0.1 mg/L. 
In the same study period, average phosphate concentrations are more than 0.5 mg/L in north¬ 
western Trinity Bay, in the upper Houston Ship Channel, and in western Galveston Bay. Consid¬ 
erable dilution is evident near the Trinity River. West Bay has extremely low phosphate content, as 
does East Bay near Rollover Pass. 
The north-to-south nutrient gradients in the Galveston Bay System, encompassing more than two 
orders of magnitude and the plumes flowing out Bolivar Roads, deserve continued monitoring, as 
do seasonal concentrations approaching eutrophism. 
Active Processes 
The interconnected active processes of today are the same as those that occurred in past geologic 
time and that first formed the Galveston Bay System. Continuously changing magnitudes and rates 
of sediment influx, sea-level change (Figure 9), subsidence, faulting, and erosion and accretion are 
demonstrated by gains and losses of land, bay or Gulf. In contrast to active geologic processes, human 
activities rapidly alter or overwhelm the short-term effectiveness of some of the natural active pro¬ 
cesses in sculpting the bay system. 
Sediment Influx, Natural Subsidence and Sea-Level Change 
Sediment influx is significant where streams enter the bay system. Continuous sedimentation, in 
the absence of sea-level rise and subsidence, causes shoreline accretion and provides both stable land 
and nutrients for new marsh growth. Decreased rate of sediment influx with a concomitant rise of 
sea level or increased subsidence produces shoreline erosion and removes marsh. 
Records from the Trinity River near the delta from 1935 to 1980 show a continuous decline in the 
suspended sediment load beginning in 1950, coincident with the increased dammed reservoir 
capacity (17). The upstream reservoirs trap not only bed load but also a considerable fraction of the 
suspended load of streams. For the interval (1904-1980), combined tidal records at Galveston (18) 
show a relative sea-level rise of nearly 1.5 feet (Figure 9). 
Recently the bayhead deltas of the principal streams feeding Galveston Bay have begun to lose 
land and elevation. The loss of land between 1956 and 1979 reflects decreased influx of sediment and 
natural subsidence related to compaction of deltaic sediments; a rise of sea level, although possibly 
involved, is not documented. Figure 9 illustrates the loss of fluvial woodlands, swamps and marshes 
in the San Jacinto delta area (7). 
As noted previously, subsidence is a continuing natural process in which thick sedimentary 
deposits compact over long periods of time. An overprint of additional subsidence, in excess of 10 
feet at some locations in the Houston metropolitan area, has occurred since 1906 as a result of with¬ 
drawal of subsurface fluids. A large bowl-shaped area more than 80 miles in diameter has subsided 
principally because of ground water removal (19). Subsidence along the bay at Clear Lake Bayou near 
17 
