Roads. The tide tables illustrate the slow progression of the tides between the inlets and the upper 
bay tidal stations (Table 3). 
Freshwater Inflow 
Not eveiy stream entering the bays has a stream gauge; consequently, Texas water agencies group 
the streams into basins in measuring and calculating freshwater inflow (15). Inflow into the Gal¬ 
veston Bay System is gauged for the Trinity and San Jacinto Rivers. Inflow is calculated for the minor 
basins composed of small streams; these basins are the San Jacinto-Brazos Coastal Basin, the San 
Jacinto-Trinity Coastal Basin, and the Trinity-Nueces Coastal Basin. From calculations for the years 
1941-1976, the average annual freshwater inflow to the Galveston Bay System from the two principal 
basins and three lesser basins was 11,340,000 acre-foot. For the same period, the maximum annual 
freshwater flow was 23,696,000 acre-foot in 1973, and the minimum annual inflow was 2,913,000 acre- 
foot in 1956, near the end of the worst Texas drought of this century. For the same years the freshwater 
inflow balanced against evaporation losses were, respectively, 22,290„000 and 1,321,000 acre-foot. 
Measurements of the average annual inflow and average monthly inflow of the major 
contributing stream, theTrinity River, show similar patterns for the years 1941-1976. Until 1970 there 
was a large difference in the fluctuation of flood stage and low stage; thereafter, the difference 
between high and low stages has been small. However, for the same time period the mean annual 
inflow equals about the same amount. The alteration of the inflow pattern correlates with the increase 
in upstream dams after the 1950 drought years. 
On the basis of exceedance frequencies for monthly freshwater inflows between 1941 and 1976 
(16), it was calculated that the Trinity River Basin supplies more than 70 percent of inflow during the 
wet months of December through June. The San Jacinto River Basin supplies about 18 percent and 
the San Jacinto-Brazos Coastal Basin supplies less than 2 percent. Inflows from the coastal basins that 
have ungauged streams are roughly calculated from the size, slopes and stream gradients of small 
streams. 
Salinities and Nutrients 
Circulation in the Galveston Bay System reflects bathymetry of the bays and tidal inlets, location 
and amounts of freshwater inflow, location and amounts of saltwater inflow, velocity and orientation 
of tides, bottom friction, wind speed and direction, rainfall history, and surface evaporation. Most 
of these variables are well known, as we described previously in this paper. However, because no 
current meters have been set in the major inlets for a long term, only brief temporal measurements 
of exchange in the inlets are available. In order to understand salinity changes and nutrient processes 
in Galveston Bay, the Texas Water Development Board has modeled tidal circulation, salinity 
changes and nutrient processes (16). The model simultaneously solves multiple tidal hydrodynamic 
equations over a rectangular grid of cells in a discrete fashion. 
Monthly vector plots of the net flow through each computational cell show similar circulation 
patterns for groups of months (16). In March, August (Figure 8) and October, the most evident 
circulation pattern in the Galveston Bay System was a northwesterly directed current in the Houston 
Ship Channel and a clockwise circulation in Trinity Bay moving along the eastern shore. The current 
in West Bay was predominantly directed in a northeasterly direction from San Luis Pass to the 
Galveston Ship Channel. In January, February, July, September, November and December, the 
current in the Galveston Ship Channel was directed southeastward, and the dominant flow in Trinity 
Bay rotated cou n terc lock wise along the northwestern shore. An internal current rotated counter¬ 
clockwise in West Bay with the net water movement from Bolivar Roads through the Galveston Ship 
Channel and through San Luis Pass via West Bay into the Gulf of Mexico. In April, May and June, 
months of largest freshwater inflow, a very strong southeasterly current prevails in the Houston Ship 
Channel. Trinity Bay flow is counterclockwise in April and May, but clockwise in June, and north¬ 
easterly moving currents dominate flow in West Bay during the same months. 
Simulated salinity gradients, calculated from the model, also display seasonality. The lowest 
salinities occur in June, whereas the highest salinities appear in August (Figure 8). In the spring and 
early summer (March, April, May and June) salinity is generally less than 5 ppt in Trinity Bay, 10 ppt 
in Galveston Bay, 25 ppt a t Bolivar Roads, 20 to 25 ppt in West Bay, and 10 to 15 ppt in East Bay. During 
these four months an intrusion of salt water is evident along and beside the Houston Ship Channel. 
15 
