Geology, Climate and Water 
Circulation of the Galveston Bay 
System 
E.G. Wermund, Robert A. Morton, Gary Powell 1 
Abstract 
E.G. WERMUND—The geology of the Galveston Bay System reflects its location in one of the 
world's largest depositional basins, the northwest Gulf Coast Basin, as well as changes in the rates 
and balance among sea level, sediment influx and basin subsidence. Sedimentary deposits of two 
ages dominate the surficial geology surrounding the bays. Deposits of the most recent interglacial 
period of the Pleistocene Epoch include (1) river sands and floodbasin muds of a deltaic plain and 
(2) sands of a barrier island system. Modem (Holocene) sediments that entrench and overlie the older 
strata are (1) fine sand and mud in rivers and bayhead deltas; (2) mud in the bays; (3) oyster reefs in 
the bays; and (4) sand composing the youngest barrier islands. Galveston Bay is extremely shallow 
(10 to 12 feet deep) compared with its large areal extent of 600 square miles. Sediment samples, 
collected a mile apart, are mud in most of the bays; samples coarsen shoreward where sand and 
reworked shell (gravel) dominate. Geochemical analyses of sediment samples indicate that abnor¬ 
mally high concentrations of barium, boron, chromium, copper, lead, nickel and zinc are products 
of anthropogenic activities and pollutants. 
The Galveston Bay System has a subhumid, subtropical climate; mean summer high temper¬ 
atures are in the upper 80s (T), and mean winter low temperatures are in the mid 40s ( # F). Mean 
annual rainfall and surface-water evaporation are approximately 50 inches. Summer winds are 
dominantly moderate and southerly; winter brings frequent aperiodic strong north winds. Droughts 
and hurricanes are frequent. Bay circulation is controlled by balances among freshwater influx, tides 
and storm winds. The Trinity and San Jacinto River Basins provide more than 88 percent of the 
freshwater inflow to the bays. Bay tides are diurnal in a 14-day cycle, and maximum tidal range is 
about 2 feet. Hurricane landings may raise the bay level to 15 feet, whereas strong north winds may 
locally lower bay level about 2 feet. 
Principal geologic processes currently altering the Galveston Bay System include (1) a relative sea 
level rise (about 2 feet in this century) and subsidence (nearly 10 feet at Johnson Space Center) in 
response to withdrawal of subsurface water, oil and gas; (2) active faulting; and (3) coastal erosion 
and deposition. Between 1850 and 1982 bay shorelines eroded at an average rate of 2.2 feet per year; 
before 1930 the erosion rate was 1.8 feet per year, whereas the post-1930 rate was 2.4 feet per year. 
Human activities commonly overprint normal natural processes and effect a loss of natural 
resources. Models of circulation, salinity and nutrients developed by the Texas Water Development 
Board indicate potential management problems. Further documentation and regular, selective 
’E.G. Wermund and R.A. Morton represent the Bureau of Economic Geology, The University of Texas at 
Austin; Gary Powell, the Texas Water Development Board. 
This paper is published by permission of the Director, Bureau of Economic Geology. 
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