is therefore characterized by a history of interdistributary bays, lakes, 

 subaerial plains, and distributary channels. 



Groundwater withdrawal in the area is limited mainly to stock or domestic 

 uses. The dissolved solids content of the water ranges from 850 to 3,100 mg/1, 

 characterizing slightly to moderately saline water. The hydrologic unit tapped 

 in the area by wells from 160 to 300 m (520 to 1,000 ft) in depth is the Chicot 

 aquifer, a major component of the Gulf Coast aquifer. The aquifer is recharged 

 northwest of Brazoria County. 



Land surface subsidence is a concern in other portions of the Texas coastal 

 area. However, due to the relatively small rates of withdrawal of ground fluids 

 (whether oil, gas, or water), subsidence is not a critical problem in this rural 

 area. Further inland, subsidence on the order of 0,06 to 0.3 m (0.2 to 1 ft) 

 has occurred in the past 35 yr associated with water withdrawal for irrigation 

 (Brown et al., 1974). In the coastal area of the refuge, however natural sub- 

 sidence (resulting from compaction of subsurface clays under pressure of the 

 overburden) may contribute a greater component of overall decrease in elevation. 

 The presence of coastal lakes with shores tending to be smooth circular outlines 

 may be taken as evidence of natural subsidence, as such gcomorphic features 

 antedate ground fluid extraction. Where marsh is dominant in low areas, ponds 

 are confined between natural levees and have irregular shapes. Where subsidence 

 has lowered marshes so that dominance of landform is lost to the expansion of 

 water bodies, ponds in the low areas merge into one another so that the number 

 jf individual ponds is reduced, but the average size of each is greatly in- 

 creased. Under these conditions each lake tends to establish rounded outlines 

 (Russell, 1936). 



The Brazoria NUR is at the eastern edge of the old Brazos River cuspate 

 delta lobe, and therefore has many geological characteristics in common with 

 the San Bernard Refuge area to the southwest. However, the Brazoria Refuge 

 area, occurring at the fringe of the delta system rather than within the delta 

 system between major river courses, has been subject to geological environments 

 different from those of the San Bernard Refuge, and which are represented today 

 by different mappable features. 



In the westernmost area of the refuge are substrates representing a 

 Pleistocene coastal upland developed between distributary channels, such as 

 Oyster Creek to the southwest and Bastrop Bayou to the northeast, former courses 

 of the Brazos River. This interdistributary area contains flood-basin, or over- 

 bank, deposits of mud, clay, and associated soils of the Pleistocene coastal 

 plain. 



The meandering channels of former courses of the Brazos River have resulted 

 in several abandoned channels in the area, including Big Slough as well as other 

 smaller distributary channels. These channels are of late Pleistocene and 

 Holocene age. The smaller channels form mud-filled topographic lows within 

 the previously mentioned low coastal plain. Flanking Big Slough are levee 

 deposits resulting from overbank flooding. The levee of silt and mud is thickest 



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