influenced by freshwater runoff comprised a 5-m deep surface layer 

 overlying, clear, intermediate salinity water between 5- and 12-m 

 depths. Waters below 13-m depth had relatively high salinities (>33 

 ppt), and a 1-m deep turbid band was evident at the 12-m deep area of 

 interface between these two water masses. This band was followed by a 

 relatively clear layer of water extending down to the bottom meter, 

 which was markedly turbid due to resuspension of sediment by bottom 

 currents . 



Turbid Water Layers 



The shelf waters of the northwestern gulf are characterized, at 

 least seasonally, by a two- or three-layered water column with bands of 

 turbidity present at the interfaces. Of these, the bottom turbid layer 

 is the most persistent, usually being detected during all seasons from 

 Louisiana (Griffin 1979) to South Texas (Causey 1969). The source of 

 much of the suspended material in the turbid layers is freshwater runoff 

 from the land, primarily from the Mississippi Drainage Basin. A process 

 model for turbid-layer transport of Mississippi River suspended sediment 

 was presented by Griffin (1979) and is summarized below. Results of the 

 EPA BGOF studies offshore of Galveston and the DOE baseline 

 investigations for brine disposal offshore of Freeport, Texas, indicate 

 that suspended sediments in those regions also appear to be mainly of 

 Mississippi River origin—corresponding well with the previously 

 mentioned effects of the river on surface salinity distribution (Figure 

 8a). Other rivers undoubtedly make proportional contributions, but 

 their combined total discharge is relatively small as compared to that 

 of the Mississippi (Figure 6). 



Griffin (1979) noted that the river discharge emanates from the 

 passes as a highly- turbid, distinct, surface plume carrying 40 to 500 gA 

 suspended sediment alone. Because of the significant density 

 differences between this plume and the clear mid-depth water in the open 

 gulf, eddy diffusivity is limited and the plume may persist for miles 

 away from the delta, perhaps even to South Texas. Slow erosion of the 

 base of the turbid plume by internal waves at the density interface 

 allows only small quantities of the suspended sediment to descend into 

 the mid-depth layer at any point. Suspended material in the plume is 

 therefore dissipated slowly into the mid-depth water and diluted as the 

 surface plume drifts with the prevailing current (generally west except 

 during summer periods when the general flow is often to the east ) . The 

 net transport is to the west and the quantity of suspended material that 

 settles through the mid-depth water at any one place is small, serving 

 to maintain the relative clarity of this water mass (e.g. Figure 9). 



The particles that pass downward continue to settle towards the 

 bottom although sinking may be interrupted by pycnoclines. Eventually 

 particles do reach the bottom where, through bottom currents, tidal 

 action, waves, upwelling, and perhaps, biological phenomena, they 

 accumulate as a nepheloid cloud of fine particles. The turbid bottom 



17 



