during the Holocene transgression, it is likely that fine-grained, marine 

 deposits cover coarser channel and valley flat fluvial deposits. 



50. Smaller streams that cut into the shelf surface may have no geo- 

 morphic expression. In many cases, the stream channel deposits are exposed or 

 are near enough to the surface to be economically dredged. Such channels can 

 often be detected on seismic reflection records by their shape, elongation, 

 and the often complex bedding of the channel deposits. 



51. An example of channel fill deposits was discussed in CERC MR- 79 -4 

 (Williams, Prins, and Meisburger 1979). In this study of the Galveston, TX 

 area, two sand- filled channels were located off Bolivar Beach north of Galve- 

 ston Inlet. These channels cut through the now submerged area during a lower 

 sea level stand and eventually filled with sandy fluvial sediments having size 

 characteristics similar to nearby beaches. In this case, however, an overbur- 

 den of 1 m of soft mud reduced the potential value as fill. 



Deltas 



52. Deltas form where sediment- laden streams enter a standing body of 

 water such as an ocean, lake, estuary, or lagoon. Delta formation occurs when 

 the amount of sediment discharge is greater than the competence of waves and 

 currents to effectively disperse the material. Thus, favorable conditions 

 occur where there is a high sediment discharge and/or comparatively low 

 nearshore wave and current energy in the standing water body. Low- energy 

 conditions usually occur in protected water bodies such as lagoons and semi- 

 enclosed bays, and in open coast areas where a low-energy wave climate pre- 

 vails or shallow offshore slopes attenuate deep water waves. 



53. Deltas are usually divided into three large components, the delta 

 plain, delta front, and prodelta. The delta plain is relatively flat and 

 consists of abandoned subaerial deposits closest to shore, an intertidal zone, 

 and a subaqueous outer zone that terminates in a comparatively steeper delta 

 front that connects the plain to the prodelta deposits on the basin floor. 



54. Deltaic sediments are characteristically muddy in most places, 

 although coarser sand and gravel occur in areas where streams have the compe- 

 tence to transport such material. Enrichment of sand- size components usually 

 occurs nearest the stream mouths and near the shoreline where waves and 



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