Many formerly productive oyster bottoms along 

 the Atlantic Coast of the United States have been 

 destroyed by a high rate of sedimentation. Dead 

 oyster reefs buried below a surface of mud in the 

 waters of Louisiana and Texas are good examples 

 of this process. The silting of estuaries may be 

 studied by the simple method of comparing the 

 depth of water shown in navigation charts of 25 

 or 50 years ago with present soundings. From 

 these dift'erences the total amount of deposit 

 accumulated over the given period of time can be 

 computed. The reduction of volume of water in 

 a basin due to sedimentation can be determined 

 from these data and from the computations of 

 the capacities of reservoirs (Dobson, 1936). 

 Brown, Seavy, and Rittenhouse (1939) success- 

 fully used this method in determining the rate of 

 silting over a distance of 19.4 miles of the York 

 River, Va. In 1857 the water volume of this 

 sector of the river, estimated at mean low tide, 

 was 227,780 acre-feet. By 1911 it was reduced 

 to 222,189 acre-feet, and in 1938 was only 206,896 

 acre-feet. The cumulative volume of sediment 

 deposited during the period 1857-1911 was 5,591 

 acre-feet, and from 1911 to 1938 reached 20,884 

 acre-feet. The annual deposition for the first 

 period of 55 years was 104 acre-feet, which repre- 

 sented 0.05 percent loss of water volume; during 

 the second period of 27 years (1911 to 1938) the 

 annual deposition increased to 566 acre-feet, 

 which corresponded to an annual 0.25 percent 

 loss of water. The increased rate of silting during 

 the later period was explained by an increased 

 erosion of soil over the watershed resulting from 

 faulty agricultural practice, deforestation, and 

 an increase in population. 



The filling of bays and estuaries with sediments 

 is a general phenomenon along the Texas coast, 

 and is particularly pronounced in Laguna Madre 

 and near the mouth of the Colorado River in 

 Matagorda Bay where some of the buried oyster 

 reefs are found under 14 feet of mud (Norris, 

 1953). Duj-ing the last 36 years silting has de- 

 stroyed 6,000 to 7,000 acres of productive oyster 

 reefs near Matagorda. These beds were described 

 in detail by Moore (1907) and resurveyed in 1926 

 by Galtsolf (1931a). In 1926 the principal reefs 

 opposite the mouth of the Colorado River were 

 sm-rounded by very soft mud but were still pro- 

 ductive. Now the nmd of the Colorado River has 

 completely buried these reefs and pushed the head 

 of fresh water seaward. For the Neuces River, 



a small stream emptying into Corpus Christi Bay, 

 the accumulation of silt recorded by comparing 

 the depths given on 1880 charts with those issued 

 in 1937 varied from 4 feet near the ship channel to 

 less than 1 foot at the south shore (Price and 

 Gunter, 1943). The average annual accumulation 

 of sediment near the ship channel was about 

 0.8 inches, only slightly less than the annual in- 

 crease of oyster shell in height. Under such 

 conditions an individual oyster, even if it grew in 

 the vertical position that oysters usually assume 

 in soft mud, in 6 to 7 years would have sunk in 

 mud for about three-quarters of its height and 

 perished. 



The soft, muddy tidal flats typical for the in- 

 shore waters of the South Carolina and Georgia 

 coast are usually devoid of oysters. U.S. Bm-eau 

 of Fisheries experiments on oyster farming in 1939 

 and 1940 in the vicinity of Beaufort, S.C., demon- 

 strated the complete unsuitability of these areas 

 for oyster culture. Reinforcement of these flats 

 by shells planted in a layer about 1-foot thick and 

 strong enough to support a man's weight was a 

 complete failm-e. The shells acted as baffles, 

 with the result that in a short time mud filled 

 all the crevices between them and in about 6 

 weeks completely covered everything with a 

 smooth layer of silt (Smith, 1949). Similar results 

 were obtained with brush and other materials 

 placed on the surface of the mud; in a few months 

 not a trace of them could be seen on the surface. 

 On the other hand, oysters grew well along the 

 opposite side of the river where a swift current 

 kept the bottom scoured. 



Accumulation of silt over an oyster bottom is 

 sometimes caused by the activity of various mud- 

 gathering and nmd-feeding invertebrates. Chief 

 among them are several species of the mud worm, 

 Polydora. The two long antennae of these worms 

 protrude from the tube in which the animal lives 

 and sweep the surrounding water. Mud particles 

 suspended in the water are caught by the epithe- 

 lium of the antennae and by ciHary motion are 

 transported toward the head to accunmlate around 

 the worm's body and thus making the tube. 

 Some of the mud is ingested and passes through 

 the intestime. P. websteri invades the shell cavity 

 of the oyster, settles on the inner surface at a riglit 

 angle to the edge, and builds a U-shaped nmd tube 

 with both orifices external. The structiu-e is soon 

 covered by a layer of conchiolin deposited by 

 oyster and becomes a semitransparent blister. 



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FISH AND WILDLIFE SERVICE 



