OYSTER BOTTOMS TN MATAGORDA BAY. 17 



ponderating set is toward the mouth of the bay, a condition neces- 

 sarily imposed by the discharge of fresh water from the several 

 streams. If there were no tributary streams, the currents would 

 be strictly conditioned by the ebb and flow of the tides, and, neglect- 

 ing the small factors of evaporation and seepage, their volumes would 

 be equal in the two directions; but the Colorado and its sister 

 streams drain vast areas of the country, discharging a volume of 

 water which relatively to the cross section of the bay is very con- 

 siderable, and as essentially all of this water finds its way into the 

 gulf through Pass Cavallo, the downward currents must conse- 

 quently be stronger than those flowing toward the head of the bay. 

 This gives the upper margins of the reefs a decided advantage in 

 the matter of conditions favorable to spat fixation and the growing 

 and fattening of the oysters, inasmuch as the cultch is kept cleaner 

 and more food is carried within the reach of the oysters setting on it. 

 It also appears reasonable to invoke the current characteristics as 

 an explanation of some of the physical peculiarities of the long reefs, 

 especially that diversity which occurs between the two sides. The 

 water of the Colorado Kiver, which, especially in times of freshet, 

 is heavily charged with mud, flows into the bay just above Dog 

 Island Reef. As it spreads out after leaving the channel, its velocity 

 is promptly checked and the coarser and heavier particles of sand 

 and mud are deposited to produce a fan-shaped shoal surrounding 

 the mouths of the river and Buffalo Bayou, w^hile the finer particles 

 remain in suspension. At high water, when the crest of the reef 

 is covered, the outward flow of the bottom stratum of this water 

 is largely checked by the barrier of Dog Island Reef and some part 

 of the suspended matter is thrown down on the bottom close to the 

 reef as silt, while over the crest there is flowing a current of sufficient 

 velocity to keep the top of the upstream portion of the reef cleanly 

 scoured and in condition to receive fresh accretions of young oysters. 

 As the crest of the reef is crossed, the velocity is again lessened by 

 reason of the larger cross section of its available channel in deeper 

 water, and there is a deposit of silt upon the downstream side of the 

 bed, rendering it less adapted to a set of spat. When the level of 

 the water is below the crest of the reef, a generally similar result is 

 brought about by somewhat different means. Then the entire dis- 

 charge passes through the several channels by which the reef crest 

 is traversed, especially those at Dog and Tiger islands. There is a 

 current of varying strength running lengthwise of the northeast 

 side of the reef and a swift current through the channels, but as 

 soon as it passes the barrier the silt-laden water spreads out and 

 eddies after leaving the channels, and there is again a tendency to 

 the deposit of mud. 



