excluding other flow characteristics, in order that the results may be 

 easily comparable to observations in the field — emphasis having been 

 placed on biology rather than hydraulics. 



Although practically all the clams caught in square-foot trays by 

 Smith (1952), during two seasons of observations, were under 12 mm., the 

 upper size limit was increased to 2 2 mm. in the present experiment in order 

 to obtain additional data. Three hundred and fifty clans were measured to 

 the nearest millimeter and separated into seven size groups of 50 each: 

 2-[i, 5-7, 8-10, 11-13, lU-16, 17-19, and 20-22 mm. in length. Because of 

 the limited number of juvenile clams available in winter, the same clams 

 were used for each current velocity run. 



The experimental procedure consisted of adjusting the flow to produce 

 the desired current velocity. Then one of the groups of 50 clams was 

 carefully "poured" into the water at the upstream end of the flat, and 

 observations were recorded at 5-minute intervals for one-half hour. This 

 procedure was followed for each of the seven size groups of 50 clams at the 

 three current velocities and at zero velocity, as a control, making a total 

 of 28 "runs." Clams which were moved by the current were noted as to type 

 of movement, such as sliding , rolling , or jumping. The velocity of move- 

 ment was not measured, owing to the shortness of the test area and the 

 large number of individuals, but the rate of movement was noted in descrip- 

 tive terms. The number of clams which were washed into the sediment trap 

 and the number which burrowed were recorded. A clam which had burrowed to 

 a point at which it was not likely to be dislodged by the current — or 

 about to the umbones — was considered to have burrowed and was removed from 

 the flat. This was necessary in order to recover the clams for further use 

 before they were able to lose themselves in the sediment. 



Water temperatures, recorded at frequent intervals, varied from U.0° 

 to 506" C. These low temperatures appeared to have no adverse effects on 

 the clams, which anpeared to be as active as they have been observed to be 

 at summer temperatures. 



Results 



Where clams were moved by one of the three currents, the predominant 

 type of movement was rolling , in which case the clams were usually oriented 

 with the longest axis at right angles to the direction of flow. Sliding 

 occurred rarely and only for distancesof one-half inch or less. At 2$ 

 cm/sec current velocity, some of the smallest clams appeared to move in a 

 combined jumping and rolling motion. In general, the smaller clams were 

 carried downstream more easily and more rapidly than the larger, which is 

 in agreement with the eroding velocity for coarse sand and gravel 

 (Hjulstrom 1939). 



