pieces from less than three. Gashes in the aluminum sheet metal 

 and reinforcing angle indicated that the corrals had been struck 

 by propellers. Most of the pieces were bent and crumpled. There 

 were several l-2m-deep pits in the substrate which we believe were 

 created by prop wash. We have observed towboats working as long 

 as three hours to pull a grounded barge off the shore. When a 

 towboat operates at full throttle in one place in shallow water, 

 it undoubtedly scours away the bottom. Sand and mud were probably 

 washed away from our ground anchors and the aluminum corrals 

 subsequently drawn into the props, or props may have actually 

 struck the substrate and our corrals. Prop wash probably 

 scattered and buried some of our marked mussels. 



We found all the corrals near the pilings. The diver said 

 the top edges of the corrals were bent over and had either been 

 covered with sediment to a depth of 2 cm or pushed down into the 

 mud. A barge evidently settled on the corrals during low water or 

 had been pushed over the corrals. 



Corrals in the upstream control plot had been damaged, 

 apparently by smaller propellers. Debris and a campsite left on 

 the bank indicated that the area was used heavily by recreational 

 boaters during the summer. Corrals in the downstream control plot 

 were intact and slightly silted in on their upstream edge. 



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