PERILS AND ENEMIES OF MUSSELS 63 



Among these the freshets are very prominent. Dur- 

 ing the freshets, entire beds of mussels sometimes become 

 mudded over or covered with gravel or sand. Sometimes 

 the erosion of the water causes large banks of earth to 

 fall into the stream and cover the mussel beds. In other 

 cases, the channel of the stream may be entirely changed 

 leaving large areas of mussels exposed to the air and 

 sun. Droughts are very disastrous in many instances, as 

 the mussels, being nearly uncovered, are devoured by 

 hogs, or, being entirely uncovered are exposed to the air 

 and sun, so death results. 



Another cause of injury is drift ice, which sometimes 

 plows into the mussel beds and crushes the mussels. It 

 is probable, however, that this is not a very destructive 

 agent. 



In some States the logging industry is a great menace 

 to mussels. The large number of logs which pass down 

 the rivers make it practically impossible for the mussels 

 to live. 



Water pollution is one of the worst causes of destruc- 

 tion. This is especially noticeable in the upper part of 

 the Illinois River. The Chicago drainage canal has a 

 very bad effect for many miles. The dissolved ox*ygen is 

 drawn from the water by the organic matter in the sew- 

 age and mussels cannot live in it, especially during warm 

 weather. 



The refuse from factories which is turned into the 

 rivers destroys the mussels. Along the Potomac and 

 Shenandoah Rivers, and near Harper's Ferry are a num- 

 ber of mills which grind wood into pulp for making 



