374 THE SALMON RIVERS OF SCOTLAND 



Of recent years Glasgow has commenced to set its house in 

 order to this respect, and, like other things Glasgow undertakes, 

 the business is being well done. Already down the river there 

 is a noticeable change for the better ; the water is less opaque, 

 the deposit of sludge is less heavy, and the floating impurities 

 which used to form a horrid scum, jare now less noticeable. 

 It is premature to say much about the prospects of salmon 

 returning to the river, but I believe there is now better chance 

 for salmon in the Clyde than there has been for very many 

 years. The sine qua non is the sufficiency of dissolved oxygen 

 in the purified water, the avoidance of a great mass of prac- 

 tically unoxygenated water a sewage zone as it is called 

 moving up and down the dredged channel with the flow and 

 ebb of each tide. 



In this connection it has to be realised that fresh and salt 

 water are of very different densities and do not mix freely, and 

 that even when mixed in a tidal estuary the salt water contains 

 nearly 20 per cent, less oxygen than the fresh, although 

 deaerated salt water is capable of taking up oxygen almost 

 three times as fast as fresh. The danger to fish of pollution, 

 apart from the active toxic properties which may be present, 

 is the rapid absorption of oxygen which goes on. The water 

 bacteria can do a great deal to devour and convert the 

 impurities, but there must be a balance of oxygen in the 

 effluents from sewage works, introduced by dilution with a 

 sufficiency of natural water, or else fish cannot breathe. 



The river Leven, flowing from Loch Lomond, enters the 

 Clyde at Dumbarton, 13 miles below Glasgow, and in spite of 

 many pollutions here also, salmon have never quite forsaken 

 this channel. It is foul enough in all conscience, but it is 

 improving, and thanks to its great body of pure water from 

 Loch Lomond the dilution of impurities is very considerable. 



The Vale of Leven is famous as a centre of the Turkey-red 

 industry, and quite a number of large works for both dyeing 

 and calico-printing are situated on the banks of the river at 

 such towns as Renton and Alexandria. Dumbarton, again, 

 is a shipbuilding centre, and in the whole vale there is a large 

 and thriving population. Sewage works on an extensive scale 

 are now completed for the whole district of Alexandria, with a 

 population of about 25,000, but at the other two places all 



