360 THE SALMON RIVERS OF SCOTLAND 



the sewage system, and I have no doubt when this is done a still 

 further improvement will follow. 1 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 practically 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 twenty 

 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 pro- 

 perties 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 pollu- 

 tions 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 Eenton and Alexandria. Dumbarton, again, is a shipbuilding- 

 centre, and in the whole Yale there is a large and thriving popula- 

 tion. 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 pollutions other than a small amount 

 of the dyeing bye-products are as yet poured straight into the river. 

 The Orr-Ewing works of the United Turkey-red Company, Limited, 

 are provided with settling tanks, which retain a certain proportion 

 of the heavier matter in suspension and as much grease as can be 



J The Burgh of Motherwell has recently acquired 10 acres of ground for Sewage 

 Purification Works, and the Burgh of Hamilton is likewise extending her sewage 

 treatment* 



