UK ECN site. The catchment area of the River Clyde is about 2000km2 and the river changes in character a great deal in its 121km journey to the tidal weir in Glasgow. In its upper reaches, it is used to fill the Daer reservoir which supplies drinking water to much of South Lanarkshire; there is also sheep farming and commercial afforestation in this part of the catchment. The river is joined by tributaries of various sizes and quality reflecting the land uses of their catchments: there is much opencast coal mining in some, whilst others are urban or agricultural. The Clyde passes through a fertile valley in its middle reaches where there is extensive market gardening, fruit growing and garden centres. In its lower reaches the river receives a considerable amount of treated sewage effluent from large regional sewage works. The river is quite sluggish in its flow because of the flat landscape. As a result of this and the BOD of the effluents, there is serious oxygen depletion in the lower reaches during the summer months. The ECN sampling site is situated in the lowest reach, where average flow is 41 cumecs.