This is a view of Portrack Lake which was the meander that was cut off from the straightened section of the River Tees when the river was improved in the 1830s. Courtesy of Fred Starr. 
Picture Stockton Archive
A Pictorial History of Stockton-on-Tees
The Lake was tidal, as I think, there were sluices which allowed the river to flow in and out, through the embankments which formed the side of the river. Over the years the lake sludged up and as you can see it got in a dreadful state. I also think sewage from the Malleable Steelworks was drained into the lake.