Possible Clues:

• River breaks in rounding marsh, finally
• Tower Bridge spans it
• The Ma's in London
• Morning dipping in the Southern river
• English setting for a series of Impressionist paintings by Monet
• The longest river entirely in England
• River with its source near Kemble in the Cotswolds
• What has London banks acting badly during trial, mostly
• River which has its source in the Cotswolds and flows into the North Sea
• River by Parliament