Related Clues:

• The Suir, Nore and Barrow
• Exe and Wye, say (but not Zed)
• King splits trousers? They're in all across solutions here
• The Nile, etc
• Missouri and Ohio
• Seine, Nile, Tigris, etc
• Tweed and Mersey?
• Fourth, twenty-fourth and twenty-fifth letters, all going to the third, reportedly?