Possible Clues:

• Scottish river and firth
• The Australian yachts, primarily, that can be seen in Perth
• Dundee's river
• 'The ___ Bridge Disaster' (McGonagall poem)
• Wait to drop two rivers — for one?
• Buster of rock
• River of Scotland
• It flows, steady at intervals
• River described as “silvery” in at least four of William McGonagall’s “poetic gems”
• Occasionally steamy Irish brew