The top luxury river cruise lines are probably Tauck, Scenic and Uniworld, in that order.
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Barbara

12/1984 - American Hawaiian Cruises
8/2002 - Celebrity Infinity Alaska
2/2004 - Carnival Inspiration Western Caribbean
3/2005 - Carnival Spirit Mexican Riviera
8/2006 - NCL Crown Bermuda
2/2007 - Crown Princess Southern Caribbean
5/2007 - RCCL Serenade of the Seas Pacific Northwest
8/2007 - Island Princess Alaska
10/2008 - Star Princess Mexican Riviera
8/2009 - RCCL Mariner of the Seas - Mexican Riviera
10/2009 - Crown Princess Canada/New England
8/2010 - NCL Epic Eastern Caribbean
9/2010 - Grand Princess Transatlantic
3/2011 - Dawn Princess Australia
5/2011 - Coral Princess Panama Canal
7/2011 - HAL Volendam Alaska
9/2011 - RCCL Oasis of the Seas W Caribbean
5/2012 - Emerald Princess Baltic
11/2012 - Uniworld River Queen Rhine & Moselle
1/2013 - Grand Princess South America
4/2013 - Avalon Vista Tulip Time
10/2013 - Royal Princess Transatlantic
1/2014 - Ocean Princess Tahiti
4/2014 - Ruby Princess Transatlantic
8/2014 - MSC Divina Eastern Caribbean
3/2015 - Crown Princess Calif Coastal
4/2015 - Quantum of the Seas Bahamas
9/2015 - NCL Getaway Caribbean
5/2016 - HAL Westerdam Repo
2/2017 - Viking Star West Indies to Barcelona
4/2017 - Majestic Princess Rome to Athens
6/2017 - HAL Koningsdam Midnight Sun