This wonderful procession of Royal Barges is part of the celebration of the 3 month long Buddhist Lent period. It is from Rama VII Bridge to the Temple of the Dawn (Wat Arun).
I paddled the Chao Phraya River last Christmas with John Gray (of the World Respected John Gray Sea Canoe) which was fun and our family visited Wat Arun last month.
More information from Richard Barrow - thanks again Richard.
He has even made a map.
View Royal Barge Procession in Bangkok in a larger map