Here is a video about kayaking on the Arda River from last autumn, preparation for spring kayaking and rafting on Arda.
Here are details of the two-day program: https://rafting.bg/en/rafting-on-arda-river/
Dates for rafting on Arda river: 19th-20th of March, 16th-17th of April – confirmed group 7th-8th of May