I was there paddling this spring. Its a short season, pretty much every thing is natural snow melt runoff, common wisdom is all of April and first half of May is your best chance. There is a reasonable website you should be able to google with a few guesses set up by some of the small number of Greek paddlers. The main guide book is now a little dated and written by a German. I've only seen the German language version but the book said there was an English version. I copied the following info from inside the cover:
martin_bettinger AT t-online.de (email address obfuscated)
Franz Bettinger/Ulla Eich,
66386 St Ingbert, Obere Kaiserstrasse 127
My advice, these rivers are mostly in the North of the country, its very quiet and rural and utterly beautiful. Shuttle roads are tiny and obscure and you should buy good topo's. Two vehicles would be very helpful as few shuttles we did had any traffic nor were they river level but climbed high up. There is some hard paddling there but the group I was with was not upto that so I never got to see it, but I paddled some of the most spectacular class III and IV I have ever seen. Much of the rock was limestone and gorges are incredibly deep and cliffed out. Water quality in general was very high, though small villages surround and I suspect sewage systems are primitive. I was traveling there with a Czech group but most paddlers we saw were Austrian and quite friendly and helpful.
There are hydro scheme's going in every year so like so many places...get it whilst you can.