Just got back from Kauai 2 days ago - I spent some time trying to figure out paddling potential before going but the info was sparse. It definitely helped to have a look around and think about potential runs. As far as I could see, there are a few rivers that have potential - and the AWA website indicates a class IV run through Waimea Canyon.
The rivers are characterized by being completely flat within a few miles of the ocean, then have moderate gradient punctuated by very tall waterfalls for the next few miles inland. Closer to the headwaters the rivers are vertical. Mt. Wai'ale'ale, at 5148 feet, is the wettest spot on earth - and there are waterfalls coming off every aspect.
A few places with potential:
Hanalei River - north side near Hanalei Bay
Kalihiwai River - north / northeast
Wailua River - largest river on any of the islands - east central
Hanapepe River - ? this is on the south side & looks like one of the longer drainages
Waimea River - Southwest side of the island, flows through Waimea Canyon a.k.a the Grand Canyon of the Pacific (this valley is very impressive, especially considering the size of the island. It looks like it should be on the scale of Utah / Arizona rather than an island 30 miles in diameter.) This side of the island is much drier than the east side.
Access to the Waimea River looks like a real challenge, but if you're motivated, there are a few trails to the bottom. You would have to descend about 3000 feet over several miles. Trails are good though, just steep.
You can download the DRG (USGS Topo set) from:
and take a closer look at what looks do-able yourself.
The only vessels that I could find for rental are sit on tops - we paddled up the Wailua River to some waterfalls - tough to get the stern under in the eddies though. I recommend stowing a 6 pack on board and renting your own kayak if you do this (rather than guided tour). The place to rent is about a mile north of the Wailua River mouth - left side of the main highway and across the street from the "guided kayak tours" place. It's just past the stoplight.
On the other hand, if I were going back tomorrow I would not hesitate to bring my playboat, or maybe a creeker if I was going to do some exploration. The river volumes weren't very high when we were there - it would probably be kind of a bony run on any of these right now - it wasn't really raining very much. However, these rivers respond very quickly to increased rainfall. The surf is kind of small right now, and I think you would have a great time on the waves. Just be aware of where the reefs are shallow - these places could ruin your day in a kayak if you were to be upside down.
Here's the story of a first decent through Waimea Canyon:
Also - I saw a number of signs around waterfalls & rivers warning about Leptosporosis. You'll probably want to bring your noseplugs and keep mouth closed going over the big drops.
If you have questions, I'll do my best to provide more specific info. Good luck and have fun. Recommendations: hike the Nuololo Trail & Kalaulau Trail (Na Pali Coast), surf Poipu & Hanalei Bay, snorkel with Turtles at Ke'e Beach.