Crayfish?

Whilst sitting a the top of the waterfall this little fella crawled out for a look. I'm not sure what he is, but a Crayfish sounds the most plausible idea... to give you an idea of scale it's about 6" (15cm) long. We sat there and watched it for ages crawling along the weeds that were attached to the side of the waterfall... What it was doing we have no idea, but it was battling against a very strong current to crawl from the bottom of the waterfall to the top... didn't know Crayfish acted like Salmon :-)