Is it not on the monsal trail... ? Could find that and walk along...
I'm fairly sure (after reading an article about Owen Paterson trying to control his river for his own benefit and not let those pesky proles all over his prime fishing land) that rivers and streams are not owned by anyone and are a public right of access so you can't be done for trespassing (until you get out of the river, in which case you can be if you're on private land). I'm not sure how accurate this information is though.