Lyric discussion by Corvid 

Cover art for Annan Water lyrics by Decemberists, The

The correct lyrics are "You may render me a wrack when I come back"

:definition: Wrack: seaweed or other vegetation cast on the shore.

Basically he is begging the river to calm itself and let him pass just this once and in turn the river can be as strong as it wants when he returns and do what it wants with him as he tries to cross back over.

My Interpretation

The lyrics from the vinyl jacket read:

"You may render me a wreck when I come back"

Trivial matters!