A wee nano stuck in a bridge only accessible from the riverside.
Simples...
...well, if you have a kayak and a good sense of balance! Please note: There are two caches for to account for the varying river height. Spoiler pics now included
Gur pnpur vf ba gur jrfg fvqr bs gur evire naq gur fbhgu fvqr bs gur oevqtr, jurer gur aneebj yrqtr zrrgf n fznyy pbeare ba gur snpr bs gur jnyy.