At Fishtek, a great deal of our work focuses on designing appropriate solutions to improve fish passage past structures and barriers to the migration (both upstream and downstream) of fish. The vast majority of these structures are man-made and a legacy left from the industrial (and pre-industrial) past.
Nature, however, also conspires to put barriers in the way of upstream-migrating fish and the current issue of the fantastic Catch Magazine has a beautiful image on the front cover that illustrates the battle that some fish face as they head upstream to spawn: http://www.catchmagazine.net/Magazine/cmoct2013#section=0&page=0