Here is another great piece of footage demonstrating the impact that man-made structures have on fish:
https://www.youtube.com/watch?v=h0mkWWaEiec
The footage was shot at Norbury weir, on the River Dove. The weir is almost 3 m high and is a very significant barrier to the migration of salmon, as well as resident brown trout and grayling within the river.
The difficulty the fish (which are mostly salmon and/or large trout) have in moving up the weir is evident and it clearly shows how important a fish pass can be in ensuring the free passage of fish past anthropogenic obstructions such as this.