In this case the otter's run along the wall to deposit spraint.
Close-up of spraint
The spraint in this picture's the dark blob at the bottom on the left hand side of the kerb.
This water vole came up from a burrow right by my feet.
I've never seen an otter in the wild, but I do know how to check where they hang out. Under bridges is a favourite spot, and these two bridges at Grindley Brook are good examples. The second one's right by the busy lock and canal shop, but I presume the otters must come at night when it's quiet and mark the walls and kerbs with their spraint.
Must share this excellent Youtube footage of some fighting female water voles my friend posted yesterday. It's the best example I've seen. http://www.youtube.com/watch?v=Z4HahC7W6-k&list=FLnxzCfi9DQN0pH40VIkLeGg&feature=mh_lolz