Brush and ink + Photoshop
Funnily enough, the picture that inspired me was a detail of the picture! Here are some photos of the original painting:
[update: here they are!]
Original detail shot
The complete painting I found afterwards
Also, browsing through the internet, I discovered a few pictures of the river by Hiroshige. Interesting to see how different artists focus on different elements of a scene.
And to finish of with, the original ink drawing. A pain to scan - 4 separate scans on my A3 scanner!
Brush and ink