Sluice Gate

This object has puzzled me ever since I came to live here in Leith, six or seven years ago now. It is very obviously a sluice gate, but what is not so obvious is its purpose, and it must have, at some point in time, had a purpose. The puzzling thing is that this is the only remaining piece of evidence of what must have been here in the past, there is absolutely nothing remaining that could give a possible clue as to why this sluice gate is where it is.

Update:-

On thinking about it, this is even more puzzling because all of the surrounding ground is higher than the river itself...

Update on the Update:-

Mystery solved, thanks to a link from Shandonner, it seems that there was once a flour mill on this bend of the Water of Leith. So, I can only assume that it had a water wheel which was lower than the actual mill and the the power generated was somehow fed up into the mill itself.

Further:-

For those interested here is the map with the position of the sluice gate marked with an X.

I also discovered an artist's impression from the early 1900's