Holding back the water

I did a 17 mile run today by repeating a 3 mile loop round by the canal and up the Taff trail. It was actually quite a pleasant way to do it, breaking it down into manageable chunks, and it's a nice route. The canal is no longer used as is now a nature preserve (it's where that blasted heron from yesterday likes to hang out). This is an old lock, that I guess has been holding that water back for years, but is never filled.