An ancient stone sluiceway which includes a turtle-shaped stone basin has been excavated from among the Sakafuneishi ruins in Asuka. The sluiceway was situated 2.5 meters to the south of the oval- and turtle-shaped basins.
Turtle-shaped water facility unearthed at Sakafuneishi ruins
Archaeologists believe that the ancient stone sluiceway channeled water from a spring (that still flows today) to the turtle-shaped basin.
Various sculpted stone relics believed to be evidence of the existence of a water source near the ruins of the seventh-century Asukakyo palace had also already been unearthed at the site. The opening of a square shaft in the direction of the spring measuring 1.8 meters x 2.4 meters was surrounded by stone blocks. At the center of the shaft was a tower 1.3 meters high. The discovery of the sluiceway is thought to support the view that the Sakafuneishi ruins were used by Empress Saimei (594-661) for religious services using water from these channels.
Sakafuneishi ruins
These findings were the first time archaeologists succeeded in tracking down a water source and the path that water would have taken.
Heritage of Japan 
very interesting the boulder with what appear to be carved channels for water? any other photos of that?
Other sources say the structure resembles those in ancient Korea used for playing a game involving floating cups of sake down the channels. There are more photos I’m sure, if I track them down I’ll point you their way or post them.