Playing Area
It is permitted to move the River as a whole unit into the centre of the playing area before the commencement of laying any other tiles. All players must agree on the River’s placement if this is the case. The playing area extends to the edge of the table being used, the table edge being the boundary for tiles. No tile may overhang the table’s edge...
Adding to that: No tile may overhang the table or free float in empty space!
I think I remember reading that those are official rulings actually. It is permissible to move all tiles on the table as a whole, and the table edge is the edge of the permitted playing area.
