In the Old Testament, the mercy seat on the ark in the Most Holy Place (in the back of the tabernacle) was pictured as the throne of God (2 Samuel 6:2; Psalm 99:1). As God himself is the temple in the New Jerusalem, his throne is not pictured as standing in some specific location but instead is wherever God is: everywhere. The point is that this river comes from God himself.
1 Then the angel showed me the river of the water of life, bright as crystal, flowing from the throne of God and of the Lamb