The river flows from the throne of God and the Lamb--that's the source. And it flows right through the middle of the street of the city. What begins with God sustains the world.
On either side of the river is the tree of life.
This tree has leaves for the healing of the nations. The word "nations" means that the healing is not just for us insiders, but that is for them too.
I found an April 1999 Interpretation that had survived at least three moves. In it, Gail A. Ricciuti writes about Revelation 21:22-22:5 in the section, Between Text and Sermon.
I hope you also can find a copy and read her entire article. Here's an excerpt:
The end of things will come not by a cosmic catastrophe but a revealing, not from the worst we can imagine, but from the best we dare to hope. The psalmist records that it was by the rivers of Babylon we sat down and there we wept when we remembered Zion (Psalm 137:1). But whereas the rivers of Babylon represented exile, John's river-vision is of a homecoming....
Rome's power proves no match for the Power embodied in the River flowing from the throne of the God-Lamb, and the Tree rooted by those waters. The ultimate triumph of God is best imagined, paradoxically, in the organic, ecological realm, which proves at last enduring and indestructible in a way that all the the earthly powers were not. The final assurance we are given that God will preside over the end of history as over the beginning of creation, and really preside over it so much as dwell within it....
The final denouement is not a threat but an invitation to us, as inheritors of a blessed future, to begin to build on earth the reality toward which our hope reaches out!