What is meant by the title of Chapter 7 "As Long As Grass Grows or Water Runs" in A People's History of the United States?

The point of
this chapter in Zinns book is that the United States government has consistently abused the
Native Americans.  The title refers to a phrase used by Andrew Jackson in giving instructions to
an envoy to the Choctaws and the Creeks.  The envoy was to tell them that they should accept
Indian Removal and move out beyond the limits of the United States.  If they did so, they would
own the land in that area as long as grass grows or water runs.

Of course,
the US government did not keep that promise.  Native Americans were later pushed off lands west
of the Mississippi as soon as white settlers wanted that land.  Therefore, Zinn uses this phrase
as a title to emphasize how Native Americans have been (in his mind) mistreated and lied to by
the US government.