The speaker , when using<em> henceforth</em>, refers to the fact that if the land is sold to the settlers, as a consequence, the rivers will also be given to them. Chief Seattle means that if his people sell the land to the white men , the settlers, that equals giving all what is in nature to them. Selling the land is just the begining of what will happen from then -now- on; the rivers will come next. Everything will end up being in the hands of the settlers.