Three versions of this fragment exist: B12 | 49a (see below) | B91
Because of this fact, there is much debate on which version is authentic (if any) and what it means.
B49a: "We step and do not step in to the same rivers; we are and are not."
B12 is a fragment about the unity/diversity inherit in reality. Reality is changing and in flux (like the the different waters), yet it is all stable and unified (like a river). B49a assigns change and stability to the one stepping into the river as well as the river. B91 focuses just on the diversity and not on the unity. I think that B49a above is authentic (at least the first half). B12 also seems authentic, giving the reason for B49a: "different and different waters flow" upon those who step into a river.