  We propose to use the hadron number fluctuations in the limited momentum
regions to study the evolution of initial flows in high energy nuclear
collisions. In this method by a proper preparation of a collision sample the
projectile and target initial flows are marked in fluctuations in the number of
colliding nucleons. We discuss three limiting cases of the evolution of flows,
transparency, mixing and reflection, and present for them quantitative
predictions obtained within several models. Finally, we apply the method to the
NA49 results on fluctuations of the negatively charged hadron multiplicity in
Pb+Pb interactions at 158A GeV and conclude that the data favor a
hydrodynamical model with a significant degree of mixing of the initial flows
at the early stage of collisions.
