  In the context of the decoherent histories approach to quantum theory, it is
shown that a class of macroscopic configurations consisting of histories of
local densities (number, momentum, energy) exhibit negligible interference.
This follows from the close connection of the local densities with the
corresponding exactly conserved (and so exactly decoherent) quantities, and
also from the observation that the eigenstates of local densities (averaged
over a sufficiently large volume) remain approximate eigenstates under time
evolution. The result is relevant to the derivation of hydrodynamic equations
using the decoherent histories approach.
