  The hydrodynamic formulation of quantum mechanics is used to elucidate the
mechanism for decoherence, the suppression of interference effects in a system
evolving from an initial coherent superposition. Analysis of time-dependent
trajectory ensembles, flux maps, and elements of the stress tensor for two
composite systems, in one of which the system is uncoupled to the environment,
leads to the decoherence mechanism. For the uncoupled case, the quantum force
acting on the fluid elements directs flux toward an attractor where the
interference feature arises. For the coupled case, the classical force acting
on each fluid element counters the quantum force and leads to gradual
separation of the components of the initial superposition. Concomitantly, fluid
stress is relieved when flux vectors diverge from a repellor in the mid-region
between the separating wavepackets, thus suppressing formation of the
interference feature.
