  We review the apparently hydrodynamic behaviour of low transverse momentum
particles (p_T < 1.5 GeV/c) produced in central and semicentral (b < 7 fm)
heavy ion collisions at RHIC. We investigate the impact parameter dependence of
various observables, elaborating on radial and elliptic flow and particle
multiplicities. We also discuss possible ambiguities in the initialization of
the hydrodynamic system and present observables that should allow for their
resolution.
