  We study and numerically compute the scattering coefficients of shallow water
waves blocked by a stationary counterflow. When the flow is transcritical, the
coefficients closely follow Hawking's prediction according to which black holes
should emit a thermal spectrum. We study how the spectrum deviates from
thermality when reducing the maximal flow velocity, with a particular attention
to subcritical flows since these have been recently used to test Hawking's
prediction. For such flows, we show that the emission spectrum is strongly
suppressed, and that its Planckian character is completely lost. For low
frequencies, we also show that the scattering coefficients are dominated by
elastic hydrodynamical channels. Our numerical results reproduce rather well
the observations made by S. Weinfurtner {\it et al.} in the Vancouver
experiment. Nevertheless, we propose a new interpretation of what has been
observed, as well as new experimental tests.
