  We reexamine the scattering coefficients of shallow water waves blocked by a
stationary counter current over an obstacle. By considering series of
background flows, we show that the most relevant parameter is $F_{\rm max}$,
the maximal value of the ratio of the flow velocity over the speed of low
frequency waves. For subcritical flows, i.e., $F_{\rm max} < 1$, there is no
analogue Killing horizon and the mode amplification is strongly suppressed.
Instead, when $F_{\rm max} \gtrsim 1.1$, the amplification is enhanced at low
frequency and the spectrum closely follows Hawking's prediction. We further
study subcritical flows close to that used in the Vancouver experiment. Our
numerical analysis suggests that their observation of the "thermal nature of
the mode conversion" is due to the relatively steep slope on the upstream side
and the narrowness of the obstacle.
