  In general relativity, the interaction between a black hole and the fields
around it (a process known as backreaction) proceeds via the evolution of the
black holes mass and angular momentum. Analogue models of gravity, particularly
fluid mechanical analogues, have been very successful in mimicking the
propagation of fields, and the effects they experience, around black holes.
However, hydrodynamic black holes are externally driven systems whose effective
mass and angular momentum are set by experimental parameters, and as such no
significant internal backreaction processes are expected to take place. We
show, using a rotating draining vortex flow, that a fluid system of finite size
exhibits a memory that keeps track of scattering processes in the system. This
memory is encoded in the total mass of the system and hence, the backreaction
arises as a significant global change in the background parameters, as opposed
to a small local correction. More importantly, this backreaction is
encapsulated by a dynamical metric, raising the possibility of studying
wave-background interaction around evolving black hole spacetimes.
