  Using quantum gas microscopy we study the late-time effective hydrodynamics
of an isolated cold-atom Fermi-Hubbard system subject to an external linear
potential (a "tilt"). The tilt is along one of the principal directions of the
two-dimensional (2D) square lattice and couples mass transport to local heating
through energy conservation. We study transport and thermalization in our
system by observing the decay of prepared initial density waves as a function
of wavelength $\lambda$ and tilt strength and find that the associated decay
time $\tau$ crosses over as the tilt strength is increased from
characteristically diffusive to subdiffusive with $\tau\propto\lambda^4$. In
order to explain the underlying physics we develop a hydrodynamic model that
exhibits this crossover. For strong tilts, the subdiffusive transport rate is
set by a thermal diffusivity, which we are thus able to measure as a function
of tilt in this regime. We further support our understanding by probing the
local inverse temperature of the system at strong tilts, finding good agreement
with our theoretical predictions. Finally, we discuss the relation of the
strongly tilted limit of our system to recently studied 1D models which may
exhibit nonergodic dynamics.
