  We develop a hydrodynamic description of the resistivity and
magnetoresistance of an electron liquid in a smooth disorder potential. This
approach is valid when the electron-electron scattering length is sufficiently
short. In a broad range of temperatures, the dissipation is dominated by heat
fluxes in the electron fluid, and the resistivity is inversely proportional to
the thermal conductivity, $\kappa$. This is in striking contrast with the
Stokes flow, in which the resistance is independent of $\kappa$ and
proportional to the fluid viscosity. We also identify a new hydrodynamic
mechanism of spin magnetoresistance.
