  We use exact diagonalization to study the eigenstate thermalization
hypothesis (ETH) in the quantum dimer model on the square and triangular
lattices. Due to the nonergodicity of the local plaquette-flip dynamics, the
Hilbert space, which consists of highly constrained close-packed dimer
configurations, splits into sectors characterized by topological invariants. We
show that this has important consequences for ETH: We find that ETH is clearly
satisfied only when each topological sector is treated separately, and only for
moderate ratios of the potential and kinetic terms in the Hamiltonian. By
contrast, when the spectrum is treated as a whole, ETH breaks down on the
square lattice, and apparently also on the triangular lattice. These results
demonstrate that quantum dimer models have interesting thermalization dynamics.
