Valley Hall effect is an appearance of oppositely directed fluxes in two valleys of the Brillouin zone. The valley flux is perpendicular to the initial flux of particles which can be induced by an external field and temperature gradient or caused by a drag of charge carriers and excitons by phonons or photons. We present a theory of the valley Hall effect in two-dimensional crystals induced by the drag [1]. It is shown that there are three contributions to the valley current: (i) the skew-scattering contribution and the anomalous contributions caused by (ii) side jumps of the wavepackets at scatterings and (iii) anomalous velocity. Partial cancellation of the anomalous contributions is demonstrated, and its physical reason is discussed. We also study the valley separation of electrons in ultra-clean two-dimensional channels where the mean free path exceeds by far the channel width and the conductivity is limited by the edge scattering. In these systems the anomalous velocity contribution dominates inside the sample while the side-jumps are important at the sample edges. As a result, a non-monotonous profile of the valley polarization across the channel is formed.
 [1] M. M. Glazov and L. E. Golub, Phys. Rev. B 102, 155302 (2020) and Phys. Rev. Lett. 125, 157403 (2020).