  A theory of the valley and spin Hall effects and resulting accumulation of
the valley and spin polarization is developed for ultraclean channels made of
two-dimensional semiconductors where the electron mean free path due to the
residual disorder or phonons exceeds the channel width. Both ballistic and
hydrodynamic regimes of the electron transport are studied. The polarization
accumulation is determined by interplay of the anomalous velocity, side-jump
and skew scattering effects. In the hydrodynamic regime, where the
electron-electron scattering is dominant, the valley and spin current
generation and dissipation by the electron-electron collisions are taken into
account. The accumulated polarization magnitude and its spatial distribution
depend strongly on the transport regime. The polarization is much larger in the
hydrodynamic regime as compared to the ballistic one. Significant valley and
spin polarization arises in the immediate vicinity of the channel edges due to
the side-jump and skew scattering mechanisms.
