  We theoretically study the effects of strong magnetoelastic coupling on the
transport properties of magnetic insulators. We develop a Boltzmann transport
theory for the mixed magnon-phonon modes (magnon polarons) and determine
transport coefficients and spin diffusion length. Magnon-polaron formation
causes anomalous features in the magnetic field and temperature dependence of
the spin Seebeck effect when the disorder scattering in the magnetic and
elastic subsystems is sufficiently different. Experimental data by Kikkawa et
al. [PRL 117, 207203 (2016)] on yttrium iron garnet films can be explained by
an acoustic quality that is much better than the magnetic quality of the
material. We predict similar anomalous features in the spin and heat
conductivity and non-local spin transport experiments.
