  We use data from the DESI Legacy Survey imaging to probe the galaxy density
field in tomographic slices covering the redshift range $0<z<0.8$. After
careful consideration of completeness corrections and galactic cuts, we obtain
a sample of $4.9\times 10^7$ galaxies covering 17 739 deg$^2$. We derive
photometric redshifts with precision $\sigma_z/(1+z)=0.012 - 0.015$, and
compare with alternative estimates. Cross-correlation of the tomographic galaxy
maps with Planck maps of CMB temperature and lensing convergence probe the
growth of structure since $z=0.8$. The signals are compared with a fiducial
Planck $\Lambda$CDM model, and require an overall scaling in amplitude of
$A_\kappa=0.901\pm 0.026$ for the lensing cross-correlation and $A_{\rm ISW} =
0.984 \pm 0.349$ for the temperature cross-correlation, interpreted as the
Integrated Sachs-Wolfe effect. The ISW amplitude is consistent with the
fiducial $\Lambda$CDM prediction, but lies significantly below the prediction
of the AvERA model of R\'acz et al. (2017), which has been proposed as an
alternative explanation for cosmic acceleration. Within $\Lambda$CDM, our low
amplitude for the lensing cross-correlation requires a reduction either in
fluctuation normalization or in matter density compared to the Planck results,
so that $\Omega_m^{0.78}\sigma_8=0.297\pm 0.009$. In combination with the total
amplitude of CMB lensing, this favours a shift mainly in density:
$\Omega_m=0.274\pm0.024$. We discuss the consistency of this figure with
alternative evidence. A conservative compromise between lensing and primary CMB
constraints would require $\Omega_m=0.296\pm0.006$, where the 95% confidence
regions of both probes overlap.
