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We present a pipeline based on a random forest classifier for theidentification of high column-density clouds of neutral hydrogen (i.e. theLyman limit systems, LLSs) in absorption within large spectroscopic surveys ofz>3 quasars. We test the performance of this method on mock quasar spectra thatreproduce the expected data quality of the Dark Energy Spectroscopic Instrument(DESI) and the WHT Enhanced Area Velocity Explorer (WEAVE) surveys, finding>90% completeness and purity for N(HI)> 10^17.2 cm^-2 LLSs against quasars of g< 23 mag at z~3.5-3.7. After training and applying our method on 10,000 quasarspectra at z~3.5-4.0 from the Sloan Digital Sky Survey (Data Release 16), weidentify ~6600 LLSs with N(HI)>10^17.5 cm^-2 between z~3.1-4.0 with acompleteness and purity of >90% for the classification of LLSs. Using thissample, we measure a number of LLSs per unit redshift of 2.32 +/- 0.08 atz=[3.3,3.6]. We also present results on the performance of random forest forthe measurement of the LLS redshifts and HI column densities, and for theidentification of broad absorption line quasars.

Detecting neutral hydrogen at z > 3 in large spectroscopic surveys of quasars