We address the problem of inferring chemical parameters of river water qual
ity from biological ones. This task is important for enabling selective che
mical monitoring of river water quality. We apply machine learning, in part
icular regression tree induction, to biological and chemical data on the wa
ter quality of Slovenian rivers. Regression trees are constructed that pred
ict values of chemical parameters from data on the presence of bioindicator
taxa at the species and family levels.