The hydrogeological model of Latvia, version LAMO4, was used as the driver for the MODPATH program. The attempt was made to find sources of the river base flow of the Iecava river. Unforeseen results were obtained: many sources were located very far from the river drainage basin; even within the basin, the shape of pathlines is very complex. The results of this numerical experiment demonstrate usefulness of applying a large regional HM, like LAMO4, for investigating the complex processes of nature.