In 2010-2012, the hydrogeological model (HM) of Latvia (LAMO) was established by scientists of Riga Technical University (RTU). In 2013-2014, LAMO has been upgraded. In 2015, the recent version LAMO4 was developed. In LAMO4, the plane approximation step was decreased from 500 metres to 250 metres. The base flows of rivers for HM have been adjusted by accounting for data that were provided by measurements of river streams LAMO is run by the commercial program Groundwater Vistas (GV), version 6. It contains the MODFOW and MODPATH programs for supporting HM and for tracking groundwater particles, respectively. By using MODPATH, the case of the Iecava river was investigated, in order to find sources that support groundwater inflow into the river (its base flow). Unforeseen results were obtained. Due to appliance of the large regional HM, it was discovered that some part of the river base flow comes from areas that are located very far from the river drainage basin. It also was not expected that even within the basin the particle traces were very complex. Results described in the paper, serve as demonstration for usefulness of applying large complex HM for investigating processes of nature.