Investigation of Latvian Rivers in Lowland and Hilly Areas by Using the Hydrogeological Model of Latvia The Water Framework Directive of European Union recommends applying the river drainage basin principle for defining boundaries of groundwater bodies. This principle has been used in Latvia for establishing boundaries for the main water bodies of the country. By using the hydrogeological model of Latvia (LAMO), scientists of Riga Technical University have tested applicability of the principle for lowland (Iecava) and hilly (Malta) river drainage basins. The groundwater mass balances of the basins were examined and the MODPATH particle tracking was applied. It was discovered that only for the Quaternary system, the principle can be used, because there the groundwater flow through the basin border was relatively small. The principle does not hold for the primary strata. In Latvia, drinking water is mostly provided by the primary strata. For this reason, the boundaries of the main groundwater bodies should be revised. The investigation was supported by the Latvian State research program EVIDEnT Keywords: hydrogeological model, river drainage basin, balance of groundwater flows, MODPATH