The quality of surface water within a region is mainly influenced by natural processes such as precipitation, soil erosion and weathering, geology, vegetation and anthropogenic activities. The most important factor is geological formation in watershed and water flow path. In this study, the spatial variations of physical and chemical parameters of surface water including EC, SAR, Cl and TDS and factors influence on water quality were investigated in the Seimareh and Kashkan rivers in Ilam and Lorestan provinces. For this purpose the water quality data sets collected from the 14 hydrometric stations along the rivers during the 1-year (2009-2011) and analyzed. So the accuracy of the geostatistical interpolation methods such as kriging (OK), cokriging, and deterministic methods like inverse distance weighting (IDW) and radial basis function (RBF) in mapping of the studied parameters were evaluated. The result showed that based on the RMSE criteria, for all studied parameters, kriging and cokriging methods were showed lower RMSE while between deterministic methods, RBF is better than other interpolation techniques. The investigated variatins of physical and chemical parameters of surface water quality revealed the causes of increasing amount of physical and chemical parameters in Kashkan River and its tributaries were related to impact of geological formations in the region especially Gachsaran and Aghajary.