This study investigated whether the representative elementary area (REA) concept can be adopted in catchments with multiple geologies. We observed stream discharge at 65 points and water chemistry at 157 points in a 55 km² catchment that included multiple geologies. At observation points with uniform geology, stream chemistry became constant beyond about 1 km² in granodiorite and volcanic rocks. At observation points with multiple geologies, spatial variability remained large beyond a few square kilometers. SiO₂ and Mg²⁺ concentrations became constant above 10 km², but Ca²⁺ and electrical conductivity did not become constant until 55 km². Our calculations revealed that for areas of 1–17 km², almost all of the observed variables were explained by mixing based on geological percentages. However, at greater than 17 km², the observed values were higher than the calculated values. Therefore, in regions with multiple geologies, the range of the REA with single-parameter geology was confirmed. In our catchments, the REA concept was applicable to areas of 1–17 km², but areas larger than 17 km² was outside the range.