The determination of maximum water discharges of floods and rain floods at small mountain streams is relevant for engineering and construction purposes. However, currently, the accounting of the surface runoff of small watercourses is often neglected when construction works are being carrying out. This fact leads to various problems with the exploitation of constructed objects. The purpose of this work is to assess the impact of the construction of the biathlon complex in the recreational area of Yuzhno-Sakhalinsk on the maximum surface runoff from the territory that it occupies. Maps of natural and anthropogenically transformed catchments of the territory were created with the usage of the data, collected during the field work in the summer of 2018. Hydrographical characteristics of both kinds of catchments were determined in accordance with generally accepted recommendations. On the basis of the obtained data, there was carried out the comparative analysis of the current state of the study area and its char