The purpose of the present paper is to highlight the hydrologic role of Alnus glutinosa - the black alder, which is spread in two stands within the hydrographic basin of Râul Mare – Cugir. As they are pure stands, meaning stands made of the same species (alder) and equi (trees with the same age), they serve as a typical example of a homogenous statistical collectivity. In the P.U. III Râul Mare, the forestry formation “alders made of black alder” is covering a surface of only 1.2 ha out the total area of forest fund, situation that leads to significant amounts of sludge collected from the entire hydrographic network and the hydrographic basin taken into study. Within the present study, 2 administrative units were selected, covering 1.2 ha surface within the Forestry District of Cugir – P.U. III Râul Mare where one established 8 plots, 4 in each administrative unit, and measurements were carried out on a total number of 220 trees. Within the 1B administrative unit, 84 trees were measured, predominantly from the quality class IV, having diameters between 12 and 42 cm and heights between 14.2 and 19.2 m, while within the 41E administrative unit a number of 136 trees were measured, being in the majority in the same quality class IV, with diameters between 8 and 34 cm and heights between 12.7 and 13.5 m.