Cuejdel Lake, the largest natural barrage water body in Romania and the most recent natural dam lake in Europe, is situated in the Oriental Carpathians. The first small lake in this region formed in 1978 and a second one, which still exists, formed in 1991. Measurements were made in the period 2010 – 2012, with a Hach Lange multi-parameter instrument to identify the physico-chemical parameters of the water. Two basic ecological parameters were assessed, including temperature, pH and LDO (luminiscent disolved oxygen). Cuejdel Lake acts as a dimictic reservoir with direct thermal stratification during the summer, spring and autumn, and reverse stratification during the winter. In the winter, the water freezes with an ice cover of between 25 and 90 cm thick. At the lowest depths, the temperature is constant all year (5°C). The surface water is influenced by the air temperature. In the summer, the water is between 2 and 3°C cooler than the surrounding air. The water is slightly alkaline with values that oscillate around 8. The alkalinity of the water is caused by the elements that are transported in the water as the hydrographic basin drains. The water in the upper section of the lake is very rich in oxygen. However, below 10-11 m, dissolved oxygen is not present. The temperature, pH and dissolved oxygen are correlated during the spring, summer and autumn, but not during the winter (this phenomena is induced by the ice cover). In addition, the relationships between temperature, pH and dissolved oxygen are occasionally disrupted by local factors, such as high depths, high input values, and the existence of vortex currents. The linear tendencies of these three indicators are interrupted by the presence of two thresholds, one at 2 m and one between 4 and 5 m.