Río Cruz del Eje winds through the semi-arid landscapes of northwestern Córdoba Province, Argentina, draining an active basin of approximately 1,700 km² with an average flow rate of 3 m³/s. Formed by the confluence of the San Marcos Rivers, the river receives major tributaries including the Candelaria and Quilpo Rivers before ultimately reaching the Salinas Grandes salt flats during major flood events. The basin's endorheic character means that connections to downstream salt pans occur only during wet periods, giving the river a distinctly seasonal personality shaped by the surrounding sierras and the semi-arid climate of the region. A defining feature of the system is the Cruz del Eje Dam and its reservoir, which creates a range of aquatic habitats — from flowing riverine stretches influenced by the sierra topography to the still, deeper waters of the impoundment itself. This diversity of environments supports a surprisingly rich fish community, with the reservoir alone hosting 17 species across seven orders and 12 families, representing the highest recorded species richness among regional reservoirs in the area.