Embalse Dique de Ullum is a large reservoir on the Río San Juan, situated approximately 18 to 20 kilometers west of the city of San Juan in the Ullum Department of San Juan Province, Argentina. The dam was inaugurated on December 3, 1980, and the reservoir covers an area of 32 square kilometers with a total volume of 440 million cubic meters. Average depth is 15 meters, with a maximum depth of 40 meters. The reservoir sits at an elevation of 768 meters above sea level at the head of the Quebrada de Ullum canyon.

The water is notably turquoise in color, set against a backdrop of dramatic mountain terrain. The reservoir is accessed via Ruta Provincial 60, and the approach road from the city of San Juan is lined with palm trees along a divided highway. The primary purposes of the dam are irrigation of the Valle del Tulum agricultural zone, hydroelectric power generation, and potable water supply for the greater San Juan area, all of which are worth keeping in mind when planning a paddling visit, as water levels can fluctuate with operational demands.

The reservoir is an established venue for water sports, including kayaking, windsurfing, and sailing. The area just upstream of the reservoir on the Río San Juan, known as Aguas Blancas Park, features an artificial wave that has hosted national and international freestyle and slalom kayak competitions, including a freestyle kayak event in 2017. This section is specifically prepared for whitewater kayaking disciplines and is distinct from the flat-water paddling available on the reservoir itself.

Sport fishing is also available on the reservoir almost year-round, with rainbow trout and brown trout among the species present. Fishing requires a license, obtainable at local fishing supply stores. The reservoir is part of a broader tourism circuit called the "Ruta Interlagos," which also includes Dique Punta Negra further along the route, covering more than 100 kilometers of mountain scenery. Trekking circuits of varying difficulty are available in the surrounding area. Visitors are advised to contact the Secretaría de Turismo de San Juan for current provider listings, schedules, and any applicable regulations for water activities.