Dique de Ullum: San Juan's Desert Reservoir

Dique de Ullum sits 18 km west of San Juan city, a 32-square-kilometer sheet of turquoise water held back by a 1980s dam. The reservoir is free to enter, and the drive from the provincial capital takes about 25 minutes on paved Ruta Provincial 60. What you get for that short trip is a sharp contrast: bare, rust-colored hills on every side, water cold and clear in the middle, and a shoreline dotted with balnearios (beach clubs), paradores, and a growing cluster of water-sports operators. It is the closest thing San Juan has to a beach — and locals treat it that way, packing the eastern embarcadero on summer weekends.

The dam itself was built for water regulation and hydroelectric power, and that utilitarian origin shows in the best way: the lake is long, deep (up to 40 meters in places), and calm enough for kayaks, paddleboards, and small sailboats. Winds pick up in the afternoon, which is why windsurfers and kiteboarders gravitate here from September through April. You do not need a boat to enjoy it. A towel, mate, and a spot on the rocky shore will do.

Getting There and Getting on the Water

By car from San Juan

Ruta Provincial 60 runs straight from the city's western edge to the dam. The road is fully paved and in good condition; the only tricky part is the final stretch around the embarcadero, where signage is sparse. If you are driving yourself, aim to arrive before 11 a.m. on weekends — the parking area near the main beach fills by midday in January and February. There is no gas station along the route, so fill up in San Juan before you leave.

By bus

The Red Tulum network connects the city to the reservoir, though service is not as frequent as the schedule suggests. Lines 160, 161, and 162 reach the embarcadero at Dique de Ullum, with the 160 running from the city center and the 161/162 covering suburban stops. Buses run hourly on weekdays but thin out on Sundays. From the bus stop, it is a 500-meter walk to the water — a detail that matters if you are carrying coolers or kids. A taxi from San Juan costs roughly 8,000–12,000 pesos one way (fares change; confirm with the driver).

Boat rentals and launches

The only authorized boat launch is at the Embarcadero de Ullum, on the eastern shore. Navigation is permitted Wednesday through Sunday, 7 a.m. to 8 p.m., and all craft must comply with provincial law 7562 (life jackets, registration, and a safety check). Rental outfits on site offer kayaks, stand-up paddleboards, and small motorboats; prices are not posted consistently, so ask before you commit. The hydro-pedal (pedal boat) concession that started in 2019 still operates in high season, with six boats that fit three adults or two adults plus three children.

What to Do at the Reservoir

Swim, sunbathe, and use the balnearios

The embarcadero area is the only stretch officially habilitated as a balneario, meaning it has lifeguards and marked swimming zones. Entry is free. The beach is rocky rather than sandy — water shoes are worth the luggage space. Clubs along the shore charge for lounge chairs and shade structures, usually a few thousand pesos for the day. The public beach on the eastern side of the ASER property is the most accessible free option, though it gets crowded.

Hike the Quebrada de Ullum and Colorado Canyon

A short trail from near the dam leads into the Quebrada de Ullum, a dry gorge of volcanic rock that glows orange at sunset. The walk to the canyon rim takes about 40 minutes at an easy pace, with little shade — bring a hat and at least a liter of water per person. Local guides offer a longer trek to the Cañón Colorado and Cuevas Encantadas, combining geology with a bit of scrambling. The views over the lake from the top are the best in the area, and you will likely have them to yourself on weekdays.

Visit La Usina and the clay-limp park

La Usina, a renewable-energy-themed park at the embarcadero, opened in late 2024 and has become the reservoir's most surprising attraction. The immersive exhibit on clean energy runs Tuesday to Sunday, 10 a.m. to 1 p.m. and 5 p.m. to 9 p.m. General admission is 2,000 pesos (promotional rate; fees are subject to change — check the official site for current rates). The site also includes a walk through natural clay-limp formations that look lunar, plus a food and recreation area. It is a solid half-hour stop, especially with kids.

Eat, drink, and stay for sunset

Paradores around the embarcadero serve simple food — empanadas, sandwiches, grilled meat — at prices slightly above city levels. The real draw is the late afternoon: the water turns deep blue, the hills go pink, and the wind drops. Several establishments transition into bars and music venues after dark, particularly on Saturday nights in summer. If you want a quieter experience, drive a few kilometers west along the lake's northern shore, where the road is unpaved but the crowds disappear.

When to Go and What to Know Before You Arrive

Seasons and weather

The best window is October through April, when daytime temperatures sit between 25°C and 35°C and the water is swimmable. December to February is the busiest period — expect 12,000 vehicles per day at the reservoirs on peak weekends, according to provincial figures. Spring (September–November) and autumn (March–May) offer a better balance: warm afternoons, cooler mornings, and far fewer people. Winter (June–August) is quiet; the water is too cold for most swimmers, but the landscape is at its sharpest for photography.

Safety rules and practical tips

Navigation is allowed only Wednesday to Sunday, 7 a.m. to 8 p.m., and only from the authorized embarcadero. Swimming outside the balneario zones is risky — the reservoir has steep drop-offs and cold currents. The provincial government requires life jackets for all boat occupants and conducts checks at the launch. Sun protection is non-negotiable: the arid climate means constant sun and almost no shade. Bring more water than you think you need.

Combine it with Punta Negra and the wine valleys

Dique de Ullum is the first stop on the so-called Ruta de los Lagos. Continuing west on the same road for another 35 km brings you to Dique Punta Negra, a smaller but equally scenic reservoir with its own beach clubs. The drive between them takes about 40 minutes and passes through the Ullum valley, where several boutique wineries offer tastings by appointment. Most visitors do both dams in a single day, leaving San Juan around 9 a.m. and returning by 6 p.m.

Frequently Asked Questions

Is there an entrance fee for Dique de Ullum?

No. Access to the reservoir and the public balneario is free. You pay only for optional services: club chairs, boat rentals, food, and the La Usina exhibit (2,000 pesos at the promotional rate).

Can I get there by public bus from San Juan city?

Yes. Red Tulum lines 160, 161, and 162 stop at the embarcadero, with the 160 running from the city center. Buses are hourly on weekdays but less frequent on Sundays, and the stop is a 500-meter walk from the water.

What is the best time of year to visit?

October to April for swimming and water sports. Spring and autumn offer the best weather-to-crowd ratio. Summer weekends are the busiest — up to 12,000 vehicles per day — so arrive before 11 a.m. if you visit in January or February.

Are there lifeguards at the reservoir?

Yes, but only in the habilitated balneario zone at the embarcadero. Swimming outside marked areas is not recommended due to steep drop-offs and cold currents.