Lake Arenal, Reservoir and wetland near Arenal Volcano, Costa Rica

Lake Arenal is a reservoir in Costa Rica's northern highlands, about 30 km long and nearly 5 km at its widest, with a surface of roughly 85 to 88 square kilometres. Its depth shifts seasonally between 30 and 60 metres. The Sangregado Dam closes its southeastern end, and the shoreline is cut by inlets and wooded points, with low hills beyond.

Lake Arenal began as a small natural lake. In 1979 the Sangregado Dam was built at its southeastern end, tripling the surface to 85 square kilometres. The government expropriated all land below 550 metres; the towns of Arenal and Tronadora were submerged, their residents resettled in the new town of Arenal to the northeast. The dam also displaced the Hacienda La Rosita cattle operation, owned by P. Eckrich & Sons, a subsidiary of Beatrice Foods.

The lake's name comes from the old town of Arenal, now submerged; residents were resettled northeast in the new town of Arenal. Today the lake is known for rainbow bass fishing, and kayak fishing in shallow coves is a growing activity. The Ramsar designation of 7 March 2000 covers the lake and its surrounding wetlands.

The dependable winds that draw windsurfers and kite surfers arrive from November through April and are strongest at the lake's western end, not the northern shore. Kayaking and stand-up paddling are more usual on the eastern part near the earthen Sangregado Dam. Hiking, biking and horseback riding are possible in the surrounding hills.

The water is used to generate electricity and supplies a meaningful portion of the country's overall power needs. The water surface and surrounding areas support over 300 bird species, making it a hotspot for birdwatchers.