Açude Castanhão, officially known as Açude Público Padre Cícero, is the largest multiple-use public reservoir in Brazil, impounding the Jaguaribe River in the municipality of Jaguaribara, Ceará. The reservoir extends across parts of four municipalities — Alto Santo, Jaguaribara, Jaguaribe, and Jaguaretama — and sits approximately 307 kilometers from Fortaleza. With a total capacity of 6.7 billion cubic meters, the sheer scale of this water body defines the paddling experience: open water stretches across a vast inland sea in the semi-arid sertão of northeastern Brazil.

The reservoir was constructed between 1995 and 2002 and serves multiple purposes including drinking water supply, irrigation, industrial use, and fish farming. It also maintains year-round flow in the Jaguaribe River through controlled releases. The surrounding landscape reflects the semi-arid Caatinga biome characteristic of interior Ceará.

Water levels can vary dramatically depending on rainfall cycles. During the prolonged drought of 2012–2016, the reservoir dropped to just 5 percent of capacity, which would significantly affect paddling conditions and access. Paddlers should check current water levels before planning a trip, as extreme low-water periods can expose large areas of dry lakebed and alter access points considerably. The reservoir receives supplemental inflow from the São Francisco River transposition project, which began delivering water to Castanhão in 2021, providing some buffer against drought conditions.

Açude Castanhão is used as a fishing and paddling destination with identified water access points. Given its size and multi-use designation, paddlers should be aware of potential boat traffic associated with fishing and other activities. Contacting local authorities or DNOCS (the reservoir's operating agency) before a visit is advisable to confirm current conditions and any paddling regulations or restricted zones.