The Río Alerzal, also known as the Río Esperanza, is a steep mountain river in the western Chubut Province of Argentine Patagonia, draining eastward from Laguna Esperanza through rugged Andean terrain before joining the Río Turbio on its left bank, just upstream of where the Turbio opens into Lago Puelo. The river runs approximately 13 kilometers and descends around 230 meters over that distance, a gradient that produces numerous rapids throughout its course.

The river originates from the meltwater of Glaciar Esperanza Norte, the longest glacier in Chubut Province, and the upper valley sits within the Río Turbio Multiple Use Provincial Park and Reserve. Laguna Esperanza, the source lake, lies at an elevation of 496 meters and is of glacial origin, surrounded by dense Andean Patagonian forest dominated by Nothofagus species. The valley is characterized by glacial moraines, outwash terraces, and forested slopes, with peaks reaching above 2,400 meters at the Cordón de las Agujas. Annual precipitation in the Andean foothills of this region typically ranges from 1,000 to 2,000 mm, driven by Southern Hemisphere westerly winds, with water inputs coming from rainfall, snowmelt, and glacial melt.

The consistent gradient and the presence of many rapids along the full length of the river indicate a technically demanding watercourse. The remote location within a protected reserve, the dense surrounding forest, and the glacially fed hydrology are defining characteristics of the paddling environment. Access to Laguna Esperanza requires a long approach on foot. The only designated camping area in the vicinity of the lake is on its western shore, near a private estancia.