Lago Rosario is located in the Futaleufú Department of Chubut Province in Argentine Patagonia, approximately 22 kilometers from the Welsh settlement of Trevelin and close to Los Alerces National Park. The lake sits at coordinates 43°15′01″S, 71°20′56″W, in a region where waters drain the Andes toward both the Atlantic and Pacific oceans.

The surrounding landscape is characteristic of the Patagonian Andean foothills, with clear-flowing waters and dramatic mountain scenery. The lake and its watershed sit atop two significant aquifers, contributing to the clarity and productivity of the water system in central Chubut Province.

Small motorized inflatable boats are used on the lake by recreational visitors. Camping facilities are available at the lake for paddlers looking to base themselves directly on the water. The Lago Rosario Lodge operates from November 1 through May 1 and is situated directly on the lake, catering primarily to fly fishing visitors. This suggests the warmer months of the Southern Hemisphere spring through early autumn represent the primary visitor season.

The waters support rainbow trout, brown trout, brook trout, landlocked salmon, and Pacific king salmon—relevant context for paddlers who may encounter fishing activity on and around the lake. The region's network of clear streams and rivers connecting to the lake system offers additional waterways worth exploring in the vicinity.