A narrow land bridge separates the southern end of Lake Pueyrredón from the smaller, shallower Lake Posadas. Its drainage basin is bordered by the Ghio Lake basin to the northeast. Cochrane Lake drains into the Baker River through the short Cochrane River on its western edge. The 12,156 foot (3,706 meter) high Monte San Lorenzo has a small stream named Oro River, originating on its north flank that flows generally northeastward into Lake Pueyrredón. The vast semi-arid Patagonian Plateau lies east of the lake. The Ecker River and the Blanco River cut canyons across the plateau. The former is the main tributary of the Pinturas River.