This park owes its name to the Simpson River which flows through it on its way to the West, and is one of the most desired rivers for fly-fishing.
This reserve owes its name to the Rio Simpson, the river which crosses the park on its journey westward, and which is one of the best rivers for fly-fishing.
The protected area is 40,452 hectares (88,994 acres) of rugged geography, with some peaks reaching higher than 1,600 m (5,248 ft) above sea level, creating valleys and narrow canyons that shelter the descending rivers. As well as the Simpson, there are the Correntosos and Cascada de la Virgen rivers, branches of the Simpson, both surrounded by beautiful scenery and lush vegetation.
In terms of vegetation, there are mainly forests of coigües, tepas, mañío, ciruelillo, canelo and tepú. There is also chilco, which is the most abundant tree of the zone. In terms of fauna, the huemul is found in the eastern sector, where the geography is rocky, and on the steep slopes of the Andean and Huemules hills. The puma, on the other hand, lives in areas of medium altitude, spending the summer at a higher altitude and the winter on the plains. Other species which live in the reserve are the pudú, fox, güiña, bandurrias, queltehues and a large variety of wild ducks.