Question: A large tributary of the Clearwater River flows within a forest with how many acres?

Context paragraphs:
Clearwater River (British Columbia): The Clearwater River is the largest tributary of the North Thompson River, joining it at the community of Clearwater, British Columbia.  The Clearwater rises from glaciers in the Cariboo Mountains and flows in a mostly southerly direction for 201 km to the North Thompson.  Its entire course, except the last 5 km , is within Wells Gray Provincial Park.  Its confluence with the North Thompson is protected by North Thompson River Provincial Park.

South Fork Clearwater River: The South Fork Clearwater River is a 62 mi long river in north-central Idaho in the United States.  Draining about 1175 mi2 , the South Fork joins with the Middle Fork Clearwater River to form the Clearwater River, a major tributary of the Snake River.

Swan Lake (Montana): Swan Lake is a lake located east of Flathead Lake and the town of Bigfork, Montana.  The Swan River comes from the south and fills the lake.  The lake is similar to Moyie Lake in southern British Columbia.  It is a narrow, small, and twisty lake that is hard to navigate.  There is a small divide that separates the Swan River Valley from the Clearwater River Valley.  The Clearwater River flows south through a series of lakes, including Seeley Lake and empties into the Blackfoot River.  The two valleys are known locally as the Seeley/Swan.  Swan Lake runs parallel to Montana Highway 83.

Clearwater River (Queets River): The Clearwater River is a river situated on the Olympic Peninsula in Washington.  It is the main tributary of the Queets River.  The Clearwater River is 39 mi long.  Its drainage basin is 61.5 sqmi in area.  The Clearwater's main tributaries are the Snahapish River and the Solleks River.

Middle Fork Clearwater River: The Middle Fork Clearwater River is a short, but high volume river in northern Idaho and is the major source of the Clearwater River, a tributary of the Snake River.  The Middle Fork flows west for 23 mi from the confluence of the Selway River and Lochsa River at Lowell, to the town of Kooskia where it joins with the South Fork Clearwater River to create the Clearwater River.  The Middle Fork's entire length is within Idaho County, although a small portion of its watershed extends into Clearwater County.

Clearwater River (New Zealand): The Clearwater River is on the West Coast of the South Island of New Zealand.  The river originates on the northern slopes of Mt Mitchell on the eastern end of the Victoria Range, only one valley north of the Fox Glacier and Fox River.  Clearwater River and its tributary creeks drain land to the north of the Cook (Weheka) River.  The river flows under a bridge of just north of the Fox Glacier township and passes close to Lake Matheson shortly after before flowing into Cook River, which drains into the Tasman Sea.

Clearwater River Dene Nation: The Clearwater River Dene Nation is a Dene First Nations band government in the boreal forest area of northern Saskatchewan, Canada.  It maintains offices in the village of Clearwater River situated on the eastern shore of Lac La Loche.  The Clearwater River Dene Nation reserve of Clearwater River shares its southern border with the village of La Loche.

Selway River: The Selway River is a large tributary of the Middle Fork of the Clearwater River in the U.S. state of Idaho.  It flows within the Selway-Bitterroot Wilderness, the Bitterroot National Forest, and the Nez Perce National Forest of North Central Idaho.  The entire length of the Selway was included by the United States Congress in 1968 as part of the National Wild and Scenic Rivers Act.

Bitterroot National Forest: Bitterroot National Forest comprises 1.587 million acres (6,423 km²) in west-central Montana and eastern Idaho, of the United States.  It is located primarily in Ravalli County, Montana (70.26% of the forest), but also has acreage in Idaho County, Idaho (29.24%), and Missoula County, Montana (0.49%).

Murtle River: The Murtle River is a river in east-central British Columbia, Canada.  It rises from a large unnamed glacier in the Cariboo Mountains at an elevation of 2300 m and flows southwest for 18 km to the head of gigantic Murtle Lake.  The river also drains Murtle Lake then flows southwest for 36 km into the Clearwater River.  The Murtle River is the longest and largest tributary to the Clearwater.

Supporting paragraphs: Selway River Bitterroot National Forest
Answer: 1.587 million