Flagstaff Lake is located in Somerset County and Franklin County, Maine, in the United States. The North Branch Dead River and South Branch Dead River join in the lake, forming the Dead River.
The lake’s surface area is 20,300 acres (82 km2), and it has a storage capacity of about 275,482 acre-feet (339,802,000 m3). It is extremely shallow (more so during drought years). Its deepest point is 48 feet (15 m) Landlocked salmon and brook trout can be caught. Some points on the lake are very marshy and have to be negotiated with a canoe or kayak; even then, wading may be necessary in the marshy areas to get through to the rest of the lake.
Bigelow Mountain Range overlooks from the lake’s southern shore, which makes the lake popular for outdoor activities. The lake is mostly undeveloped with relatively few boats and is optimal for kayaking.
