[2] The reservoir was formed behind Mossyrock Dam in the late 1960s and submerged the communities of Kosmos, Nesika, and Riffe; residents of the area were relocated due to the planned inundation.

[10] In 2017, Tacoma Power lowered the maximum water level of the lake by 30 feet (9.1 m) due to concerns that the piers on Mossyrock Dam could fail during a major seismic event and cause catastrophic flooding.

[11] Located on the eastern shore of the lake is the Kosmos Unit, a part of the Cowlitz Wildlife Area.

Also known as Kosmos Flat, the 520-acre (210 ha) site contains a mix of habitats, such as meadow, riparian, and wetland, and is split by Rainey Creek.

The unit contains a small hardwood forest and is known for abundant wildlife, especially various species of birds including pheasants, ospreys, and raptors.