About 45 miles (72 km) long, it originates in the Cascade Range and flows generally west and northwest, emptying into the southern end of Lake Washington.

The Cedar River originates in the Cascade Range near Abiel Peak, Meadow Mountain, and Yakima Pass, along the King and Kittitas countyline.

Several headwater streams join in the high mountains fed from glacial run-off, then the Cedar River flows generally west.

The Cedar River Watershed provides drinking water for 1.4 million people in the greater Seattle area.

About two-thirds of King County uses water from the Cedar River Watershed, over 100 million US gallons (380,000 m3) per day.

[5] Chester Morse Lake is the main storage reservoir of the Cedar River Watershed system.

[citation needed] The upper Cedar River flows through a region of deep and porous glacial outwash.

[8] The watershed also has an education center where visitors can learn about the issues that involve the region's drinking water, forest and wildlife.

The city constructed a ninety-foot concrete dam and used bedrock to the west and a glacial moraine to the east as natural barriers.

Between the hours of 12AM and 2AM on December 23, 1918 a large section of the Cedar Reservoir failed and spilled between 800,000 and 2,000,000 cubic yards of water.

The discharge flowed down Boxley Creek Valley, destroying the town of Edgewick, sawmills (owned by North Bend Lumber Company), and parts of the Milwaukee Railroad.

In 1924 the City of Seattle began the process of managing the Cedar River Watershed with a plan of ensuring water quality for the future.

The 1962 Cedar River Watershed Cooperative Agreement began the process of transferring the remaining privately owned land to the City of Seattle.

[5] Due to logging in the early 20th century, only about 17%, or 14,000 acres (57 km²), of the Cedar River Watershed consists of old growth forest.

Starting in the late 20th century efforts began to be made to protect and restore the river and its watershed.