The West Fork Millicoma River is a tributary, about 34 miles (55 km) long, of the Millicoma River in the U.S. state of Oregon. Rising in northeastern Coos County near the Douglas County line, it flows generally southwest through the Elliott State Forest of the Southern Oregon Coast Range to the community of Allegany. There it joins the East Fork Millicoma River to form the Millicoma. In turn, the Millicoma, which is only about 9 miles (14 km) long, joins the South Fork Coos River to form the Coos River. The Coos River, itself only about 5 miles (8 km) long, empties into the eastern end of Coos Bay, which connects to the Pacific Ocean.

Coos Bay is an estuary where the Coos River enters the Pacific Ocean, the estuary is approximately 12 miles long and up to two miles wide. It is the largest estuary completely within Oregon state lines. The Coos Bay watershed covers an area of about 600 square miles and is located in northern Coos County, Oregon, in the United States. The Coos River, which begins in the Oregon Coast Range, enters the bay from the east. From Coos River, the bay forms a sharp loop northward before arching back to the south and out to the Pacific Ocean. Haynes Inlet enters the top of this loop. South Slough branches off from the bay directly before its entrance into the Pacific Ocean. The bay was formed when sea levels rose over 20,000 years ago at the end of the Last Glacial Maximum, flooding the mouth of the Coos River. Coos Bay is Oregon's most important coastal industrial center and international shipping port, with close ties to San Francisco, the Columbia River, Puget Sound and other major ports of the Pacific rim.