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.

The Coos River flows for about 5 miles (8.0 km) into Coos Bay along the Pacific coast of southwest Oregon in the United States. Formed by the confluence of its major tributaries, the South Fork Coos River and the Millicoma River, it drains an important timber-producing region of the Southern Oregon Coast Range. The course of the main stem and the major tributaries is generally westward from the coastal forests to the eastern end of Coos Bay near the city of Coos Bay.