South Slough National Estuarine Research Reserve (South Slough NERR) is located in a sheltered arm that forms the southern end of the Coos estuary in southwestern Oregon. The Reserve and its watershed are significant as a relatively undisturbed area representative of coastal ecosystems in the Pacific Northwest. This overview of the South Slough watershed and the surrounding Coos region illustrates the natural and cultural complexity and richness of the South Slough area.
The South Slough watershed is a 19,295 acre sub-basin of the Coos watershed drainage. Covering an area of approximately 20 sq miles at MHW, the Coos estuary is the sixth largest estuary on the Pacific coast of the contiguous United States and the largest estuary completely within Oregon state lines. Like most estuaries found in Oregon, the Coos estuary is a river mouth that "drowned" as sea level began to rise 20,000 years ago. In drowned river mouth estuaries, heavy winter precipitation results in high discharge of fresh water and sediment; seawater inflows dominate during the summer when terrestrial stream flows are low.
The Coos estuary divides just inside its ocean opening. The main branch, Coos Bay, arches to the north-northeast, and the narrowly enclosed arm of South Slough drops almost due south. Coos Bay’s main tributary, the Coos River, enters at the southeastern end of the "U." At the southwestern end, the estuary opens to the sea. The ocean mouth of the estuary is defined on the north by the tip of a seven-mile long sand spit (North Spit), and on the south by a rocky headland (Coos Head).
The shoreline of the Coos estuary is bordered by the municipalities of Charleston, Barview, North Bend, Coos Bay, Millington, Eastside, and Glasgow, with a collective population of approximately 36,000 people in 2000. The estuary is an important industrial center and shipping port, with the navigational channel routinely dredged to maintain adequate depths for commercial shipping. Extensive tidelands, primarily sand flats, mudflats, and saltmarshes, constitute about 60-70% of the estuary.
The area of land that drains into South Slough is roughly shield shaped and approximately twice as long as the Slough itself. It is generally characterized by steep slopes and sandy bluffs. The
watershed boundary is defined on the east, south, and west by prominent ridges with many small streams draining into South Slough. The southern half of the watershed, beyond the Reserve boundaries, contains the springs and creeks which feed Winchester Creek, the Slough’s largest tributary stream.
Many streams enter the Slough near the narrow peninsula of Long Island Point. This north-pointing ridge separates the Slough into an eastern branch, Sengstacken Arm, and a western branch, Winchester Arm. These segments of the Slough are fringed with stream-fed marshes.
Just north of Long Island Point lies 23-acre Valino Island, which like much of the surrounding land, it is essentially a consolidated dune remnant. The northern boundary of the Reserve is immediately north of the island. The shoreline of the northern end of South Slough is periodically marked with small coves and marshes. The watershed boundary extends eastward around Joe Ney Slough and its tributaries. Most of the land in this area of the watershed is more gently sloped, but with abrupt drops to the water.
Approximately 70% of the South Slough watershed is in private or county ownership with lands that are actively managed for timber production, and 5% is zoned for rural residential occupation. The remaining one-quarter of the watershed, or 5,000 acres, comprises the South Slough National Estuarine Research Reserve.