Course
Marys River arises in western Benton County at 659 feet (201 m) above sea level and falls 452 feet (138 m) between source and mouth to an elevation of 207 feet (63 m). The main stem, formed by the confluence of the East Fork Marys River and the West Fork Marys River, begins at about river mile 40 (RM 40) or river kilometer 64 (RK 64) north of Marys Peak in the Central Oregon Coast Range. The West Fork Marys River, which is 6 miles (9.7 km) long, rises at 44°44′00″N 123°34′15″W / 44.7333333°N 123.5708333°W / 44.7333333; -123.5708333 (West Fork Marys River source) and flows south to join the East Fork. The East Fork, which is 5 miles (8.0 km) long, rises at 44°42′00″N 123°30′55″W / 44.7°N 123.5152778°W / 44.7; -123.5152778 (East Fork Marys River source) and flows southwest to join the West Fork. Marys Peak rises to 4,022 feet (1,226 m) above sea level at 44°30′15″N 123°33′04″W / 44.5042870°N 123.5512165°W / 44.5042870; -123.5512165 (Marys Peak summit).
From its source, the river flows south along the east side of Marys River Road past the unincorporated community of Summit and under Oregon Route 180 at about 38 miles (61 km) from the mouth. Turning southeast, the river flows just south of Route 180, receiving Devitt Creek from the left before reaching the unincorporated community of Blodgett. Here, about 32 miles (51 km) from the mouth, it crosses under U.S. Route 20 and in quick succession receives Norton Creek from the left and the Tumtum River from the right. Turning south and then east, it passes the unincorporated community of Harris at about RM 26.5 (RK 42.6) before reaching the unincorporated community of Wren. Near Wren, it turns north, crosses under Route 20 again, turns east, receives Gellatly Creek and La Bare Creek, both from the left, and passes under Oregon Route 223 at about 22 miles (35 km) from the mouth.
Just east of Route 223, the river receives Read Creek from the left, then Blakesley Creek from the left at about RM 20 (RK 32) and soon turns sharply south. About 14.5 miles (23.3 km) from the mouth, it flows under Route 20 for the third time, immediately receives Woods Creek from the right and turns southeast. It soon passes under Oregon Route 34 at the unincorporated community of Flynn, where it receives Greasy Creek from the right. Marys River then flows southeast by Philomath, which lies to its left (north), receives Newton Creek from the left, and passes a United States Geological Survey (USGS) stream gauge at 9.4 miles (15.1 km) from the mouth. At about 5.5 miles (8.9 km) from the mouth, the river receives Muddy Creek from the right and turns northeast toward Corvallis. In Corvallis, it passes Marysville Golf Course and Mill Race on the right, receives Squaw Creek and Oak Creek from the left, and passes Avery Park on the right right about 1 mile (1.6 km) from the mouth. Shortly thereafter, it flows under Oregon Route 99W and enters the Willamette River near the larger river's RM 132 (RK 212).
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