The Muddy River is a stream, about 29 miles (47 km) long, southeast of Mount St. Helens in the U.S. state of Washington.

[8] Muddy River originates at about the 5,000-foot (1,500 m) level on the southeast side of Mount St. Helens in the Cascade Range.

[8] Flowing northeast through the gorge, the stream plunges over three waterfalls before emerging from the narrows and curving sharply again to the southeast.

[8] Turning south, the river descends less rapidly, passing under Forest Road 25 and then receiving Clear Creek from the left about 5 miles (8 km) from the mouth.

[7] Below the confluence with Clear Creek, the river meanders through Cedar Flats, leaves the national forest slightly more than a mile from the mouth,[8] and passes a United States Geological Survey (USGS) stream gauge, on the left 0.5 miles (0.8 km) from the mouth.