It is approximately 115 miles (185 km) long,[1] draining an area of the Cascade Range southeast of Eugene, which is at the southern end of the Willamette Valley.

The Middle Fork Willamette River is formed in northeastern Douglas County by the joining of several small headwater streams near Emigrant Pass.

[5] Named tributaries from source to mouth are Beaver, Tumblebug, Royal, Fizz, Indigo, Found, Skunk, Swift, Mac, and Echo creeks.

[4] Downstream of Hills Lake Reservoir come Salt, Salmon, Shortridge, Gray, Spot, and Chillo creeks, followed by the North Fork Willamette River.

[4] Entering the Middle Fork via Lookout Point Reservoir are Duval, Carpet Hill, Schweitzer, South, North, Armet, Harper, Cain, School, Rhodes, Crale, Goodman, Fern, Bannister, Hazel, Rolling Riffle, and Minnow creeks.