[2] The South Fork originates from South Cascade Lake and flows west briefly before turning north quickly and flowing in that direction until it converges with the Middle Fork to form the Cascade River Proper.

About halfway between its source and its mouth the river flows through a gorge called Box Canyon.

It flows briefly north, then west to join the South Fork, forming the Cascade River Proper.

It picks up several large glacier fed streams shortly below its source.

At the mouth of Marble Creek, the river turns west again until its confluence with the Skagit.