The upper reaches of the Styx River are in the Tasmanian Wilderness, south west of Maydena.

The Styx River rises below Mount Mueller at an elevation of 1,110 metres (3,640 ft) above sea level and flows generally east by north, joined by five minor tributaries, before reaching its confluence with the River Derwent near Macquarie Plains, west of New Norfolk.

[1] The Styx Valley contains old growth forests including the tallest hardwood trees on earth, Eucalyptus regnans.

The Wilderness Society and Senator Bob Brown have campaigned to save the forest from harvesting for sawn timber and woodchips.

[3] The name has no classical associations; early colonial visitors noted the many fallen trees, stripped of bark and bleached by years of exposure, laying across the river and along the banks.