Stygia

Stygia likely refers to the River Styx . In Greek mythology, the River Styx separates Earth from Hades , or the underworld. 
=="The River of Rivers in Connecticut" ==
Stygia appears in Stevens' poem "The River of Rivers in Connecticut": 
 There is a great river this side of Stygia,
 
 Before one comes to the first black cataracts
 
 And trees that lack the intelligence of trees. 
 
 
 In that river, far this side of Stygia, 
 
 The mere flowing of the water is a gayety,
 
 Flashing and flashing in the sun. (ll. 1-6a) [1]
==References ==
1. Wallace Stevens, Collected Poetry and Prose, Library of America, 1997. Page 451.