The Collaborative International Dictionary of English v.0.48:

Subterfluent \Sub*ter"flu*ent\, Subterfluous \Sub*ter"flu*ous\,
   a. [L. subterfluens, p. pr. of subterfluere to flow beneath;
   subter under + fluere to flow.]
   Running under or beneath. [R.]
   [1913 Webster]