The Collaborative International Dictionary of English v.0.48:

Declivitous \De*cliv"i*tous\, Declivous \De*cli"vous\, a.
   Descending gradually; moderately steep; sloping; downhill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

declivitous
    adj 1: sloping down rather steeply [syn: declivitous,
           downhill, downward-sloping]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "declivitous":
   collapsing, deciduous, declined, declining, declivate, declivous,
   decurrent, descendant, descending, dipping, down, down-reaching,
   downcoming, downfalling, downgoing, downgrade, downhill,
   downsinking, downward, drooping, dropping, falling, inclining,
   leaning, oblique, on the descendant, on the downgrade, pitched,
   plummeting, plunging, sagging, setting, sinking, sloped, sloping,
   submerging, subsiding, tilted, tipped, tottering, tumbledown