Synonyms for Stagnant (same or very similar meaning)
WordNet sense 1 (not circulating or flowing):
deadWordNet sense 2 ((of fluids) not moving or flowing):
standingWordNet sense 3 (not growing or changing; without force or vitality):
moribundWordNet sense 4 (characterized by an absence of force or forcefulness):
adynamic, undynamicFrom the ODE community, based on WordNetadd/edit