( ( ( The ( ( most important ) fact ) ) ( that ( defines ( an estuary ) ) ) ) ( ( is ( ( the mixing ) ( of ( ( fresh and ) ( salt water ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (ADJP (RBS most) (JJ important)) (NN fact)) (SBAR (WHNP (WDT that)) (S (VP (VBZ defines) (NP (DT an) (NN estuary)))))) (VP (VBZ is) (NP (NP (DT the) (NN mixing)) (PP (IN of) (NP (NP (JJ fresh)) (CC and) (NP (NN salt) (NN water)))))) (. .)))
The most important fact that defines an estuary is the mixing of fresh and salt water.
(ROOT (S (NP (JJ Fresh) (NN water) (NNS biomes)) (VP (VBP are) (VP (VBN defined) (PP (IN by) (NP (JJ low) (NN salt) (NN concentration))))) (. .)))