(S (S (BASENP (PRP It)) (VP (VBZ is) (NP (BASENP (DT a) (NN gravity) (NN dam)) (COMMA ,) (SBAR (WHNP (WDT which)) (S (VP (VP (VP (VBZ holds) (PRT (RP back))) (BASENP (DT the) (NNS waters))) (PP (IN through) (BASENP (JJ sheer) (NN mass))))))))) (PERIOD .))