The first two stanzas were extremely similar. The rhyme pattern was the same. Each image in the first found a parallel in the second. The third stanza, however, varies the pattern in several ways. The rhyme scheme will shift to a,b,a,b,c, leaving the last line unconnected to this stanza and to the previous ones.
Force is replaced by hand.
Force, as an abstract and general term, is easier to understand as a controller of human destiny than the very specific word, hand. In the previous stanzas, the contrasts were clear. Water may be life-giving, but as the hand whirls it in the pool, the words join to convey a sense of danger, of the whirlpool. The first four words of the second line previously connected the speaker's growth with nature's. In this line, the pronoun is left out.