BOOK REVIEW : A Lyrical Tale of the River Is Piloted by Language--Not Plot : DIVINING BLOOD <i> by Maureen McCoy</i> ; Poseidon Press; $20; 260 pages

In a novel as lyrical and poetic as Maureen McCoy’s “Divining Blood,” it’s difficult to single out one event as the novel’s turning point, the moment at which all that has gone before suddenly comes into focus. The closest thing, however, may be the scene during a casual summer party in which Johnny Melody, the shy Mississippi riverboat engineer, is beaten at a simple pea-and-shell game by his host, Dr. Skylar Walsh.

Johnny sees again that Delana, his lover and Walsh’s daughter, is “that pea,” and, in shaking Walsh’s hand in congratulations, feels compelled to break a few of his fingers. “The bones snapped so wonderfully loud,” think Johnny’s shipmates, already uncomfortable at being so long on land; “maybe doctors’ bones break especially hard. The healer’s hands had not given up easily to humility.”

The tugboat Pat Furey has docked at Delana’s hometown near St. Louis to drop her off, the birth of Delana and Johnny’s child, Robin, having brought her seven years on the water to an end.

And what a time it has been: Delana--short for Magdalena--came to the tug as a 17-year-old runaway, beginning as a cook but leaving a full-fledged pilot.

Cheramie, the captain of the Pat Furey, is sorry to see her go, for she’s got, he says, “a pilot’s eye,” an eye allowing Delana, and apparently only Delana, to win at her father’s shell game. Walsh replies, of course, that Delana’s skill is in her blood. “She got it from me. I taught her,” he says, attempting to claim an influence he lost long ago--years before Johnny’s appearance, back when he abandoned Delana’s mother, Dovie, for a nurse.

This exchange between father and captain makes clear that “Divining Blood” is elaboration upon a familiar saying--that blood is thicker than water.

McCoy, author of the novels “Summertime” and “Walking After Midnight,” doesn’t assess the saying’s accuracy. True, Johnny goes back to the tug alone, but that’s because Delana has laughed off his frequent proposals of marriage; she chooses to remain on land, staying home with her child and childhood family (what remains of it, Dovie having died and Walsh having married the nurse, Joy).

She does so, it seems, to address--and dress--old wounds, largely derived from her parents’ troubled marriage: Delana was its last gasp, conceived when Walsh returned to Dovie, temporarily, a few weeks after their other child drowned in the Mississippi. Delana, as a result, has considered herself a living accident, a replacement, always drawn to the river that indirectly brought her into being . . . and equally indirectly, Robin. With Robin’s birth, and finally her baptism on the water, a circle has been brought to a close.

It’s an unusual, agreeable story, but “Divining Blood” is carried by language more than plot. McCoy is an eminently quotable writer: She describes a clearing as being “where the first spring violets always look terribly polite amid the layers of old leaf”; nature as having “a hostess heart: the minute you are out of sight, its mild sympathy deserts you”; Dovie as sending gifts of candy because “Her coded love let sugar speak for hope.”

And here’s Delana, leaving Robin on the banks of the Mississippi before taking one last, commemorative swim in the great river: “The bright sun on bare skin seemed like a pact in favor of intensity. Dull feeling would be a sin, always, the true sin. Let eyes and skin burn. Let the mind flame on.”

McCoy’s inventive, dreamy prose has a downside, however, for sometimes it’s mannered or simply unclear. Dovie tells Walsh, angrily, “Your voice is a sausage. It holds in the guts of your lies”; the picking of apples with a cup-bearing pole becomes “the caging of fruit,” which “seemed a terrible, wounding act”; next to the above-mentioned clearing, “peeping through wherever it could, breaking its fat neck to watch, the river faced up.”

One can tease out, eventually, the meaning or import of those and similar expressions, but it’s frustrating to be tripped up by them every page or two, especially when McCoy’s prose is generally so good. Rich writing, like rich desserts, are best in small portions.