The creature is sort of "formed" from the river, finds a boy in the woods and dissects him to study him, not understanding he is killing the boy, in the end the creature fades back into the river slowly. Any ideas?
I'm pretty sure you're thinking of Theodore Sturgeon's 1940 novelette "It", first published in Unknown, August 1940 which is available at the Internet Archive; a famous story which has been reprinted many times and has its own Wikipedia page. If so, you have misremembered a couple of details, or perhaps you read one of the comic book adaptations/ripoffs of Sturgeon's classic yarn. Sturgeon's creature formed in the woods, not the river:
It walked in the woods.
It was never born. It existed. Under the pine needles the fires burn, deep and smokeless in the mold. In heat and in darkness and decay there is growth. There is life and there is growth. It grew, but it was not alive. It walked unbreathing through the woods, and thought and saw and was hideous and strong, and it was not born and it did not live. It grew and moved about without living.
It crawled out of the darkness and hot damp mold into the cool of a morning. It was huge. It was lumped and crusted with its own hateful substances, and pieces of it dropped off as it went its way, dropped off and lay writhing, and stilled, and sank putrescent into the forest loam.
It had no mercy, no laughter, no beauty. It had strength and great intelligence. And—perhaps it could not be destroyed. It crawled out of its mound in the wood and lay pulsing in the sunlight for a long moment. Patches of it shone wetly in the golden glow, parts of it were nubbled and flaked. And whose dead bones had given it the form of a man?
There is no boy for it to dissect. Instead it dissects an unidentified field-creature, then a dog, and then the dog's owner (a young man probably in his twenties), all out of intellectual curiosity:
Kimbo slowed, then flipped himself through the air at the monster's throat. His jaws closed on it; his teeth clicked together through a mass of filth, and he fell choking and snarling at its feet. The thing leaned down and struck twice, and after the dog's back was broken, it sat beside him and began to tear him apart.
[. . .]
The thing in the woods looked curiously down at what it had done to Kimbo, and tried to moan the way Kimbo had before he died. It stood a minute storing away facts in its foul, unemotional mind. Blood was warm. The sunlight was warm. Things that moved and bore fur had a muscle to force the thick liquid through tiny tubes in their bodies. The liquid coagulated after a time. The liquid on rooted green things was thinner and the loss of a limb did not mean loss of life. It was very interesting, but the thing, the mold with a mind, was not pleased. Neither was it displeased. Its accidental urge was a thirst for knowledge, and it was only—interested.
Alton is the next victim. Kimbo was Alton's dog, and Cory is Alton's brother:
It was terrible that they died the same way. Cory Drew was a strong man, but he gasped and fainted dead away when he saw what the thing of the mold had done to his brother and his brother's dog.
[. . .]
It took Cory more than a moment to recover himself from what he had seen. He turned away from it and leaned weakly against a pine, panting. Alton. That was Alton, lying there, in—parts.
It caught Cory's little girl, and no doubt would have dissected her too, but she managed to get away:
Babe opened her eyes, saw that she was free, and just as the thing turned back to her she dove between its legs and out into the shallow pool in front of the rock, paddled across and hit the other bank screaming. A vicious little light of fury burned in her; she picked up a grapefruit-sized stone and hurled it with all her frenzied might. It flew low and fast, and struck squashily on the monster's ankle. The thing was just taking a step towards the water; the stone caught it off balance, and its unpracticed equilibrium could not save it. It tottered for a long, silent moment at the edge and then splashed into the stream. Without a second look Babe ran shrieking away.
It meets its end in the brook:
The monster lay in the water. It neither liked nor disliked this new element. It rested on the bottom, its massive head a foot beneath the surface, and it curiously considered the facts that it had garnered. [. . .] There was this new cold moving thing he had fallen into. It was washing his body away. That had never happened before. That was interesting. The monster decided to stay and observe this new thing. It felt no urge to save itself; it could only be curious.
[. . .]
"I am smaller," the thing thought. "That is interesting. I could not move now. And now this part of me which thinks is going, too. It will stop in just a moment, and drift away with the rest of the body. It will stop thinking and I will stop being, and that, too, is a very interesting thing."
So the monster melted and dirtied the water, and the water was clean again, washing and washing the skeleton that the monster had left. It was not very big, and there was a badly-healed knot on the left arm. The sunlight flickered on the triangular silver plate set into the pale skull, and the skeleton was very clean now. The brook laughed about it for an age.