You're right, it IS still criminal.
But, lets play that out: Say she fell off the grand canyon, and as you say, you hike down, her head is 10 feet from her body, any reasonable person could tell from the scenario the girl is dead
First issue is this; It's illegal not report a dead body now. You had a case in Michigan that set this off a decade ago. It depends on the circumstance and the wording of the statute in Utah, which I am too lazy to pull at the moment, but you are looking at minimum a misdemeanor crime for failing to report the death, especially a death you were a direct witness off.
But, like you said, you drive away. And what did was say Involuntary Manslaughter is?>a death that results during the commission of ***or flight from*** a misdemeanor or felony
So you knowingly and willingly hiked down to check on the body, saw they were dead by any reasonable standard, and then fled the scene. Not only did you fail to report a dead body, you fled the scene of a fatal accident, also a misdemeanor crime in all 50 USD states. So we're back to Involuntary Manslaughter.
But hell, lets play it out further, you don't report the body, so it becomes a missing person investigation. You've now knowingly obstructed a police death investigation, not reported a dead body, and fled the scene of a fatal accidents you witnessed. All misdemeanor crimes that fit the guidelines for Involuntary Manslaughter