When I was in grade 4 or 5 we read a sci-fi book about some guy who had been sent to this planet to find out what had become of a scientist if I'm remembering correctly. This alien planet had a colony living on it. The air wasn't breathable. The scientist had a daughter who he had genetically altered to be able to breathe the air and survive the planet's environment. She had reptilian features because of this altering. The colony was near a mesa. That's all I remember. Would you happen to know the name of this novel? It's irked me for years that I can't remember the title.
I think you might be remembering Keeper of the Isis Light by Monica Hughes. It's appropriate to the age you recall reading it, and features a girl who has lived in isolation until a new colony is established near the Mesa where she lives. She was adapted to suit the environment, and the colonists react badly to her appearance until she rescues one of the colonist's children.