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Short version

• Planet colonized, several generations passed, knowledge of tech & being a colony is gone


• Robot guardian has been trying to help by giving the tribe replacement tech


• Culture has "taboo" around many things, including ascending a nearby mountain (discovered by the main character to be a result of altitude sickness)


• There's a... rising river?


• Some of the original colonists (or their children?) who know the history are still alive, but are seen as senile & not taken seriously

Longer Version

I read this probably in the late 80s or early 90s when I was young - I'm not sure if it's young adult or not, and I don't remember if it was a short story, novella, or full length.

It follows a character - a young person (?) in a primitive-ish "tribe," who is interested in exploring the world. She knows she's not supposed to go up the mountain, because it's taboo. (Many things are, which she's frustrated by).

She does eventually do so anyway, and gets sick. She's been taught that ill would befall her for violating the taboos of her culture, but, at some point, she has the line "is the sickness because of the taboo, or is the taboo because of the sickness?"

I believe she is rescued by a robot guardian at some point. He takes her to a cache in a cave, with a computer and such. He's been giving the tribe various things to replace broken components, such as a flourescent light tube, a replacement microchip, and surveying rods, because there's a river rising or such.

Back at her tribe, with the striped rod, an elder says "that looks like one of the surveying rods we used when we came here!" But, everyone considers these elders - probably the original colonizers - to be senile, and they don't take them seriously.

Sorry about the rambling nature of this!

I believe this is The Guardian of Isis by Monica Hughes. Not a girl protagonist, but Jody is an androgynous name.

One review has a pretty good synopsis that I've added emphasis to address some of the points:

Second volume in the Isis trilogy. In this, the third generation of children have been born and a rigid, taboo bound society has developed in the colony that was established in the first volume. Survivors of the original colonists are labelled 'Firsts', their children 'Seconds' and so on down to 'Fourths'.

Jody is the youngest Third and finds himself a misfit, partly because of the enmity between his grandfather of the same name, who was rescued by the Keeper of the Isis Light, Olwen, in the first volume, and Mark London, Olwen's lost love who has become the President. Mark presides over a primitive society that he has deliberately made so, cutting them off from knowledge of who they really are and from technology, so that when Olwen's Guardian robot supplies 'gifts' over the years to fix things that don't work any more, such as the communication device, they are totally ignorant of the purposes of such things and instead revere them as artefacts only viewable by the favoured few. They now believe the Guardian is a god - pretty ironic when the first book showed how the colonists looked down on him as a robot - and London's machinations have worked so well that Fourths now don't even believe that the colony came from Earth, and think the stars are just decorations in the sky.

The river which once drained through sinkholes and emerged in another valley, has become blocked and the valley is becoming flooded, a serious problem as the rariefied atmosphere of the high passes is almost unbreathable to the humans who have been told not to go to those places anyway by the taboos Mark has created in the wake of his disappointment and anger about Olwen's true nature. Jody tries to alert people to the danger but Mark does everything possible short of murdering the young man, to ensure that his warnings are ignored - pretty illogical but we are meant to feel Mark's pride is too strong for him to unbend even for the survival of his people. It is only when he engineers things so that Jody has to journey out of the valley to seek help from the Guardian, that the young man finally learns the truth.

I wasn't totally convinced that the society would have lost all its knowledge in this time scale. Hughes tries to overcome this by saying that Mark has taught everyone to ignore what the 'elders' say about the old days, and yet he is an elder himself! Also why is Mark not able to e.g. change a lightbulb in the 'Sacred Cave' as they now call the cave housing the computer that was meant to keep them connected with the Guardian . . .