The story starts with a murder victim, Whitney, who was poisoned while visiting a dude ranch in Arizona. Castle and Beckett head there to investigate her death.
The Indians come up when Castle and Beckett learn a Yavapai tribe lives nearby and had a river dammed in the 1920s. Whitney had been fooling with dynamite, and I wondered if she planned some eco-terrorism against the dam. No, as it turns out.
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There actually are three recognized Yavapai tribes: the Yavapai-Apache Nation, the Yavapai-Prescott Indian Tribe, and the Fort McDowell Yavapai Nation. None are called simply the Yavapai Tribe, but that's a minor detail. That Castle used an actual tribal name is impressive enough. Using an appropriate name for southern Arizona, where the ranch presumably is, and avoiding the commonplace "Apache" and "Navajo," is a real feat.
It's kind of farfetched that the tribe would have a sign identifying itself in the middle of nowhere, but that's another minor detail.
The "consultant"--more properly called an elder--is played by Eloy Casados. His biography doesn't say anything about his being Native, although he's presumably Latino. But he has played a Native at least a couple of times. The most notable example is Ishi: The Last of His Tribe (1978), a made-for-television biopic.
River or stream?
According to one site, the word in question is aha gah hel’la. For all I know, that could be a real Yavapai word.
The word is a clue that Whitney was searching for gold bars stolen by the Peacock Boys gang. But the puzzle doesn't quite make sense.
Castle and Beckett deduce that people were searching in the wrong place because of the mistranslated word. But are they searching for the present-day river or stream, or the original river or stream? They don't say--or if they do say, it isn't clear.
Any stream they found now would be the wrong one because the dam changed the water's course. They'd have to look for the original river or stream, not the present-day versions. Since they never set eyes on any actual water, I guess that's what they're doing.
But finding an old stream bed after a century of erosion would require geology expertise and advanced technology. It's not something Castle and Beckett could do on their own. And yet they do find whatever it is they're looking for.
The whole dam thing seems like an unnecessary complication. Just say everyone searched near the river when they should've searched near the stream. If the film location doesn't have a stream, say the drought dried it up. Or it's just over the hill.
Incidentally, there actually was a Yavapai dam conflict:
Orme Dam conflict
Yavapai boy remembers
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The boy eventually told the story to missionaries, who wrote it down. So the Yavapai are responsible for remembering an incident that otherwise would've been forgotten.
This bit is unnecessary since a gang member could've told the story to anybody. I suppose it serves to integrate the Yavapai into the plot a little better. And the details, such as they are, are plausible. White folks did use Indians as servants or slaves, and missionaries did preserve the Indians' lore in writing.
All in all, Once Upon a Time in the West was a decent episode. The Indians didn't play much of a role, but the details were reasonably accurate. Castle may be the prime-time network leader when it comes to including Native lore on TV.