Can you please tell me what you would call the object in the pictures below? (They're all part of the same thing).
A gully? A ravine?

I'm not entirely clear about the difference between gorge, gulch, gully and ravine.
My understanding is that gully is the smallest of the family, and tipically on a hillside. A gorge is bigger, but not as big as a canyon. A ravine is very deep and (possibly) narrow. A gulch, I honestly don't know.

Can you please clarify the matter for me?
Thank you

In US usage "ravine" and "gulch" usually imply large geological formations (and these located in drier areas). "Gully" would be the closest to what's shown in the pictures, but it's usually used for formations in drier areas, like deserts and prairies, while the picture seems to show a wet area, like in a rain forest. I personally would describe the pictures as a runoff stream (when the water is running after a rain) and a dry stream bed (when there's no rain).
Thank you Anon.

So there's no name for a gully in a wet area.
Got it.
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What ??

I would call that a little ravine

All over Hawaii, wet ravines are labeled as "gulch(es)". I taught geography for 5 years and was honestly the first time I saw this intriguing word! Came back from the trip and watched NOPE and the synopsis write up uses the word gulch to describe the setting, although dry and out west in the USA . Super interesting...

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The pictures show a small ravine. A gorge is bigger. A gully is an impermanent affair made by rushing water in loose earth. "Gulch" is one of theose words that has synonyms in English from many languages, used for the thing in different locales. A gulch is a water channel that is dry most of the time and only fills when it rains. In Arabia, that's a wadi. In Australia, it's a billabong. In Los Angeles, they're made of concrete, and they call them drainage canals and name them as rivers. But none of this is very well defined. People call these things what they call them, and it will vary from place to place, even within one country like the USA. The trick is to pick the one that goes with what you have in mind and will create the image you want in the most people.