Assuming that a waterfall is a thing, rather than a process, (Got that?), a "Ribbon Waterfall" is one that is much taller than it is wide — its height is much greater than its "crest width", the width of the stream at the top, or the beginning point of the waterfall.
So the stream of water going over the falls forms a relatively thin "ribbon" of water that falls a long, long way before it finally goes Splat!.
A ribbon waterfall, then, being much taller than it is wide, looks like a ribbon of water if you squint hard enough from far enough away, though it cannot be worn in the hair or used to wrap birthday gifts. (Too wet, eh? And cold too, likely smells of fish and/or bear poop, and is inconvenient to get ahold of when you need it most, etc.)
This type of waterfall tends to be seasonal, or even "ephemeral", coming and going with the seasons, or appearing and vanishing depending on the amount of water flowing in a particular year. As with the rest of life, ribbon waterfalls are not all that dependable, which can add to their charm, if you like that sort of thing.
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