Roughly, about ten miles upstream from the waterfall at Henneth Annûn.
A slight correction to your assumptions: There is no cliff from which Sam watches the (single) Oliphaunt. Instead, Sam climbs a tree and watches the Rangers' fight with the Haradrim, including the Mûmak, from there. This was one of the bay trees "close by" the bed of fern "a little way back above" the "small clear lake in a shallow dell" where Frodo and Sam made camp, and where Sam made the cookfire whose smoke was spotted by Faramir and his men. This was "less than ten miles" from Henneth Annûn where Faramir marched them to for the night.
In The Atlas of Middle-Earth, it's a little easier to see this on the detail map of Henneth Annûn than the overview map of Sam and Frodo's path. It's labeled "burned circle". Karen Wynn Fonstad is correct in placing Henneth Annûn downstream from the pool: They come to a gorge which "was the same stream that trickled far above out of the round pool". After which they are blindfolded and follow the river to the refuge.
However, KWF's depiction of this stretch of stream as straight is a simplification caused by insufficient information. Faramir leads Frodo and Sam through thick woods, so the stream and the gorge it drops into might follow a more crooked path than that.
(All quotes come from the chapters "Of Herbs and Stewed Rabbit" and "The Window on the West" from Book IV of The Two Towers, but they are too fragmentary so I'm foregiong the traditional blockquote and hand-drawn red circle. We can piece the rough location together out of the quotes).