Well, I feel like there could be both rivers that meander through valleys into the lake, and one that falls down a cliff from the mountains right into the water. Maybe there is one arm of the mountains that comes right to the shore? I dunno, I like that you put a space of coastline between the shore and the mountains, and it could even be expanded in places with lowland penninsulas. What's beyond the mountains, or should we not care? Are their peaks snow-covered?
I was reading the very late text "Cirdan" and it says some stuff about boat-building. It agrees with what you all said earlier:
"...the Teleri developed a craft of boat-making; first as rafts, and soon as light boats with paddles made in imitation of the water-birds upon the lakes near their first homes, and later on the Great Journey in crossing rivers..."
I'm not sure if this contradicts my assumption that the Elves at Cuivienen never cut down trees. Could they have made rafts and boats solely out of fallen logs and reeds? Ancient Egyptians made reed-boats that could carry multiple people. But I don't know how you'd make paddles without cutting down at least small trees. Hm. They could have invented
pollarding or
coppicing, to harvest wood without permanently killing entire trees.
Coppicing and Pollarding both keep trees young and make them longer-lived. Pollarding produces wood that's good for boat-building and poles, but also makes trees ugly. Maybe Elves invented their own unique way to prune trees to keep them youthful and beautiful and also get useful wood?
Not that they knew how to make seaworthy ships that could survive an ocean crossing, as the Valar later warn Cirdan when his first impulse is to sail across Belegaer to catch up with Eressea.
Interestingly, Cirdan is implied to have lived at Cuivienen and said to have "ever been the foremost and most inventive and skilful" boat-builder. "Cirdan" is a nickname, but nobody knows his original name. It might have been Nōwē. Oh, and Elwe is Olwe's
older brother, whatever that means. He woke up 5 minutes earlier? I mean, if they had parents, why weren't those people leaders instead?
(Even some of the Valar are elder siblings to other Valar, despite being created
before time itself.)