How does the Mirrormere Work?

[Inkling]

I was reading FOTR again and I noticed that when Gimli, Sam, and Frodo go to look in the Mirrormere after traveling through Moria, the book says "There like jewels sunk in the deep shone glinting stars, though sunlight was in the sky above. Of their own stooping forms no shadow could be seen." Earlier, when Gimli is speaking in the mines of Moria during his song/poem about Durin, he says

"He stooped and looked in Mirrormere,
And saw a crown of stars appear,
As gems upon a silver thread,
Above the shadow of his head."

So, if Durin looked in the Mirrormere and saw his reflection with the jewels crowning it, why isn't everyone else reflected in the Mirrormere?

 

[Culaeron]

I’ve always interpreted that passage as describing the water depth being so great that no shadows on the bottom can be seen. In my younger days, I was employed in the oil industry in the Gulf of Mexico. The colors of the waters there, and the depth that one can see to, are vastly different when viewed over the side of a boat vs. the rich colors seen when looking down from the top of a production platform nearly a hundred feet above the water. I always read this passage as describing the Mirrormere as vastly deep, and absorbing more of the sun’s light. Of course this is just a personal observation, and may not be what was intended by the author at all...,