This drabble is such a treat that I re-read it at every opportunity -- and I was pleased to see it featured on the HASA front page....
I've commented on it before, but please bear with me while I burble on again.... LOL!
The most surprising part is the PoV: what an inventive idea! And your Bruinen indeed has much to say.
Starting with "Drifts of rain or drifts of snow" is brilliant; it's just water, not even coalescing into a rivulet yet, or emerging as springs... yet it is already a conscious, sentient entity.
The language is so picturesque and evocative, I could almost feel the Bruinen's revulsion at being sullied by the feet of the Nazgûl horses.
I love how Elrond is referred to, as simply the Master or the Mariner's son... the river is busy taking messages to Ulmo, and so is privy to the more mythic aspects of Elrond's background.
And "Haven of the Mariner's son" is a perfect example of the many layers of meaning you put into this, suggesting Eärendil's ports of call as well as directly describing Imladris.
Sorry for the relatively inarticulate babbling, but I just had to let you know how much I enjoy reading Loudwater. I have added it to my Inspiring Drabbles playlist... and I'd like to
subvert your muse, er, encourage you to write more drabbles in the future, since you clearly have such a talent for the form.