Learn English – beat me over the head with it

I'll take a crack at this, although I agree with chrylis that it's a little hard to figure out for sure what you are asking.

"Always the same, isn't it? 'Poor old Cuthbert, doesn't listen to a word you say...head in the clouds again...always gets the wrong end of the stick.' And on and on and on and on and on!" ranted Cuthbert.

Okay, forget about the airport and the context of the story. You have a group of people together, and one of them feels like the others think he's foolish. So he starts a rant:

Always the same, isn't it?

That means: No matter what we're doing, you think the same thing of me.

'Poor old Cuthbert, doesn't listen to a word you say...

Cuthbert is talking here, so he is saying what he thinks everyone else is thinking about him: namely, that he doesn't listen.

...head in the clouds again...always gets the wrong end of the stick.'

means: Cuthbert doesn't pay attention to what's going on around him; Cuthbert always finds himself confused in life.

And on and on and on and on and on!" ranted Cuthbert.

Means that he could keep going with his grumbling rant, if he wanted to, adding things like: Cuthbert always has his head up his ass, Cuthbert never does anything right, Cuthbert couldn't find his way home if you gave him a map and paid for his taxi fare, etc.

_{If I've not answered your question correctly, let me know, I'll delete my answer.}

Indeed artistic or poetic language, as is often found in novels, can be hard to understand by someone who isn't yet completely fluent in the language.

The incomplete sentence

Through the sopping grass and down towards the river.

This is an incomplete sentence, used for artistic effect. You could fill in the rest of the sentence like this:

She walked through the sopping grass and down towards the river.

A way of speaking, recognized by all fluent speakers, is to say a direction, without a verb, as a command. For example, a sergeant can order soldiers in formation to march by saying "Forward!" It would be very abrupt, but you could order someone to go into a car by saying "Into the car!" The incomplete sentence you found doesn't give an order, but these examples illustrate how a mere prepositional phrase describing a path without a verb easily suggests motion in English: motion along that path. Given the preceding context, which suggests that "she" isn't going to stay indoors, and has just gotten her camera, a (very fluent) reader easily understands that motion is suggested: "she" is moving—probably to a place where she can do something with the camera.

I took a quick look at the surrounding context and found another incomplete sentence like this, a paragraph earlier:

A night of heavy rain which left the trees dripping.

I get the impression that the author intends these incomplete sentences somewhat like "establishing shots" in movies. The author used each of them as transitions. After each one, the author added full sentences that go into more detail about the new scene or location.

"It"

Each it that you marked in bold refers to the river. The first It is a little jarring even to a fluent speaker, because the phrasing parallels the way people usually describe the weather: "It was raining", "It is sunny today", etc. Maybe the author intended that parallel, because of the word "today". The ambiguity clears up when you reach "rippled and churned". The only thing mentioned nearby that could be "wide and brown" and could "ripple and churn" is the river.