Fantasia in F "River Fantasia" : Feedback?

I am writing a fantasia where I am representing the flow of a river from a little stream out to sea(I even nicknamed the piece River Fantasia), and because it is a fantasia, I'm not really focusing on the themes, motives, etc. like I would for a sonata. I'm just improvising the melody and bass as I go along. But I do have an arc for my fantasia which goes like this:

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Little stream - High pitched, even the "bass" is in the treble clef here, slow, quiet, feels like it is starting to flow

Like how a stream gets bigger, my melody becomes more ornamented and my bass becomes more rich(going from a single melodic line with more scales towards more arpeggios and chords), gradually sinking down from the treble clef into the bass clef(contrary motion being part of how the bass line sinks), and my dynamics gradually get louder(I mean, even at the beginning, there are forte moments, but like the average dynamic gets louder)

Rapids - Lots of bass motion, especially in fast octaves, to get across the turbulence, changes from major to minor, lots of loud dynamics, faster notes in general

As you get further away from the rapids, the turbulence leaves behind a grandiose sound to the bass and the melody, it changes back to major, and the notes slow down,

Widening river - Grandiosity, left hand continues to play in the bass clef, rapid, turbulent motion(along with the change to minor and everything else in the first instance of the rapids) may or may not come back(I haven't decided yet, I will decide when I get there), overall note speed slows down, even if the tempo doesn't budge

River gets quite wide - The grandiosity that was there before starts diminishing, dynamics get quieter, notes get even slower, music heads towards a final cadence

Out to sea - The diminuendo continues, if the tempo hasn't already slowed down, a ritardando occurs, music ends at least as quiet, possibly even quieter than it began, the final cadence has an extensive arpeggio that is played by both hands and then a final chord

 

As you can see, the contrary motion is part of how the bass line sinks from the treble clef into the bass clef, and scales are used more towards the beginning of the piece. I have reached my first Rapids moment of the piece. And as you can see, in the 6 measures before, I prepare the major to minor motion with downward moving chromaticism. And since the Rapids moment is going to have a lot of octaves, loud dynamics, and faster notes in general, I decided to do an accellerando in the 3 measures before the Rapids moment. What do you think of what I have written so far? And also, what do you think of the F major to C minor motion? I know I have prepared for it well, but should I move to C minor? Or, if I have Db in my minor key preparation anyway, should I go to F minor? Or should I try both and see which version I prefer, possibly asking for your input if I'm stuck(I will probably write the C minor version and then transpose it to F minor, saving the F minor as a separate file)? The parallel minor is more common to show up than the minor dominant, but at the same time, if I am aiming for the turbulent sound of rapids, then moving to the minor dominant might accentuate that turbulence.

It looks promising. I admire your ability make such detailed planning for composition. It's very good...

Do you intend to write it for piano?

Perhaps, although it is impressionism, you can get inspiration from Debussy's La cathedrale engloutie, in which he "describes" the rising of the cathedrale and its sinking.

I think moving from F maj to Cm is quite possible in different ways and could work.

Sorry, I meant to put in the mp3 and pdf of what I have so far when I posted it. Well, I have it there now.

And yes, I am writing my fantasia for piano.

I think what you have so far is very sweet and evocative. I can really picture a river. Some little things:

The sequence at bar 11 and onwards is a nice idea. I like how the right hand moves downwards in a chromatic scale and it's a great touch. I think it sounds a little awkward towards the end. There could be a little tidying up with the left hand countermelody.

I would encourage you to use the very top of the piano a little more to get a greater feel of a small river just growing. The timbre on the very high notes is suitable for this.

Moving on, for the rest of the sections, I think it would be better to follow a musical form but be free within it. Perhaps a ternary form could work like the one I have below:

Introduction - This little bit that you have posted

A - Introduce a main "river theme"

B - Now you can expand and go through the rapids, widening river etc.

A' - The recapitulation of the main theme could be very short. I think a form would just help your audience make more sense of the piece as a whole, although I can see what you are doing.

Keep going with this! It's a lovely piece, and there are so many possibilities within the piano medium that you have given yourself. Maybe at a later date you could try orchestrating it, even just as an exercise.