I'll start by saying that I'm new to bonsai, so take this all with a grain of salt...
I love the idea of making a penjing style pot resemble a river! And I'm glad that you're keeping an open mind, and considering your options when making a new one, because I see such great potential in a river themed penjing pot. You might want to consider spending some time traveling a mile or two along a river for inspiration though.
I've been going on long walks and swims along creeks and rivers lately, looking for plants with exposed root systems to scoop up and train to resemble a bonsai tree. Every Sunday for the past 6 weeks I've been going on these adventures that I've come to call NEBARI SAFARIS or NEAGARI SAFARIS (depending on my goal) and I've taken in a lot of the scenery. Rivers offer such a wide range of potential for creating a miniature realistic scenery, and I'll do my best to touch on as much of that as I can.
Rivers carry tons of sediment downstream. The blue that most people associate with water is a reflection of the sky, but in a river, the current gives a lot of variance in the reflection (depending upon the speed and resistance from the landscape), which often reflects quite a bit of the surroundings, like the trees along the river bank. The water you depicted looks way too blue for a river. Think about a real place in the world that you want this river to depict, and look at pictures online if it helps, so that you can dial in on a realistic color.
The river you've depicted has a lot of debris. There's only 2 occurrences I can think of where you'd see that much debris so close together. 1- if there was a flood, and in that case there would be a significant amount sediment, (which brings me back to the color) and 2- if there was a significant amount of rocks in the landscape that were preventing the driftwood from floating downstream. What you created looks more like a flood. The sticks seem to resemble debris more than driftwood, because they're all over the place and in calm water. They are also quite dark. You would achieve a better result with authentic driftwood that has had time to soften, form small holes, become bleached by the sun, and gain character. Consider which direction your river is flowing, and place the driftwood in a place that it would be stopped by the landscape despite the flowing current.
The trees along a river have a lot of erosion to deal with, so not only do their root systems get exposed by the soil erosion, but often this causes them to lean toward the river, and create an overhanging effect. The trees that overhang the river may effect the pathway of reflecting the sky, and effect the perceived color of the river. The exposed root system potential along a river is quite significant. If you plan on using that tree in your pictures, I recommend trying to expose the roots a bit more.
When it comes to tree placement, if you choose to have the tree in the center, your best bet to making it look realistic is to create a high enough rock planter for your tree to avoid most of the extremes of the river's current. The realistic approach for facing your tree, would be one that explains why your tree is bent. I would have your first bend go upstream. This can be explained by the initial planting position when the seed or sapling became caught on the face of the rock, and grew there. Then your second bend in the tree, curving downstream can be explained by a long flood that pulled the tree with the current. And finally, your third bend can be explained by your tree correcting itself the best it could after the flood. Looking at your picture... I would recommend placing the left hand side as your upstream side. Nonetheless, if you choose to place your tree in the center of the river, I would forego the traditional pot, and create a custom rock planter that has its opening on the upstream side of your penjing display.
There are also lots of other interesting variables that you can consider making a part of your river. There could be a bright bleached white sand bar... a fallen tree that crosses the river... the river could split in 2... in fact, here is a prime example of how much a root system can be exposed by a stream that splits in 2... I found it on one of my neagari safaris. (see attached photo) Anyway, before you take a second attempt at your "river penjing display" I highly recommend visiting a river in your area if you can, and spend some time traveling along it, taking in as much of the scenery as you can. Who knows, you might just find the perfect piece of driftwood, an amazing rock, or just a handful of sand and pebbles. I find that its usually good to experiment a little before tackling a serious art project in order to make a proof of concept.
Anyhow, those are MY thoughts on the matter. Let me know if you want my address or PayPal to send me a penny.