This is Word of AN.
He said that the trace runoff from our Black Soil had caused the lowlands along the river to become abnormally rich over the thousands of years.
Erosion happens period. We minimize it, but there is literally no way to stop fine particles from washing downriver. Also keep in mind that while the particulates themselves might not wash that far down the water, we're adding enriched nutrients to water, which causes increase in fish and other riverine life, which in turn enriches everything downriver.
And AN isn't an expert on agriculture. He knows a hell of a lot, but I can't honestly expect that he knows everything in detail.
What happens is that some runoff occurs as plant available nitrogen is highly water soluble along with particulates. These particulates are a mixed bag, but the organic parts are not a huge amount.
What basically happens with Black Soil is as follows:
The Charcoal and Clay shards mix in such a way that they create a kind of sponge that is able to bind truly impressive amounts of nutrients and water through colossal surface area and chemical bindings that plant roots (and/or soil life like bugs and bacteria that process it for the plants) are able to overpower. That alone limits N washout. Especially as the concentration is simply not the same as with modern agriculture where a wheat field may get 200 kg pure N per hectare. We are maybe at 50 or so as the volume of Black Soil is simply that much bigger and thus harder to apply without machines.
And all that Nitrogen bound up in the organic mass is not permanent. Everytime you give it air via the plough or other, the bacteria process it and the plants eat it. The storage capacity of Black Soil, however, is much higher, binding a lot. In short, the Nitrogen which is what actually boosts the plant growth the most runs out very, very quickly. You can expect to lose 1 percentage point per season (of those 5%).
The particulate washout that happens is indeed a boon to the HK, but like the Nile, it needs to happen every single year. The much smaller amount they get is used up much faster than what happens to us.
And no, they won't get a Black Soil layer from this. The Charcoal is unlikely to survive the conditions and be broken down by bacteria while the clay-silt is nothing special on it's own.
Further compounding the issue is that the Nitrogen they get washes out just as easily. So without the reapplication they get from our runoff and the usual silt, the floods are going to decrease fertility fairly rapidly as they are left with only the substrate but no fertilizer.
What I expect to happen when the Dam provides a pool where the particulates can fall out is that the HK see increasingly poor harvests along that river after 3-4 year delay.
The fish will help some, but you truly underestimate the sheer amount of fertilizer required and how much dry mass the fish and their waste actually provide.