But silt isn’t exactly sand.
When water erodes mountains up yonder, it does this partly by dissolving the mineral content of the rocks. Quartz and feldspar are sort of what’s left over because these two things don’t dissolve in water but when the other stuff holding them in place gets dissolved, they get swept downstream.
The crystals get deposited differently based on their size. Big ones sink and get stuck, smaller ones get carried and bounced along some distance before getting deposited. The smaller crystals get mixed in suspension and carried farther. That suspended load will un-suspend itself when the water slows down in the bottom lands.
So, civilization got its start partly because small sand crystals don’t dissolve in water but can also be held in suspension for a while.
It’s kind of ironic, then, that the Sumerian civilization eventually ended in part because a lack of rain in their bottomlands meant the dissolved salts in the irrigation river water were never washed away further downstream, but just concentrated as the sun evaporated the water there.
They slowly switched from growing mostly wheat to barley which is more salt-tolerant. But eventually the salt got to be too much. You can even see it today as a white crust on the surface of parts of modern-day Iraq.
The Yellow River in China gets it name from the massive amount of silt suspended in its water. Its known as the siltiest river in the world.
In fact, about every 100 years the silt causes the river bottom to exceed the depth of the river in places. This causes massive flooding as the water just spills out everywhere (as there is no more riverbank) until it finds a new course and digs in. This has caused major problems for the Chinese throughout history.
So, silt depth changes as it is suspended and deposited, but what about sand depth? What about all the sand in the big deserts around the world? How deep is that sand? This is a hard question to answer. Look at the Sahara, for example.
We call it one desert, but its not really a continuous feature. In fact, only 20% of the Sahara is covered by sand! The rest is basically just rock. The sand is mostly formed and moved by wind erosion.