The source of the animals food
Answer: Steep, high velocity, no silting. The high water speed picks up silt from the bottom instead of letting it settle to the bottom. When the land gets level like often happens at the mouth of rivers where the water spreads out and slows down, silt settles to the bottom and you could get a wide, shallow delta.
(Put in own words this is from online hope it helps)
Fish poop.
Tropical white beaches get their pearly white color from the excrement of fish far off of the coast which eat coral and other things that reside on the ocean floor. When they eat they swallow some sand into their mouths as well, and process it with the food they just ate. Their stomachs can't really digest the sand, tiny minute rocks essentially, and thus turn their color white. Or the rock forming the sand can be naturally pigmented a whitish tone as well.
