Short answers, pulled from the story.
Sediment is loose solid material broken from rocks and soils through weathering and erosion, then transported by wind, water, ice, or gravity to a new location where it is deposited. It occurs naturally and can eventually be buried and compacted into sedimentary rocks such as sandstone and siltstone through a process called lithification.
The Phi scale is a logarithmic (log base 2) measurement system geologists use to classify sediment particle sizes. It ranges from colloids smaller than 1 micrometer all the way up to boulders exceeding 256 millimeters, with named classes including clay, silt, sand, granule, pebble, cobble, and boulder.
Sediment is most commonly transported by water through fluvial processes, producing beach sands and river deposits. Wind carries fine particles to form desert sand dunes and loess deposits. Glaciers transport a wide range of sediment sizes, leaving behind moraines and till.
The Exner equation expresses the balance between sediment in transport and sediment deposited on a riverbed. It states that the rate of increase in bed elevation due to deposition is proportional to the amount of sediment falling out of the flow, and it explains patterns of erosion and deposition throughout a stream.
Lavakas are large erosion gullies that form on devegetated land on the Madagascar high central plateau, which covers roughly ten percent of that country's land area. A typical lavaka is 40 meters wide, 80 meters long, and 15 meters deep, and lavakas may account for 84 percent of all sediment carried off by Malagasy rivers, turning waterways dark red-brown and causing fish kills.
Sediment can harm coral reefs by physically smothering coral tissue, abrading surfaces, forcing corals to spend energy on sediment removal rather than growth, and triggering algal blooms that reduce available seafloor space for juvenile coral polyps to settle. Watershed development near reefs increases the volume of land-derived sediment entering coastal waters, especially during rainfall events.