For most outdoor bodies of water, the total suspended solids will come from the surrounding area and they will build up in the water over time. Otherwise, they will come from the aquatic ecosystem itself. Lots of inorganic and organic matter will tend to flow throughout a body of water.
The decomposition of living things in the area will tend to contribute to the total suspended solids in a body of water. The lighter the particles, the more likely it is that they will continue to contribute to the total suspended solids over time.
Some of the heavier and denser particles will eventually settle to the bottom of a given body of water. This will be the case with some sand and gravel, at least in part. If there is not enough of a water flow in a given body of water, it’s even more likely that the heaviest of the particles will eventually just settle to the base of a given area. Developments like this are not necessarily positive for the aquatic ecosystem, since the formerly suspended solids will choke certain organisms and disrupt different small ecosystems.