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What is sediment?

 

Accelerated feeder bluff or slump due to upland clearing and impervious surfaces, to illustrate sediment as an ecological process.  Sediment is blocked by railroad at base.
Sediment refers to the particles (such as sand and other soils) which settle, or are deposited, on the sides and bottom of water bodies. It is important in the formation of beaches, spits, sand bars and estuaries and provides substrates for aquatic plants and animals. Sediment also provides nutrients and minerals vital to the health of downstream ecosystems.

 

Sediment reaches aquatic areas in three main ways:

  1. watershed erosion,
  2. mass wasting events, such as landslides, and
  3. shoreline erosion.

The key places that sediment comes from are:

  • steep slopes with unstable or unprotected soils (such as feeder bluffs),
  • landslide hazard areas,
  • and unarmored channels.

Why is sediment important?

Sediment processes are an extremely important part of many ecosystems, as well as of primary importance to particular species.

For example, various organisms in both marine and freshwater environments rely on replenishment of sediment for their reproductive habitat. Changes to sediment (either too much or too little) can change substrates or cause sediment not to be deposited in the appropriate locations.

What affects the amount of sediment and its movement?

Land uses causing excessive amounts of sediment to enter the aquatic ecosystems and uses causing major reductions in sediment delivery can be detrimental to shorelines.

Land uses that can affect sediment include:

  • the removal of vegetation on erodible soils, leaving them exposed;
  • soil disturbance and clearing adjacent to the shoreline;
  • filling with foreign soils;
  • roads within 200 feet of the shoreline;
  • high road density or impervious surface in the basin;
  • shoreline armoring;
  • channelization of streams; and
  • increases in stream flows.

Sediment moves through the ecosystem and is sometimes stored in wetlands, floodplains, streams, lakes, and the banks of the shorelines. The amount of sediment reaching these areas is primarily altered by draining or filling wetlands, loss of shoreline roughness (for example, the removal or loss of large woody debris), channelization of streams, shoreline armoring, dams, and the development of structures like boat ramps and groins which are oriented perpendicular to the marine shoreline. Dredging and bulkheads can also affect how much sediment is present in aquatic shoreline areas.

References: Edwards 1998; MacDonald et al 1994; Washington Forest Practices Board 1997; Nelson and Booth 2002; Williams et al 2001; Kadlec and Knight 1996; >;Macdonald et al 1994; Dube 2003; Williams et al 2004; and Johannessen et al 2005 (Acrobat pdf files)