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Stream substrate (sediment) is the material that rests at the bottom of a stream. There are several classification guides. One is:

• Mud – Comprised of silt and clay.
• Sand – Particles between 0.06 and 2 mm in diameter.
• Granule – Between 2 and 4 mm in diameter.
• Pebble – Between 4 – 64 mm in diameter.
• Cobble – between 6.4 and 25.6 cm in diameter
• Boulder – more than 25.6 cm in diameter.

Stream substrate can affect the life found within the stream habitat. Muddy streams generally have more sediment in the water, reducing clarity. Clarity is one guide to stream health.

Marine substrate can be classified geologically as well. See Green et al., 1999 for a reference.