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

• Mud – 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.