Gradient is low. The substrate consists of sand and mud. Oxygen deficits may occur. Upper Perennial - Gradient high, water velocity fast. Water flows throughout the year.
Substrate consists of rock, cobble, gravel, and sand.
Dissolved oxygen content high. Intermittent - Flowing water present only part of the year. When water is not flowing, it may remain in isolated pools or surface water may be present. Flowing water present all year. This system includes many of the same classes as the palustrine and lacustrine systems.