Typically, rills join to form gullies, which join to form streams, which join to form a river.
Riffles are the sites of primary production.
Typically, organisms attach to the substrate to avoid being swept away by the current. Riffle areas have a high O 2 content because of the churning. They serve as catch basins for organic materials and produce the CO 2 used by the producers in the next riffle. Bogs depend heavily on precipitation to supply nutrients, so they usually are low in mineral salts and are acidic. Salt Water Salt concentration is a limiting factor for organisms which live in salt water.