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Physical Characteristics of a Stream
Three important characteristics of a stream are salinity, temperature, circulation, and dissolved oxygen. Salinity is the amount of dissolved salts present in water. It is measuered in parts per thousand (ppt). Fresh water is usually around or under .5 ppt while salt water generally contain 35 ppt. Streams tend to have the most saliniity at their mouths and decrease closer to the source. Salinity also changes with depth because salt water is more dense than fresh water. Streams are usually shallow so they cannot store much heat energy and their temperature fluctuates. There are, however, temperature differences in different depths of the water. Circulation affects the distribution of nutrients, sediments, and plankton. Dissolved oxygen is measured in miligrams per liter of water (mg/L). Worms, clams, and other benthic organisms need about 1 mg/L, fish, crabs, and oysters, need 3 mg/L, and spawning migratory fish and their larvae and eggs need 6 mg/L (Physical Characteristics 2008).

Types of Organisms
A variety of species live in streams. Among them are resident freshwater fish that spend their entire lives in freshwater streams and rivers. Catfish and sunfish are resident freshwater fish. Anadromous fish such and shad and sturgeon spawn in streams but spend most of their adult life in saltwater bays or oceans. Bacteria, worms, and clams are benthic organisms, which live at the bottom of streams. Frogs, turtlesm and salamanders spend a portion of their life in rivers and streams as well.Many insects live in sreams and provide food for many fish, birds, and amphibians. Eels are also found in streams (Streams and Rivers 2008).

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4.1 Watersheds pd2