There are two types of water that is naturally found in streams, lakes, oceans, and rivers. These are saltwater, which makes up most of the bodies of water, and freshwater, which is much harder to find because it is mostly stuck in glaciers or comes from the atmosphere. Freshwater in streams comes mostly from precipitation but the stream might have its source coming from a freshwater lake or spring.
Every stream has three to four parts or zones in it that allow for different life forms to survive. At the source of the stream, the water will be cooler in temperature and easier to see down to the bed. At this point, there is more oxygen in the water which leads to life forms like trout and heterotrophs, which are organisms that cannot make their own food and need organic carbon to survive. In this zone, there may be waterfalls or rapids and the only fish that live here will be flat bodied. There may also be some simple algae or moss in this area.
In the middle of the stream, there is a zone which is wider and slightly warmer. There is less oxygen in this area and phytoplankton and more diverse green plants can be found here. This zone is deeper and flatter without a lot of obstacles in the bed.
At the mouth of the stream, often it will turn or run into a river. This is the warmest zone and contains the least amount of oxygen. This water will look like it is muddy or filled with silt; the bed can no longer be seen for the most part. Due to the low amounts of oxygen, this is where catfish and carp dwell. There may be some plants and algae but there is less light to help cause a large diverse environment.
At the very end of the now probable river, there will be most likely be a dispersion zone where saltwater mixes in with freshwater. This happens around the mouth of the river or estuary as it meets the ocean. This area will cater to a wide array of wildlife as the stream has carried many nutrients that have not been dispersed into saltwater yet.
Unfortunately, very little freshwater, about 1% of it, is actually accessible to life forms. Most of it is frozen or in the ground. Many fish, aquatic plants, and some other wildlife need freshwater to survive. Humans use freshwater in many important ways to survive and it can only be replenished through the atmospheric water cycle.