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The Water Molecule
The water molecule is made up of two hydrogen atoms and an oxygen atom. Water molecules trade bonds with each other about once a millisecond at their slowest, which is relatively long for hydrogen bonding, so the water molecule's structure is treated as "permanent." Since water molecules are so prone to hydrogen bonding, water is relatively unreactive, meaning that is not prone to explosive or violent reactions. At their freezing point, water molecules bond together in a hexagonal pattern that is relatively space-inefficient, so water expands and becomes less dense as it freezes, which is why ice floats on the surface of freshwater instead of sinking. Rapidly-moving freshwater, which doesn't have a calm or even surface for ice to float on, is unlikely to freeze.
Carbon, Oxygen and Hydrogen from the Atmosphere
Rainwater brings carbon dioxide from the atmosphere into freshwater. As carbon dioxide dissolves into rainwater, it releases an excess hydrogen ion, which has a negative charge, and brings down the pH of the rainwater. Rainwater then combines with the rest of the freshwater, and brings the pH of that water down, too, making freshwater slightly acidic. Moving water, whether it's mixing because of high winds or because it's a body of flowing water such as a stream or lake, has more oxygen -- from the air and from unbonded oxygen atoms -- circulating through it than still water does.
Minerals (TDS)
When flowing into a body of water, rainwater must flow over rocks and the ground. The rainwater's slightly acidic pH dissolves some of the minerals from those rocks and soil. The different mineral compositions of the rocks and soil result in different mineral compositions for freshwater rivers and streams that flow over it. Usually, the total dissolved solids (TDS) present in groundwater is less than 250 mg/L. The more arid the area's climate or time of year, the more groundwater makes up the overall river composition. Since groundwater doesn't move as quickly or as often as surface water, it tends to spend longer in contact with any particular rock or ground it passes over, which leads to more minerals dissolving into it --- which leads to an increase in the ratio of minerals to freshwater molecules within the body of water in general. Evaporated water also leaves behind the minerals dissolved into it, so the more water evaporates from a body of water, the higher the ratio of minerals to water molecules in the body of freshwater.
Heat and Light
The more heat and light from the sun penetrating the freshwater, the more hospitable the water is for life. The topmost layer of water, called the littoral zone in a pond or lake, is the warmest and brightest, so it usually has a relatively large amount of diversity in terms of plant and animal life. It may be home to rooted plants, free-floating plants such as algae, shellfish, aquatic insects, terrestrial insect larvae and eggs, amphibians and fish. The second layer of water, called the limnetic zone, is still relatively warm and well lit, and is hospitable to microorganisms and fish. The deepest part of the water is called the profundal zone in lakes and ponds. Since little light and heat from the sun penetrates that far, it maintains a fairly constant temperature---usually at around 4 degrees Celsius. With little heat and light, the zone is relatively inhospitable to life, and it's mostly home to animals that feed on the dead plankton that floats to the freshwater bed.