In a watershed, when water begins flowing from its source along the surface, they form streams and rivers. The differences between the two are streams (narrow, less water volume) and rivers (wider, more water volume). Sources of flowing water include glacier and snow melts, springs and rainwater runoff.

Flow rate affects biological and chemical properties of rivers. Faster flowing water makes it difficult for aquatic plants to root; the fewer plants there are, the smaller the food webs are in the ecosystem. Additionally, there is less detritus (decaying organic matter), resulting in even less organic matter for food webs. However, faster flow can result in more mixing of water (i.e. rapids) and increase the levels of dissolved oxygen and decrease the temperature of water.

Slower flowing water tends to be warmer in temperature; warmer water has a lower capacity to dissolve oxygen. Another phenomenon in slower flowing water is more sedimentation. This provides a medium for plants to set roots and nutrients for algae. 

All rivers and streams are freshwater. Two threats to rivers and streams are (1) pollutants and (2) an excess of nutrients.