Large, swift rivers have a greater ability to dilute and degrade runoff pollutants, unlike smaller streams. Flow measurements help determine how much each subwatershed is contributing to downstream flows and the extent to which development is changing the natural hydrology in the watershed. Water Temperature Temperature of the water directly affects biological and chemical processes. Some fish species prefer colder waters than other species. Benthic macroinvertebrates will move in the stream in order to find their optimal temperature.
Aquatic organisms rely on the presence of oxygen in streams.
In water, oxygen is in a dissolved form. Water temperature and altitude, time of day, and seasons can all affect the amount of dissolved oxygen. Oxygen is both produced and consumed in a stream. The presence of aquatic plants also affects dissolved oxygen concentrations. Green plants release oxygen underwater during photosynthesis.