As a river flows from source to sea it is subject to constant change from various physical and environmental characteristics. The resulting processes shape the nature of the river from along its entire length, to within specific sections (such as headwaters, middle and lower), and down to micro habitats in river reaches (such as riffles and pools). The key change factors in rivers are summarised below:
- interactions between water and land (hydrology);
- how the earth’s surface and underlying geology shape the river channel (geomorphology);
- chemical pollution, organic pollution, abstraction, flow management, land management, physical alteration, invasive non-native species and climate change (anthropogenic pressures);
- flow velocity, substrate composition, temperature and oxygen (abiotic factors);
- habitats and species processes (biological factors).
Downstream flow is perhaps the most central component of any river, not only because it directly (e.g., presence, absence, diversity, and abundance of plants and animals) or indirectly (e.g., river bed composition) affects every component within every river system, but also because of how the very character of rivers is shaped by the relationship between flow and river processes.