As a river flows from source to sea it is subject to constant change from various physical and environmental characteristics within the catchment. 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 factors influencing and effecting change are:
- 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).