In appropriate habitats, macrophytic vegetation is a sensitive indicator of pollution. Pollutants may be derived from point or diffuse sources. Point sources include settlements, factories and quarries; and diffuse ones, biocides, silting and fertilisers. The comparative importance of different pollutants depends both on their composition, quantity and concentration and on the characters, such as size and flow of the receiving watercourse. The effect on the river plants is also controlled by the type of vegetation potentially present. The detection and assessment of the pollution may be hindered by any disturbance or other plant-reducing factors in the habitat, such as channellisation, navigation and herbicides.