The scour of attached algae is one of the factors that makes algal biomass prediction difficult. During periods of active growth, particularly when the discharge is low and the water is clear, considerable algal populations are built up.
Every solid object in the water may become thickly covered with a brown carpet of Diatoms and trailing streamers of filamentous green algae. Silt surfaces also become brown with Diatoms and dotted with blackish-green patches of filamentous Cyaniphyta. These low water populations are, however, very unstable and a single storm may wash them away.
As to the instability, much depends on the algae themselves and on the site on which they are growing. Some such as the tough tassle like Rhodophyta and species with closely applied flattened thali are normally very little affected except by raging floods. They remain unchanged when filamentous green algae and colonial or stacked Diatoms are greatly reduced.
Similarly, algae growing on stones which are rolled and rubbed together by the water are much more affected than are those on rocks or moss.
Spring floods in areas where there are great amounts of melt water may reduce algal populations to almost nothing. Anchor ice and frazil ice may also reduce them by abrasion. This is an annual pattern common to many small streams in areas with severe winter climates.