Turbidity measures how clear the water is. Sediment or dissolved solids in the water can stick in the gills of fish, settle on top of spawning grounds, and even impair their ability to find food. Filter-feeding invertebrates such as clams and water fleas can become clogged with sediments, leading to starvation. It can even increase the temperature of the water, leading to lower oxygen levels.
Rainfall often increases turbidity in creeks, as stormwater runoff contributes to higher flows and can cause creek-bed erosion. Other sources of increased turbidity include algal blooms, waste discharge, and even animals or children playing in the water.
Fish prefer very clear water, with a turbidity of less than 10 FAU (Formazin Attenuation Units). We typically see this healthy range in San Pablo Creek, except for during and shortly after large storm events, when we have seen upwards of 100 FAU.