3.3.1 Turbidity, colour and suspended solids
As water runs off the land, there are some substances which do not dissolve but are taken along as suspended solids. Then, depending on their sizes and the velocity of the river, the solid particles may settle out at a certain point or be carried on further. Quantities are affected by seasonal changes and tend to be higher in winter because of increased storm runoff due to higher rainfall and melting snow.
The quantity of suspended solids (measured in g m−3) affects the turbidity or cloudiness of the water. Suspended solids may also contribute colour to the water. Turbidity is measured in nephelometric turbidity units (NTU). Nephelometric means that the measurement has been arrived at through the estimation of light absorption.
Particles of all sizes tend to reduce light penetration; this reduces the rate of photosynthesis and therefore causes a reduction in the growth of plant life. Very small particles which settle out on the bottom of the stream may have a blanketing effect, thereby preventing certain bottom dwellers from living there and green plants from photosynthesizing.