About 2% of the radiant energy of the sun that reaches the earth is absorbed by plants... It is estimated that it takes about 30,000 years of continuos plant growth to form a 1 m thick leans of coal.

The effect of pollution on a river depends both on the nature of the pollutant and the unique characteristics of the individual river. Some of the most important characteristics include the volume and speed of water flowing in the river, the river's depth, the type of bottom, and the surrounding vegetation. Other factors include the climate of the region, the mineral percentage of the watershed, land use patterns, and types of aquatic life in the river.