Sediments exist in rivers, lakes and oceans. Sediment particles may be derived from rocks, soils, biological and anthropogenic sources. Sediments are classified in several ways. Sediments are transported both between and within the streams, lakes, estuaries and oceans. As sediments move with water, whatever factors affect the movement of water affect the sediments as well. Sediments will move with the water masses, so long as the velocity of the water current is higher than the critical value. Sediment deposition occurs when the velocity of the current drops below the critical value. The movement and deposition of sediments in an estuary are controlled by the process of mixing of fresh water and salt water. In a well-mixed estuary, the sediments are deposited near the shore. The concentration of heavy metals in the oceanic sediments depends upon the geographical location. Coastal sediments generally exhibit higher concentrations than the deep sea sediments, because of contribution from land-based sources of pollution.