Sediment
This may seem strange, but sediment is sometimes considered a pollutant too! Sediment is considered a form of pollution when there is too much of it. Excess sediment damages river environments by smothering the organisms that live on the bottom. Sediment blocks sunlight, which means that algae cannot grow (by photosynthesis). Remember, the main sources of food in an estuary are algae and detritus. When these are covered with sediment, they are no longer accessible to the organisms that eat them. This affects the other animals in the food web, some directly and some indirectly (more discussion on this below).

What causes excess sediment?
Erosion causes excess sediment. Erosion is the loss of soil and gravel from the surface of land. It occurs when soil sediments are exposed and swept away by wind or water. Soil is most likely to erode when there is no vegetation covering it, securing it to the ground. Erosion results in excess sediment that gets carried into our waterways by rain, water, or wind.

You can tell when erosion has occurred because you will see bare, exposed ground. Streams and riverbanks erode readily as flowing water wears away their sides and bottoms.

Agriculture, development (building houses and businesses), and dredging (digging channels in shallow water) are major contributors to erosion. These practices speed up erosion by removing the grass and plants that cover the soil and disturbing sediment.

Certain farming practices lead to erosion. Clearing land to plant crops exposes bare soil to wind and rain. When hillsides are farmed, rain readily washes soil downhill. And, when windbreaks are not put up, soil is blown away. Another agricultural problem is overgrazing. When grazing animals are crowded, they may eat vegetation down to the ground, exposing bare soil.

Clearing land for development leads to erosion. Soil is exposed during the process of building homes, businesses, and parking lots. This loose soil gets blown and washed across pavement and fields. Eventually it ends up in our waterways.

Dredging is the process of digging deep channels in water to make room for large boats. Dredging stirs up large amounts of sediment. This sediment gets picked up by water currents and may settle in other areas. Estuaries often receive excess sediment from flood tides. Dredging may also result in reduced size of an estuary. Sometimes sediment that is dredged gets re-deposited along coastline habitat. Estuaries get filled in so that buildings can be built there. For example, the San Francisco Bay, a large estuary, used to harvest 6,820,000 kg (15,000,000 pounds) of oysters per year. Now they harvest none because all the oyster beds have been covered by excess sediment.

Although these practices contribute greatly to erosion, it is important to note that careful planning can reduce erosion. To learn more about land-clearing practices that do a better job at containing sediment, see Protecting our watershed: What YOU can do to help.

Excess sediment causes many problems in an estuary. It covers up algae and detritus, two very important food sources. It makes the river bottom mushy, so that oysters and clams cannot attach and live there. Excess sediment also covers fish eggs that are laid in gravel beds.

When excess sediment harms detritus, algae, oysters and fish, all the other organisms in the estuary are affected too. Animals that eat algae and detritus, such as snails, fiddler crabs, shrimp, clams and worms, no longer have a food source. When these herbivorous animals cannot survive, the animals that eat them no longer have a food source. Carnivorous striped bass, starfish, raccoons and blue crabs no longer have anything to eat. And when these animals cannot survive, wading birds, such as great blue herons, no longer have anything to eat.

In addition to losing our coastal treasures, we also lose potential income. People that fish and harvest oysters and shrimp may go out of business. It is estimated that two-thirds (2/3) of all commercial fish spend some part of their life cycle in an estuary.

Pollution is a difficult problem because it is produced as a by-product of goods and services that are valuable to us. The good news is that there are many things we each can do (yes, Y?U!) to help solve our problem. To learn more about reducing your impact on the environment, see Protecting our watershed: What YOU can do to help.

Remember, we all live downstream ? in addition to local pollution, our water contains pollutants from people living upstream. And, of course, our pollution affects the water of people living downstream from us.

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