Dredging is scraping the bottom of a river, seafloor or other marine habitat to remove silt, open up a channel, make the river floor deeper to accommodate larger boats, prevent flooding or better drain nearby land. Occasionally law enforcement officials dregde to find evidence in a crime, but in those cases, silt is usually disturbed but not removed. Any dredging efforts, although meant to improve an area, have the potential for harm.
Disturbing the Flow
Dredging changes the natural flow of a body of water. While this is sometimes the specific purpose for a dredging operation, disturbing the natural water flow may have unintended and unknown effects. Changing the water flow often destroys habitats. It may turn wetlands into dry land, and thereby eliminate flood plains. In the ocean, dredging can change the natural wave patterns. Since dredging is generally proposed as a way to solve a problem, it's important to consider the effects of not dredging vs. the effects of dredging.
Harm to Wildlife
Dredging causes harm to wildlife in a variety of ways. First, the mechanical act of dredging destroys habitats and may kill fish and other animal life. The machines used for dredging harm animals. Also, dredging disturbs sediment, causing it to swirl through the water in a way it normally does not. Additionally, dredging puts additional food, such as dead fish and fish offal, into the water. This attracts predatory fish that feed on native species and may hurt the natural population. Life forms such as corals are often destroyed by dredging, and take a longer time to repopulate than some other life forms. Dredging may allow invasive species to get a foothold while native species struggle to recover.
Harm to Flora
Dredging can crush and kill marine flora, smooth bedforms so plant life has a harder time gaining a foothold and reduce biodeversity. Dredging up sediment also decreases the amount of light that penetrates into the body of water, thereby reducing the number of plants able to grow.
Even though dredging is sometimes done in order to remove pollutants that have settled into the soil over the years, simply stirring up the contaminated sediments causes the pollutants to spread. Rather than cleaning up the body of water, dredging may transfer the pollutants to a wider area. Additionally, dredged up pollutants have to be dumped at another location, thereby increasing the chance that the contaminants will stray into areas that were previously free from contaminants.