This practice began in the 1920's and 1930's when exploration and extraction of oil and natural gas began. They are built because natural channels are not deep enough, nor are they located conveniently for industry requirements. When the canals are built, the dredged material is thrown up along the side, creating what is called a spoil bank. Spoil banks consist of marsh soil and organic material. As the banks settle, they create a levy that runs along the length of the canal.
Spoil banks disrupt the natural source of sediments needed for accretion and block overland flow.
The levies created by the spoil banks not only alter the natural hydrological flow, but can also block migration of aquatic organisms. Because the canals are deeper than the surrounding wetlands, they allow larger predators to enter areas inhabited by juvenile fish. Tides enter and leave through the canals at a greater velocity than through undisturbed marsh, resulting in erosion and widening of the banks. Saline water travels up the canals, invading deep into freshwater territory, causing vegetation to die and creating open water where dense vegetation previously existed. The greatest impact on wetland destruction has been noted in saline and brackish marshes.