Erosion

There are two types of erosion in the Bay region: watershed or upland erosion and tidal erosion. Erosion is a natural occurrence influenced by climate and geology, as well as non-sustainable land use and other human activities.

Watershed Erosion

The two most important sources of sediment from the watershed are:

• Erosion from land surfaces
• Erosion of stream banks and corridors

In the Bay watershed:

• River basins with the highest percentage of agricultural lands yield the highest overall amount of sediment each year, while basins with the highest percentage of forest cover yield the lowest amount of sediment.
• However, on a per-acre basis, construction sites can contribute the most sediment of all land uses—as much as 10 to 20 times that of agricultural lands.
• Urbanized areas can contribute high amounts of sediment because of their use of impervious surfaces—roads, parking lots and other hardened surfaces that do not allow water to pass through. The increased water flow off these surfaces can cause extensive stream channel erosion.

Since the 17th century, land use and land cover changes in the Bay watershed have severely disrupted natural erosion processes.

• During the 18th and 19th centuries, 70 to 80 percent of the watershed's original forest cover was cleared for timber and agriculture. Cleared, exposed land is especially prone to erosion.
• Deforestation peaked in the late 19th century, and reforestation took place during the 20th century. However, increased urbanization contributed to continued high erosion rates during this time.
• Studies of sediment cores in the Bay and its tributaries show a four- to five-fold increase in sediment accumulation in some parts of the Bay since the 1800s.

Tidal Erosion

The Bay's shorelines have been slowly eroding for thousands of years due to wave action and natural sea level rise. Tidal erosion can help maintain the Bay's complex ecosystem. Beaches and tidal wetlands, which are created and replenished by erosion, provide habitat for blue crabs, herons, terrapins and many other species.

Shoreline development has several negative impacts on natural tidal erosion.

• Man-made, hardened shorelines—those lined with rocks, wood or concrete—can interfere with natural tidal erosion processes by blocking the formation of wetlands and starving beaches of new sediment. They also eliminate natural shorelines, which provide habitat for many plants and animals.
• Wave action causes the bottom of the shallow water area immediately in front of the hardened shoreline to erode. This is called nearshore erosion, and it increases suspended sediment that blocks sunlight from reaching underwater bay grasses.
• Between 1988 and 2000, an average of over 19 miles of Virginia tidal shoreline was hardened each year. In Maryland, more than 300 miles of tidal shoreline were hardened between 1978 and 1997.

Accelerated sea level rise caused by climate change is another man-made cause of increased tidal erosion.