Cleaning up the Bay means more than just curbing nutrient inputs. It now means keeping dirt out as well.
The Bay Program last year set its first-ever sediment reduction goals aimed at keeping silt and dirt on the watershed—and along shorelines—so it doesn’t kill grass beds by muddying the Chesapeake’s water. Dirt, like algae blooms, clouds the water, blocking sunlight from reaching important underwater grass beds.
The sediment goals are aimed at helping the Bay Program meet its new 185,000-acre underwater grass goal by clearing the water in places where nutrient control alone may not be enough to do the job. Sediment also smothers habitats for oysters and other bottom-dwelling creatures and can clog the gills of fish.
Once, the Bay’s forested watershed contributed only small amounts of dirt to the Bay each year, but that changed as land has been converted to agriculture or developed. In colonial times, that was seen as ports such as Joppatowne, Upper Marlboro and Port Tobacco were closed to shipping when sediment filled their channels.
Today, sediment continues to wash into the Chesapeake at an accelerated rate. The Bay Program estimates that around 5 million tons of sediment is washed into the Bay from its tributaries each year, about three times what may have entered before the region was settled. More sediment is added from erosion along the Bay’s shoreline.
Sediment is also pushed in from the ocean, and other sediment is created from bones, scales and other organic material.
Sediment control actions are focused on the areas that humans can most readily control: material washing in from the rivers and material eroding from shorelines. But officials acknowledge that there is less certainty about the sediment goals than those for nutrients. This reflects the fact that scientists are far from clear about how sediment moves through the Bay and affects its water clarity.
The sediment goals, as a result, are being set in two stages.
The first step called for reducing from 5.05 million tons to 4.15 million tons annually sediment that washes into the Bay from the major tributaries. That reduction is aimed at helping to clear the water in tidal freshwater areas of the Upper Bay and its tributaries—the areas where sediment from the rivers is thought to have its greatest impact.
None of those sediment goals exceed the levels of sediment reduction that could be achieved from phosphorus control actions already needed from those individual rivers. (Because phosphorus tends to bind to sediment, efforts to reduce phosphorus runoff also reduce sediment.)
But as more is learned about sediment, it’s possible more stringent sediment goals will be set, if necessary, to clear the water for grass beds.
The second step involves shoreline erosion. In areas below tidal freshwater areas, where sources such as shoreline erosion and resuspension by waves are thought to be more important, reductions are being set on a case-by-case basis to help restore historic levels of grass beds.
In certain shallow water areas, sediment is now thought to play a more important role than nutrients in reducing the sunlight available for underwater grasses. Restoring those grass beds and the habitat they provide is considered a key part of bringing back a healthy Bay ecosystem.
Controlling sediment brings new challenges to the Bay cleanup effort. Shoreline erosion rates have increased over the past century as water levels have risen; combating that can be particularly expensive.
Reducing sediment from the rivers can be problematic as well. Research suggests it often takes decades for sediment to move downstream and into the Bay. That means control actions taken today may not yield results until far into the future.
Keeping that sediment in place does more than just help the Bay. Excess sediment is a major source of pollution to streams throughout the watershed, where it clouds the water, smothers bottom habitats and increases streambank erosion.