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Sediment

Overview

Sediment is made up of loose particles of sand, silt and clay. It is a natural part of the Chesapeake Bay, created by the weathering of rocks and soil. In excess amounts, sediment can cloud the waters of the Bay and its tributaries, harming underwater grasses, fish and shellfish.

Where does sediment come from?

Sediment forms when rocks and soil weather and erode. Around 5.2 million tons of sediment enter the Chesapeake Bay in an average year.

There are two major sources of sediment: eroding land and stream banks—called watershed sources of sediment—and eroding shorelines and coasts—called tidal sources of sediment.

Watershed sources of sediment

Eroding land and stream banks are called watershed sources of sediment. Watershed erosion increases when land is cleared of vegetation to make way for agriculture and development. Scientists estimate that most of the sediment that flows into the Chesapeake Bay comes from watershed sources.

In the Bay watershed, river basins with the highest percentage of agricultural lands yield the highest amount of sediment each year. Basins with the highest percentage of forest cover, on the other hand, yield the lowest amount of sediment. On a per-acre basis, construction sites can produce the most sediment of all land uses—as much as 10 to 20 times that of agricultural lands.

Since the seventeenth century, watershed-wide changes in land use and land cover have disrupted the natural processes of erosion:

  • During the eighteenth and nineteenth centuries, 70 to 80 percent of the watershed’s forest cover was removed to supply people with firewood and farmland. Cleared, exposed land is prone to erosion.
  • Deforestation peaked in the late nineteenth century, and while reforestation did take place during the twentieth century, increased urbanization has continued to contribute to high erosion rates.
  • Studies of sediment cores in the Bay and its tributaries show a four- to five-fold increase in sediment accumulation rates in some parts of the Bay since the 1800s.

Tidal sources of sediment

Eroding shorelines and nearshore areas, as well as the resuspension of already-eroded sediments, are known as tidal sources of sediment. Tidal erosion increases when shoreline vegetation is removed and there are not enough bay grasses growing in the offshore shallows to lessen the force of waves against the shoreline.

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 countless critters, including blue crabs, herons and terrapins.

However, human development along the shoreline can have negative impacts on the natural processes of tidal erosion:

  • Man-made, hardened shorelines—those lined with rocks, wood or concrete—can interfere with natural tidal erosion, blocking the formation of wetland habitat and starving beaches of new sediment.
  • Hardened shorelines can also lead to “nearshore erosion,” during which waves erode the shallow area immediately in front of the man-made shore. This increases the amount of sediment that is suspended in the water.
  • Between 1988 and 2000, more than 19 miles of Virginia tidal shoreline was hardened on average 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 can also increase tidal erosion.

How does sediment harm the Chesapeake Bay?

Excess sediment is a leading factor in the Chesapeake Bay’s poor health.

Because of their small size, the particles of sand, silt and clay that we call “sediment” often float through the water rather than settling to the bottom, and can be carried long distances during rainstorms. When there are too many sediment particles suspended in the water, the water becomes cloudy and muddy-looking. Cloudy water does not allow sunlight to reach the plants that grow on the bottom of the Bay’s shallows. Without sunlight, these plants—including underwater grasses—die, which affects the young fish and shellfish that depend on them for shelter.

Excess sediment can also have harmful effects on the wider Bay and the people who use it:

  • Nutrients and chemical contaminants can bind with sediment, spreading through the Bay and its waterways with particles of sand, silt and clay. Fish and shellfish that live and feed on or near contaminated sediment can become contaminated themselves, triggering fish consumption advisories in various portions of the watershed.
  • Excess sediment can smother oysters and other bottom-dwelling species.
  • Accumulating sediment can clog ports and channels, affecting commercial shipping and recreational boating.

A case study in sediment: the Conowingo Dam and Chesapeake Bay water quality

Each year, the Susquehanna River provides the Chesapeake Bay with one-quarter of its sediment loads. For decades, three large reservoirs that sit behind dams located along the lower portion of the river have held back some of the sediment pollution that would have otherwise entered the Bay. But recent studies have drawn attention to these reservoirs’ changing effectiveness as “pollution gates,” with special attention paid to the reservoir behind the Conowingo Hydroelectric Generating Station, or Conowingo Dam.

In 2012, the U.S. Geological Survey (USGS) reported that the reservoir behind the Conowingo Dam had lost its ability to trap sediment and attached nutrients over the long term.

In 2014, the Lower Susquehanna River Watershed Assessment (LSRWA) team released the results of its evaluation of sediment management options at the Conowingo Dam. It found:

  • The reservoir behind the Conowingo Dam is trapping sediment in the short-term. Because the reservoir is essentially full, it is trapping smaller amounts of incoming sediment and, during large storms, sending more silt and attached nutrients over the dam and into the Bay more often.
  • The nutrients that enter the river upstream and attach to particles of sediment are a bigger threat to water quality than sediment alone.
  • The management and mitigation of nutrients and sediment upstream of the reservoir would be more beneficial to Bay health than attempting to manage sediment at the dam through dredging, bypassing or operational changes.

While the effects of the sediment that can scour from behind the dam essentially cease once it settles to the bottom of waterways, lowering both sediment and nutrient pollution upstream of the Conowingo Dam would benefit Bay health.

To learn more, visit Learn the Issues: Conowingo Dam.

Take Action

For Chesapeake Bay restoration to be a success, we all must do our part. Our everyday actions can have a big impact on the Bay. By making simple changes in our lives, each one of us can take part in restoring the Bay and its rivers for future generations to enjoy.

To lower sediment pollution, consider combating erosion on your property. Spread mulch over bare ground, or plant buffers of trees and shrubs to capture runoff and hold soil in place.

Photos



 

Chesapeake Bay News


Sediment Loads and River Flow to the Bay

Approximately 2.71 million tons of sediment reached the Bay during the 2013 water year, which is below the 1990-2013 average load of 5.2 million tons.  The 2013 sediment load is 0.53 million tons higher than the 2012 load.

Annual average river flow to the Bay during the 2013 water year was 49.1 billion gallons per day (BGD), which is below the 1990-2013 mean flow of 53.4 BGD. The 2013 flow is 2.6 BGD less than the 2012 flow.


Sediment in Rivers Entering Chesapeake Bay: Long-Term Flow-Adjusted Concentration Trends

Twenty-eight percent of long-term stream monitoring sites in the Chesapeake Bay Watershed have improving flow-adjusted concentrations of sediment.  Between 1985 and 2012:

  • 8 out of 29 sites show improving flow-adjusted trends for sediment concentrations
  • 8 sites show degrading trends
  • 13 sites show small changes that are not statistically significant

For more information go to the U.S. Geological Survey's webpage, Summary of Trends and Yields Measured at the Chesapeake Bay Nontidal Network Sites: Water Year 2012 Update

Sediment Short-Term Flow-Adjusted Concentration Trends Measured in Watershed Streams and Rivers

Over the past 10 years, 49 percent of sites show little change while 41 percent show degrading flow-adjusted sediment concentrations. Between 2003 and 2012:

  • 4 out of 39 sites show improving flow-adjusted trends for sediment concentrations,
  • 16 site shows degrading trends, and
  • 19 sites show small changes that are not statistically significant.

For more information go to the U.S. Geological Survey's webpage, Summary of Trends and Yields Measured at the Chesapeake Bay Nontidal Network Sites: Water Year 2012 Update

Bay 101: Sediment



November 07, 2014

Scientist Allen Gellis with the U.S. Geological Survey (USGS) visits Linganore Creek in Maryland to describe how he conducts studies of sediment sources in the Chesapeake Bay watershed.

Sediment forms when rocks and soil weather and erode. There are two major sources of sediment: eroding land and stream banks—called watershed sources of sediment—and eroding shorelines and coasts—called tidal sources of sediment. An estimated 2.7 million tons of sediment was delivered to the Chesapeake Bay in 2013 and an average of 5.2 million tons comes in each year. Too much sediment can cloud the waters of the Bay and its tributaries, harming underwater grasses, fish and shellfish.

Produced by Will Parson
Music: “A Moment of Jazz” by Ancelin


Publications

The Conowingo Dam and Chesapeake Bay Water Quality

Publication date: January 15, 2013 | Type of document: Backgrounder | Download: Electronic Version

Each year, the Susquehanna River provides the Bay with one-quarter of its sediment loads. Working to curb these loads are three large reservoirs located along the lower portion of the river, each of which sits behind a dam. These…

Map: 2010 Innovative Nutrient and Sediment Reduction Grants (PDF)

Publication date: August 05, 2010 | Type of document: | Download: Electronic Version

2010 Innovative Nutrient and Sediment Reduction Grants map (PDF)

2010 Innovative Nutrient and Sediment Reduction Grants Awardees and Projects

Publication date: August 05, 2010 | Type of document: | Download: Electronic Version

2010 Innovative Nutrient and Sediment Reduction Grants Awardees and Projects backgrounder

Tidal Sediment Yield Estimate Methodology in Virginia for the Chesapeake Bay Program Water Quality Model

Publication date: June 01, 2009 | Type of document: Report | Download: Electronic Version

Water quality in Chesapeake Bay has degraded over the past 50 years with respect to oxygen depletion and reduced light attenuation. While the causes are numerous, sediment resuspension from wave and tidal action cloud the water column and…

An Introduction to Sedimentsheds:Sediment and its Relationship to Chesapeake Bay Water Clarity - STAC Workshop Report

Publication date: May 23, 2007 | Type of document: Report | Download: Electronic Version

This workshop was held to cover the topic of Sedimentsheds. sedimentshed is a new concept, and is defined as the area that contributes the sediment which directly influences water clarity in near-shore Submerged Aquatic Vegetation (SAV)…

Best Management Practices for Sediment Control and Water Clarity Enhancement

Publication date: October 01, 2006 | Type of document: Report | Download: Electronic Version

The Chesapeake Bay Program (CBP) hosted a workshop in Annapolis, Maryland on February 24-25, 2003, at which sediment experts shared information related to sediment best management practices (BMPs). The information presented on selected BMPs…

Coupling Suspended Sediment Dynamics and Light Penetration in the Upper Chesapeake Bay

Publication date: May 23, 2006 | Type of document: Report | Download: Electronic Version

The attenuation of light underwater is an important process in estuaries, directly affecting phytoplankton, submerged aquatic vegetation (SAV), visually orienting predators, and indirectly affecting oxygen depletion and other water quality…

Sediment in Chesapeake Bay and Management Issues: Tidal Erosion Processes

Publication date: May 01, 2005 | Type of document: Report | Download: Electronic Version

Sediment is the third biggest pollutant to the Bay and its tributaries. Tidal sediment comprises approximately 57% of the sediment load to the Bay. Excess sediment is a key contributor to degraded water clarity and damages critical habitats…

Directive 04-2: Meeting the Nutrient and Sediment Reduction Goals - Next Steps

Publication date: January 10, 2005 | Type of document: Directive | Download: Electronic Version

This Directive addresses next steps, each of which will advance Tributary Strategy implementation, and identifies measures to implement several Panel recommendations upon which action can be taken quickly.

Directive 03-02 - Meeting the Nutrient and Sediment Reduction Goals

Publication date: December 09, 2003 | Type of document: Directive | Download: Electronic Version

improving water quality is the most critical element in the overall protection and restoration of the Chesapeake Bay and its tributaries was the goal of this directive.

Setting and Allocating the Chesapeake Bay Basin Nutrient and Sediment Loads: The Collaborative Process, Technical Tools and Innovative Approaches

Publication date: December 01, 2003 | Type of document: Report | Download: Electronic Version

The Chesapeake 2000 agreement has been guiding Maryland, Pennsylvania, Virginia and the District of Columbia, the Chesapeake Bay Commission and the U.S. Environmental Protection Agency (EPA) in their combined efforts to restore and protect…

Economic Analyses of Nutrients and Sediment Reduction Actions To Restore Chesapeake Bay Water Quality

Publication date: June 01, 2003 | Type of document: Report | Download: Electronic Version

In developing revised water quality standards for the Chesapeake Bay and its Tidal tributaries, states may conduct use attainability analyses. This document provides economic analyses performed by the CBP related controls to meet revised…

Summary of Decisions Regarding Nutrient and Sediment Load Allocations and New Submerged Aquatic Vegetation (SAV) Restoration Goals.

Publication date: April 25, 2003 | Type of document: Policy Memorandum | Download: Electronic Version

Policy memo concerning decisions regarding nutrient and sediment load allocations and ne submerged aquatic vegetation (SAV) goals.

Benchmarks for Nitrogen, Phosphorus, Chlorophyll and Suspended Sediments in Chesapeake Bay

Publication date: April 10, 2002 | Type of document: Report

One of the Tidal monitoring and Analysis Workgroup's primary responsibilities is assessing and reporting the status and trends of nutrients and other parameters monitored within the scope of the Chesapeake Bay Program water quality and…

Chesapeake Bay Watershed Model Application and Calculations of Nutrient & Sediment Loadings, Appendix D: Phase IV Chesapeake Bay Watershed Model Precipitation and Meteorological Data Development and Atmospheric Nutrient Deposition

Publication date: August 15, 1998 | Type of document: Report | Download: Electronic Version

Chesapeake Bay watershed model precipitation and meteorological data development and atmospheric nutrient deposition.

Chesapeake Bay Watershed Model Application & Calculation of Nutrient & Sediment Loadings - Appendix F: Phase IV Chesapeake Bay Watershed Model Point Source Loads

Publication date: August 15, 1998 | Type of document: Report | Download: Electronic Version

Appendix F documents in detail the Phase IV Chesapeake Bay Watershed Model point source nutrient data assimilation process for the facilities located in signatory and non-signatory jurisdictions of the Chesapeake Bay Watershed. This…

Chesapeake Bay Watershed Model Application & Calculation of Nutrient & Sediment Loadings. Appendix B: Phase IV Chesapeake Bay Watershed Model Water Quality Calibration Results

Publication date: May 15, 1998 | Type of document: Report | Download: Electronic Version

Appendix B documents the water quality calibration of the Phase IV Watershed Model. Simulated and observed concentrations are compared for 8 years of calibration (1984-1991) at 15 water quality stations. Calibration data is shown for…

Chesapeake Bay Watershed Model Application & Calculation of Nutrient & Sediment Loadings-Appendix A: Phase IV Chesapeake Bay Watershed Model hydrology Calibration Results

Publication date: May 15, 1998 | Type of document: Report | Download: Electronic Version

Comparison of simulated and observed flow for 8 years of calibration (1984-1991).

Changes in Sediment and Nutrient Storage in Three Reservoirs in the Lower Susquehanna River Basin and Implications for the Chesapeake Bay

Publication date: March 01, 1998 | Type of document: Report | Download: Electronic Version

Changes in Sediment and Nutrient Storage in Three Reservoirs in the Lower Susquehanna River Basin and Implications for the Chesapeake Bay.




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From Around the Web

Bay FAQs

  • How does sediment affect the Bay?
  • What causes poor water clarity?

 

Bay Terms

  • Agriculture
  • Chemical contaminants
  • Deforestation
  • Ecosystem
  • Erosion
  • Nutrients
  • Reforestation
  • Reservoir
  • Sediment
  • Tributary

 

Bay-Friendly Tips

  • Plant Trees and Shrubs
  • Plant a buffer of trees and shrubs around the edge of your property to capture polluted runoff.

 

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