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Factors Impacting Bay and Watershed Health

The Chesapeake Bay is a dynamic system. Across the watershed, rainfall, temperature and other conditions change from month to month and year to year, impacting the surrounding environment. Human activities also impact the air, land and water, as we consume natural resources, produce pollution and clear land to fit our needs.

The Chesapeake Bay Program uses monitoring data to track the factors that influence the health of the Bay and its watershed. Tracking changes in these factors over time gives scientists a better understanding of natural and human-influenced variations and the long-term effects of our restoration work.

Land Use

The Bay’s decline is directly linked to population growth and development. As we consume resources and clear forests and wetlands, we offset efforts to restore rivers, streams and the Bay.

  • Population Growth: In 2016, an estimated 18.1 million people lived in the watershed: a 0.4 percent increase from 2015. Experts predict the watershed’s population will reach 21.1 million by 2040.
  • Forest Cover: In the 1600s, forests covered 95 percent of the watershed. In 2011, just 55 percent of the watershed was forested. Estimates show that we lose 70 acres of forest land to development every day.


Nutrient and sediment pollution are among the leading causes of the Bay’s poor health. Nitrogen and phosphorus can fuel the growth of harmful algae blooms, while sediment can cloud the water and suffocate shellfish.

  • Nitrogen LoadsDuring the 2015 water year (October 2014 to September 2015), approximately 217 million pounds of nitrogen reached the Chesapeake Bay. This is below the long-term average of 335 million pounds per year.
  • Phosphorus Loads: During the 2015 water year, approximately 9.8 million pounds of phosphorus reached the Chesapeake Bay. This is below the long-term average of 20 million pounds per year.
  • Sediment Loads: During the 2015 water year, approximately 1.5 million tons—or 2.9 billion pounds—of sediment reached the Chesapeake Bay. This is below the long-term average of 5 million tons—or 10 billion pounds—per year.

Natural Factors

Natural factors like precipitation and river flow have an enormous effect on the Bay. Annual rain and snowfall determine the amount of water that flows in rivers and streams. Both river flow and the concentration of pollutants in that water affect the amount of pollution entering the Bay.

  • River Flow: In 2015, river flow to the Chesapeake Bay averaged 41 billion gallons per day. This is below the normal range of flow.
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