The current Bay-wide water quality standards attainment score is 29.4%, meaning 29.4% of the Bay’s tidal waters are estimated to have met water quality standards during the 2021-2023 assessment period. This is almost identical to the 29.8% estimated during the 2020-2022 assessment period. While the current attainment score is below the record high 42.2% achieved during the 2015-2017 assessment period, it remains above those estimated in 2018-2020 (28.9%) and 2019-2021 (28.1%). Overall, the long-term trend is one of statistically significant improvement from 1985 to 2023.

Water quality is evaluated using three parameters: dissolved oxygen, water clarity or underwater grass (SAV) abundance, and chlorophyll-a (a measure of algae growth). Individual parameter assessments for the 2021-2023 assessment period show a major decline in deep water dissolved oxygen, but a moderate increase in open water dissolved oxygen. 

Because dissolved oxygen and water clarity are impacted by the amounts of nutrient and sediment entering the Bay, periods of improvement and decline in this measure of water quality correlate with significant changes in rainfall patterns across the region. For example, in 2018 and 2019, higher than average river flows entering the Bay delivered abundant nutrient and sediment loads, which resulted in low dissolved oxygen, poor water clarity and reductions in underwater grass abundance. From 2020 to 2023, more average weather patterns occurred, which led experts to anticipate the recent rebound in the Bay-wide water quality standards attainment score.

Water quality standards attainment remains far below the 100% necessary to fully support the survival, growth and reproduction of the Bay's living resources. For the Bay and its tidal tributaries to function as a healthy ecosystem and be taken off of the impaired waters listings under Sections 303(d) and 305(b) of the Clean Water Act, all applicable water quality criteria must be met simultaneously as defined in the water quality standards of Maryland, Virginia, Delaware and Washington, D.C. Segment-level assessment results of water quality standards attainment can be accessed and visualized through the Chesapeake Bay Water Quality Standards Attainment Deficit and Chesapeake Bay Water Quality Standards Attainment Indicator visualization tools.

In 2023, average river flow to the Chesapeake Bay measured 42.5 billion gallons per day, a 10% decrease from 2022. The corresponding amounts of nutrients and sediment that entered the Bay that same year were approximately 199 million pounds of nitrogen, 9.7 million pounds of phosphorus and 12 billion pounds of sediment. All of these are below both their 2022 levels and their long-term (1985-2023) average and thus represent an improvement.

Nutrient and sediment loads delivered from the watershed into the Bay are one of many factors that influence water quality. These loads, in turn, are influenced by land use, land management and river flow. Generally, when the watershed receives more rain, river flows increase, and rivers carry more nutrients and sediment into the Bay than usual.

Information about nutrient and sediment loads and river flow comes from the Bay’s nine River Input Monitoring (RIM) stations, point source discharges into the Bay’s tidal waters, and an estimate of nonpoint source discharges produced by the Chesapeake Bay Program’s Watershed Model.

Nutrient and sediment loads are monitored at 123 river and stream stations throughout the Chesapeake Bay watershed. Water quality samples collected from this nontidal monitoring network are used by the U.S. Geological Survey (USGS) to calculate trends--which describe how loads of nitrogen, phosphorus and sediment have changed over time--every year or every other year. Trends are mixed across the watershed: loads at some stations have decreased, while loads at other stations have increased. Learn more about this monitoring network and the most recent load and trend results from the USGS.

The Chesapeake Bay Total Maximum Daily Load, or Bay TMDL, Indicator combines monitored and modeled data to estimate the progress of nitrogen and phosphorus loading rate reductions (millions of pounds per year) in response to implemented management practices. The indicator addresses the following questions: 

• What reductions have been observed in the monitoring data?
• What reductions are expected from past management actions but have not been observed due to known lags between implementation action and environmental response?
• What reductions are needed from planned management actions?
• What reductions are expected but not yet observed in the monitoring data?

To provide quantitative answers to these questions, this indicator leverages data from the Watershed Model and monitoring data, including river discharge and water quality measurements, wastewater loads and the atmospheric deposition of nitrogen to tidal waters. CAST integrates knowledge of land use, nutrient inputs and watershed processes to estimate load reductions in response to implemented management practices. However, these expected reductions often differ from observed reductions due to uncertainty in the model, uncertainty in monitored trends, natural lags between the implementation of management practices and the eventual improvement of water quality, the impacts of changing environmental conditions, and the infill of the Conowingo Reservoir. By quantifying the effects of some of these factors—including lag times, changing environmental conditions and the infill of the Conowingo Reservoir—this indicator helps bridge the gap between monitored reductions and model-estimated reductions. In addition to this indicator, the Monitored and Expected Total Reduction Indicator for the Chesapeake (METRIC) offers insight into the trajectory and pace of localized load reduction, comparing monitored load trends and model-estimated load trends at the watershed’s nontidal network monitoring stations. 

The Bay TMDL indicator uses 1995 as its baseline because it marks the end of the 1993-1995 critical period used for assessing the attainment of water quality standards in the Bay TMDL. It divides the total annual loading rate reductions required to meet Bay TMDL planning targets into nine categories:

• Response:
  • Tidal Deposition Reduction Unimplemented:  Expected reductions in the atmospheric deposition of nitrogen to tidal waters.
  • Tidal Deposition Reduction Realized: Observed reductions in the atmospheric deposition of nitrogen to tidal waters.
  • Implemented and Realized: Nitrogen and phosphorus reductions that have been estimated by the model and observed in monitoring data and are the result of actions documented in Watershed Implementation Plans (WIPs).
• Time Lag:
  • Implemented But Lagged: Nitrogen and phosphorus reductions that are expected due to actions that have already taken place, but that have not been realized due to natural lags between BMP implementation and nutrient delivery to streams.
• Response Gap:
  • RIM Expected But Not Seen: Nitrogen and phosphorus reductions that are estimated by the model to account for lag times but not observed in monitoring data from the Bay’ nine River Input Monitoring (RIM) stations.
• Implementation Gap:
  • Future Implementation: Expected nitrogen and phosphorus reductions that are documented as planned in Watershed Implementation Plans (WIPs) but not yet reported as implemented.
• Planning Gap:
  • Unaccounted Additional Loads (UALs): Increased loads from Watershed Model updates that occurred after Phase 3 planning targets were set in 2018. These UALs balance load increases from changes to the model.
  • Conowingo Infill Adjustment: Additional reductions needed to offset the filling of the Conowingo Reservoir, which has increased nitrogen and phosphorus delivery to the Bay.
  • Changing Environmental Conditions Adjustment: Additional reductions needed to offset the impact of changing environmental conditions, including the increased delivery of nitrogen and phosphorus to the Bay and the decreased ability of the Bay to absorb nutrients while maintaining water quality. 

Nitrogen

To achieve the TMDL planning target for nitrogen, the annual loading rate needs to be reduced by 145.42 million pounds from this indicator's 1995 baseline. This target includes a 4.65 million pound per year increase due to changing environmental conditions, a 6.01 million pound per year increase due to the Conowingo Reservoir and a 9.66 million pound per year increase categorized as an unaccounted additional loads. As of 2023:

• Implemented management actions are yielding estimated nitrogen load reductions of 81.83 million pounds per year. (These reductions have been implemented and observed in the monitoring data.)
• Implemented management actions are expected to yield additional nitrogen load reductions of 27.21 million pounds per year. (These reductions have not been observed due to natural lags between nutrient application and delivery to streams.)
• Planned management actions are expected to yield additional nitrogen load reductions of 16.07 million pounds per year.
• The atmospheric deposition of nitrogen to tidal waters has been reduced by 6.8 million pounds per year. (In order to meet the target for tidal deposition, this figure needs to be reduced by 7.92 million pounds per year.)

From 1995 to 2023, the reduction of nitrogen loading rates has trended toward meeting the Bay TMDL. Specifically, we have seen an increase in the nitrogen reductions that have been estimated by the model and observed in monitoring data, as well as reductions in the atmospheric deposition of nitrogen to tidal waters.

Phosphorus

To achieve the TMDL planning target for phosphorus, the annual loading rate needs to be reduced by 9.35 million pounds from this indicator's 1995 baseline. This target includes a 0.56 million pound per year increase due to changing environmental conditions, a 0.26 million pounds per year increase to account for the Conowingo Reservoir and a 0.066 million pound per year increase categorized as an unaccounted additional load. As of 2023: 

• Implemented management actions are yielding estimated phosphorus load reductions of 1.63 million pounds per year. (These reductions have been implemented and observed in the monitoring data.)
• Implemented management actions are expected to yield additional phosphorus load reductions of 4.05 million pounds per year. (These reductions have not been observed due to natural lags between nutrient application and delivery to streams.)  
• Planned management actions are expected to yield additional phosphorus load reductions of 0.05 million pounds per year.

In addition, the model has estimated additional phosphorus load reductions of 2.73 million pounds per year. Because these reductions have not been observed in the monitoring data, they represent an unknown response gap and indicate unknown sources and/or processes may need to be examined and incorporated into future model refinements.

Like nitrogen, the reduction of phosphorus loading rates since 1995 has trended toward meeting the Bay TMDL. Specifically, we have seen an increase in the phosphorus reductions that have been estimated by the model and observed in monitoring data.