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Chesapeake Bay - Flow Adjusted: 2013


The Bay Health Index is strongly related to river flow

The flow-adjusted Bay Health Index represents the Bay Health Index score after removing the effect of flow. We found that about 40-50% of the annual variability in the Bay Health Index was associated with river flow entering the Bay (see Methods tab). We used this relationship to develop a “flow-adjusted” Bay Health Index, which removes the variability in the scores that is associated with changes in river flow. 

Flow adjusted scores for the Bay Health Index

Why adjust for flow?

In previous years, it was noted that The Bay Health Index seemed to go up in relatively dry years and down in wetter years. This makes sense, because higher river flow probably means that more nutrients and sediments are also entering the bay, and that should lead to lower health scores. 

Chesapeake Bay Streamflow by Year (cfs)

This effect causes the index to be variable from year to year, and it is difficult to say that the health of the bay is improving in a given year if the score goes up, or declining if the score goes down – the annual changes in scores could just be because there was more or less river flow than in previous years. To get a clear understanding of the trend in Bay health status, it is necessary to “adjust” the Bay Health Index to account for these flow changes. 

An example of the effect of flow (and of removing this effect) can be seen looking at the dry year of 2002 in the top figure. 2002 had the highest Bay Health Index on record, but also was a very “low-flow” year. Removing the effect of flow that year reveals that the score would have been a bit lower after accounting for the effect of the low flow year. Similarly in the wet year of 2003, the Bay Health Index dropped dramatically. But, taking out the flow signal meant the Bay Health Index did not drop as much.

Overall, flow adjustment reduces the year-to-year variability in the Bay Health Index, revealing that the health is neither improving nor declining (see Overview for more information on trajectories).


Flow-adjustment analysis provides a Bay Health Index that is adjusted to remove the effect of flow on the annual variability in scores. Total nitrogen, water clarity, chlorophyll a, and dissolved oxygen scores at the Bay wide scale are also related to flow, and flow adjustment was accomplished for these indicators as well. Bay-wide scores for total phosphorus, aquatic grasses, and benthic index of biotic integrity were not related to flow and flow adjusted is not necessary.    

Flow adjusted scores for the Bay Health Index

Total Nitrogen flow adjusted scores

Water Clarity Flow Adjusted Scores

Chlorophyll a flow adjusted scores

Dissolved Oxygen flow adjusted scores

Flow adjusted scores Download (Excel)


The Bay Health Index is strongly correlated with annual river flow (water-year flow; October – September) entering the Chesapeake Bay. Years with higher river flow tend to have lower scores, and years with lower river flow tend to have higher scores. While this relationship in itself does not imply that high flow causes the low scores, The relationship does make sense; higher flow years also have higher delivery of nutrients and sediments, which should lead to lower bay health scores. 

Bay Health Index and flow

Based on this correlation and using methods adapted used by USGS for similar problems, we developed a way to remove the effect of flow from the scores.

Flow-adjusted scores are calculated using the estimated annual streamflow into the whole of Chesapeake Bay as calculated by the USGS for the period from October of the previous year through September of the current year.* We used simple linear regression analysis to quantify the relationship between the BHI scores and river flow. The expected score is represented in this analysis as a straight line and can be calculated for a year based on river flow.

By subtracting the expected score for a given year from the actual score, we have the variability that is left over after accounting for the effect of river flow that year. This left-over is called the “residual.”


Bay health index and flow relationship

By adding the residual values to the overall average value, we can calculate the “flow adjusted” score. We calculated a flow-adjusted score for each of the indicators that exhibit a strong association with river flow (dissolved oxygen, total nitrogen, chlorophyll a, water clarity, and the overall BHI).

The result is an estimate of the Baywide score that would be observed if there were no annual variations in streamflow.

Flow adjusted scores for the Bay Health Index

* This period is known as the “water year” and was used because fall river flow and nutrient and sediment delivery could be expected to influence spring and summer scores the following year, and original indicator scores were evaluated using data from April through September. Some studies investigating the effect of nutrient loads on dissolved oxygen have focused on river flow and nutrient loading from January – June. We used this period also, but found that the relationship between flow in this period and BHI scores was not as strong as water year flow.

Further Information

Hirsch, Robert M., Alexander, Richard B., & Smith, Richard A. (1982). Selection of methods for the detection and estimation of trends in water quality. Water Resources Research 27(5), 803-813.

Murphy, Rebecca R., Kemp, W. Michael, & Ball, William. 2011. Long-Term Trends in Chesapeake Bay Seasonal Hypoxia, Stratification, and Nutrient Loading. Estuaries and Coasts 34(6), 1293 - 1309.