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Browse History: Water Quality Index (2011)
Indicator Icon Water Quality Index

The Water Quality Index combines the water quality threshold scores (chlorophyll a, water clarity, and dissolved oxygen) into a single overarching index. Good overall water quality is characterized by having high index scores (green). Poor overall water quality is characterized by having low index scores (orange and red).

Chesapeake Bay - Indicator Details:

  Indicator Selection Diagram

Index Map

This map shows the Water Quality Index scores for 2011. Poor overall water quality is indicated by low index scores, and is shown by shades of orange and red on the map. Most regions of the Bay had a Water Quality Index score in the poor range (score = 20-40%); this is a slight decrease compared with spatial patterns in recent years. The highest index scores occurred in the James River and Upper Bay regions. Areas of the James and Potomac Rivers also had moderate-good water quality. High index scores in these regions are largely attributed to the dissolved oxygen and chlorophyll a levels. The lowest index scores occurred in the Lower Western Shore (MD), and Patapsco and Back Rivers.

Creating this map
This map shows the Water Quality Index values for the current year. At each of the 144 monitoring stations, the percent (%) frequency that each water quality parameter meets the threshold levels was calculated to provide the Water Quality Index score. Water quality index scores between stations was then estimated by interpolation to visualize spatial patterns and trends.


Comparison of Bay Health Index scores for 2011 () compared to

Score Legend
 Score (%)
 0               20               40               60               80              100
James River  
Upper Bay  
Lower Eastern Shore (Tangier)  
Upper Western Shore  
Rappahannock River  
York River  
Overall Bay  
Potomac River  
Lower Bay  
Upper Eastern Shore  
Mid Bay  
Choptank River  
Elizabeth River  
Patuxent River  
Lower Western Shore (MD)  
Patapsco and Back Rivers  

This figure ranks each region from best to worst Water Quality Index (WQI) score for 2011. The highest ranked region in 2011 was the James River with a score of 49%, followed by the Upper Bay at 48%. The Patapsco and Back Rivers region was the lowest ranked region in 2011 with a very poor score of 15%, followed by the Lower Western Shore (MD) with a score of 17%. Ten of the 15 reporting regions decreased in WQI score from 2010, causing the 2011 Overall Bay WQI score to drop from 40% to 33%, the lowest score since 2003.

Excel Spreadsheet