The Chesapeake Bay, the largest estuary in the United States, is only as healthy as the water flowing into it. Its watershed covers more than 64,000 square miles — from Cooperstown, New York, to Lynchburg and Virginia Beach, Virginia, and from Pendleton County, West Virginia, to Seaford, Delaware, and Scranton, Pennsylvania — with 150 major rivers and streams totaling more than 100,000 miles, and more than 18.4 million people in 2020. In October 2021 the U.S. Geological Survey assembled a team to track the health of those streams through seven indicators.
Status and trend
For each indicator, the team reports status — the condition at one moment — and trend, a statistically meaningful change over an interval of five years or more. Results are to be published through frequently updated reports, maps and interactive web tools.
The seven indicators
| Indicator | Why it matters | How it is measured |
|---|---|---|
| Freshwater flow | changes the bay's temperature and salinity, the nutrients, sediment and contaminants it receives, and the habitat available in streams | continuous USGS streamflow data |
| Nitrogen, phosphorus and sediment | tied to the health of aquatic life | 123 USGS monitoring sites in 5 states, compiled since 1985, covering about 78% of the watershed; long-term trends need 30 years of data, short-term trends 10 |
| Water temperature | drives stream productivity and organisms' metabolism; shaped by weather, topography and flow, and by land use, thermal pollution, reservoirs and climate change | a multi-agency dataset of stream temperatures spanning the watershed's range of latitude and elevation |
| Hydromorphology | the interplay of flow, sediment, landform and vegetation that creates varied habitat | rapid habitat assessments, and USGS streamgage data on channel shape and hydraulics |
| Freshwater salinization | rising salt disrupts organisms' osmotic balance; sources include agricultural lime, road deicers, mining leachate, weathering of human-made surfaces and water softeners | specific conductance, from a multi-agency dataset |
| Toxic contaminants | more than three-quarters of the bay's tidal waters are partly or fully impaired by them; mercury and PCBs drive most fish-consumption advisories | uneven data — no bay-wide limits for individual toxics — make watershed trends infeasible, though amounts in fish tissue can be tracked in some reaches |
| Aquatic communities | two freshwater assemblages, bottom-dwelling invertebrates and fish, are key indicators of stream health | biological monitoring of both |
*Figure 5.Macroinvertebrates (left) and brook trout (right) are representative members of the
Chesapeake Bay watershed freshwater biological aquatic community. Photographs by Kelly
Maloney, U.S. Geological Survey (left); Jeffrey Cole, U.S. Geological Survey (right).*
*Figure 3.Maintaining naturally occurring dimension, pattern, profile, and flow regimes of stream
systems (left) helps to prevent severe bank-cutting, erosion, and sedimentation (right).
Photograph by Matt Cashman, U.S. Geological Survey.*
Suspended sediment in the Rappahannock River after heavy rain (left), and the forested, relatively pristine Rivanna River (right), both in Virginia. Credit: Chris Mason, USGS.
As population, land use and climate change across the watershed, these indicators are how scientists — and the public — can see what is happening to the bay's streams.
Sources
Based on the USGS fact sheet "Tracking status and trends in seven key indicators of stream health in the Chesapeake Bay watershed," a work of the United States government in the public domain. Pictures an earlier version of this page left out were restored on 2026-09-26.
Licence: CC0 1.0 (public domain) · Adapted from pubs.er.usgs.gov
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