“Chessie BIBI” Index for Streams

Interstate Commission on the Potomac River Basin

**🚧This website is currently being updated. Expect new information, maps, etc. in the coming weeks. Posted 1/13/2026. 🚧**

Chessie BIBI, 2018 – 2023

Chessie BIBI ratings, 2018-2023

An updated Chessie BIBI report and map was published in December 2025, Stream Health in the Chesapeake Bay Watershed, 2018 – 2023 Update.

Supporting Materials:

An estimated 66.7% of streams and small wadeable rivers in the Chesapeake watershed currently have healthy macroinvertebrate communities, with Chessie BIBI ratings of excellent, good, and fair. The percentage of healthy streams has improved 10.3% over the four 6-year intervals since 2000.

In the map to the right, a colored dot indicates the watershed has only one sampling location; the entire polygon is shaded if it has two or more locations.

**AN INTERACTIVE CHESSIE BIBI INDEX MAP EXPLORER FOR 2018-2023 COMING SOON**

Measuring Stream Health

A map showing stream health within the Chesapeake Bay.

Chessie BIBI status in Chesapeake Bay watershed (2000 – 2017)

The Chesapeake basin-wide index of biotic integrity for stream macroinvertebrates, or “Chessie BIBI,” is a multi-metric index of biological health for freshwater streams and small, wadeable rivers in the Chesapeake Bay watershed. Stream macroinvertebrates are indicators of ecosystem health because their communities reflect integrated responses to multiple environmental stressors, some of which may be unknown or not monitored. The index is composed of five or more family-level macroinvertebrate metrics (indicators) that discriminate strongly between high quality and degraded stream conditions in each of the watershed’s twelve bioregions.

State, federal, and local agencies, as well as citizen groups throughout the Chesapeake watershed collect and count macroinvertebrate samples. The Chesapeake Bay Program Data Center issues a data call roughly every six years to acquire these data. The six-year cycle accommodates the rotational sampling schedules of the state monitoring programs, which are major data providers. The Data Center quality-assures the submitted macroinvertebrate taxonomic counts, assembles them in a common database structure, and loads them into the Chesapeake Environmental Data Repository (CEDR). The Data Center also obtains the water quality and physical habitat data associated with the samples. The next data call will be in 2030 for the 2024-2029 interval.

The Interstate Commission on the Potomac River Basin (ICPRB) uses the CEDR database to calculate the Chessie BIBI index scores and ratings. Sampling sites are grouped by their 12-digit hydrologic units (HUC12) and each site’s narrative ratings (excellent, good, fair, poor, or very poor) is weighted by equal proportions of the total stream miles (1:24,000) in the HUC12. This minimizes bias caused by higher numbers of sampling sites in some areas. The weighted ratings are then summed across all HUC12s to the Chesapeake watershed scale to calculate the percentage of healthy streams.

A methodology document (Young et al. 2025) describes the data processing and analysis steps in greater detail. The four 6-year intervals analyzed to-date are: 2000-2005, 2006-2011, 2012-2017, and 2018-2023.

Developing the Index

The Chessie BIBI index was inspired by the Potomac BIBI and conceptualized in a 2008 pilot study for the Chesapeake Bay watershed. With Chesapeake Bay Program (CBP) support, the index was created in 2011 (Buchanan et al. 2011) and substantially refined in 2017 (Smith et al. 2017). Crucial to the index’s development was the involvement of several technical advisory groups comprised of regional experts and resource managers. The index has been applied to new data acquired in two data calls since 2017 and the results published in progress reports (2023, 2025).

Bioregions in the Chesapeake Bay Watershed

To develop the index, a suite of family-level metrics was calculated from each sample’s macroinvertebrate taxonomic counts. Sampling sites were grouped into one of five disturbance categories based on abiotic habitat and water quality conditions at the sites. Disturbance categories ranged from Reference (best quality) to Severely Degraded (poorest quality). Metrics for family-level community attributes (composition, richness/diversity, pollution tolerance, habit, feeding group) that most strongly discriminated between Reference and Severely Degraded  were systematically selected for the index  in each of twelve bioregions. Tailoring the index to each bioregion accounts for natural geographic differences in macroinvertebrate communities.

Values of the selected macroinvertebrate metrics at Reference sites are used to score those at other sites and generate an index score. Classification efficiency of the index was between 70.4% and 90.0%. (Indices built from genus-level metrics performed marginally better than family-level indices in some but not all bioregions.)

A common scale of five narrative ratings was applied to the Chessie BIBI index scores. The 50th, 25th, and 10th percentiles of the index scores in Reference conditions in each bioregion were used to define excellent, good, fair, and poor macroinvertebrate status. A fifth rating, very poor, was defined by half the value of the 10th percentile. The index ratings are directly comparable across bioregions and can be used to estimate percentages of stream in healthy condition in the Chesapeake watershed.

Click here to explore the interactive map of the Bioregions in the Chesapeake Bay >>>

Databases and Analysis Tools

ICPRB has produced a package containing R-scripts, shape files, spreadsheets, and documentation for calculating the Chessie BIBI index. Custom R-scripts are run through the open source Rstudio program. The raw data, calculated metrics, and BIBI scores and ratings are publicly available.

Database Versions: As demonstrated in the Chessie BIBI package, repeated runs of the program can produce slightly different index scores and ratings. This is due to the random selection of rare individuals in each program run as a sample is reduced to 100 individuals (rarefaction) in the data normalization step. Minor modifications made in 2019 to the habit assignments of a few macroinvertebrate taxa also changed the index scores and ratings at several stations. The scores and ratings used in the 2017 refinement version are called Version 2.0. Those calculated with the 2019 program from the same raw data are called Version 2.01. The database constructed with data acquired in 2020/2021 is Version 3.01. The latest database and metrics are Version 4.0.

Maps

These maps feature the Chesapeake and Potomac watersheds. Each unit on the map is a twelve-digit hydrologic unit (HUC12) and the overlapping bioregion. If a HUC12-bioregion unit is colored in, the unit has two or more sampling locations, and the color reflects the Chessie BIBI rating of the average score. A colored dot indicates only one sampling location occurred in the unit. Unsampled units are left blank (white). The “excellent,” “good,” and “fair” ratings are considered healthy communities because the ratings encompass 90% of index scores in the bioregional Reference populations. Populations with Chessie BIBI ratings of “poor” and “very poor” are considered degraded.

Maps with higher color contrast ratios for easier visibility are available below [MORE MAPS COMING SOON].

Chesapeake Watershed Maps

  • Pre-Baseline map (2000-2005) for Chesapeake Bay watershed stream health.
    Chessie BIBI Stream Health Index (2000-2005)

Stream health in the Chesapeake Bay watershed.

Potomac Watershed Maps

[MORE MAPS COMING SOON]

  • Pre-Baseline map (2000-2005) for Potomac watershed stream health.
    Chessie BIBI For the Potomac Watershed (2000-2005)

Stream health in the Potomac River watershed for the pre-baseline, baseline, and first interval periods.

High Contrast Maps

High Contrast Chesapeake Watershed Maps

High Potomac Watershed Maps

A Versatile Management Tool

Multiple studies have demonstrated the Chessie BIBI index’s sensitivity to stream nutrient concentration, flow alteration (Middle Potomac; Chesapeake Bay watershed), landscape features, urban stormwater management, and climate change. See the Reports and Publications list below for links to more of these studies.

ICPRB uses the Chessie BIBI data and maps to evaluate stream health as part of tracking overall environmental progress in the Potomac River basin.

The Chesapeake Bay Program (CBP) uses the Chessie BIBI index to measure stream health. The partnership established an initial stream health goal in response to Executive Order 13508 in 2009. The goal called for 70% of sampled streams throughout the Chesapeake watershed to be in fair, good, or excellent condition as measured by an Index of Biotic Integrity by 2025.

CBP Stream Health goal was changed in the 2014 Chesapeake Bay Watershed Agreement, which called for improving stream health and function in ten percent of stream miles above a “2008 baseline” for the Chesapeake basin. Biological communities are considered the definitive measure of stream health, and CBP selected the Chessie BIBI as its initial indicator of stream health. At a 2018 workshop, scientists and managers selected the 6-year period between 2006 and 2011 as the most practical “2008 baseline” because it overlaps all the state rotational sampling designs.

In 2025, the Agreement was revised and the goal changed to a 3% (4,340 miles) improvement every six years. Calculations for the 2025 Chessie BIBI update indicate at the current rate of improvement, it is conceivable that all three goals will be achieved by 2024 – 2029, the next 6-year interval.

Timeline

The graphic below illustrates the Chessie BIBI timeline, from the proof-of-concept analysis in 2008 to full-fledged indicator available online in 2026.

Timeline of Chessie BIBI Development.

Reports and Publications

2017 Chessie BIBI

  • Buchanan, C., R. D. Jepsen, and E. Young. 2025. Stream Health in the Chesapeake Bay Watershed, 2018 – 2023 Update. Report Prepared for the Chesapeake Bay Program Stream Health Workgroup. Available online at: www.potomacriver.org.
  • Buchanan, C., R. D. Jepsen, and M. E. Mallonee. 2023. Stream Biological Health in the Chesapeake Bay Watershed. ICPRB Report ICP23-1. Report Prepared for the Chesapeake Bay Program Stream Health Workgroup. Available online at: www.potomacriver.org.
  • Smith, Z., C. Buchanan, and A. Nagel. 2017. Refinement of the Basin-Wide Index of Biotic Integrity for Non-Tidal Streams and Wadeable Rivers in the Chesapeake Bay Watershed. ICPRB Report 17-2a (report); ICPRB Report 17-2b (appendices).
  • Buchanan, C., K. Maloney, Z. Smith, A. Nagel, and J. Young. 2018. Creating a stream health baseline for the Chesapeake basin from monitoring and model data. ICPRB report 19-7.
  • Maloney, K. O., Z. M. Smith, C. Buchanan, A. Nagel and J. A. Young. 2019. Predicting biological conditions for small headwater streams in the Chesapeake Bay watershed. https://doi.org/10.1086/700701.
  • Maloney, K. O., K. P. Krause, C. Buchanan, L. E. Hay, G. J. McCabe, Z. M. Smith, T. L. Sohl, and J. A. Young. 2020. Disentangling the potential effects of land‐use and climate change on stream conditions. https://doi.org/10.1111/gcb.14961.
  • Maloney, K. O., D. M. Carlisle, C. Buchanan, J. L. Rapp, S. H. Austin, M. J. Cashman and J. A. Young. 2021. Linking Altered Flow Regimes to Biological Condition: an Example Using Benthic Macroinvertebrates in Small Streams of the Chesapeake Bay Watershed. https://doi.org/10.1007/s00267-021-01450-5.
  • Maloney, K. O., C. Buchanan, R. D. Jepsen, K. P. Krause, M. J. Cashman, B. P. Gressler, J. A. Young, and M. Schmid. 2022. Explainable machine learning improves interpretability in the predictive modeling of biological stream conditions in the Chesapeake Bay Watershed, USA. J. Environmental Management. 322. https://doi.org/10.1016/j.jenvman.2022.116068.
  • Jepsen, R., C. Buchanan, A. Nagel, D. Caraco, and L. Fraley-McNeal. 2022. An Analysis of Pooled Monitoring Data in Maryland to Evaluate the Effects of Restoration on Stream Quality in Urbanized Watersheds. Final report to Maryland Department of the Environment. ICPRB Report 22-1.

2011 Chessie BIBI

  • Buchanan, C., K. Foreman, J. Johnson, and A. Griggs. 2011. Development of a Basin-wide Benthic Index of Biotic Integrity for Non-tidal Streams and Wadeable Rivers in the Chesapeake Bay Watershed: Final Report to the Chesapeake Bay Program Non-Tidal Water Quality Workgroup. ICPRB Report 11-1.
  • Mandel, R., C. Buchanan, A. N. Griggs, A. Nagel, and O. Devereux. 2011. Data Analysis to Support Development of Nutrient Criteria for Maryland Free-Flowing Waters. ICPRB Report 11-2.
  • U.S. Army Corps of Engineers, The Nature Conservancy, and Interstate Commission on the Potomac River Basin. 2013. Middle Potomac River Watershed Assessment: Potomac River Sustainable Flow and Water Resources Analysis. ICPRB Report PRC 13-8.
  • Buchanan, C., H. Moltz, C. Haywood, J. B. Palmer, and A. N. Griggs. 2013. A test of the Ecological Limits of Hydrologic Alteration (ELOHA) method for determining environmental flows in the Potomac River basin, U.S.A. https://doi.org/10.1111/fwb.12240.

2007 Potomac Index

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