Stream Health in the Chesapeake Bay Watershed, 2018 – 2023 Update Archives

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Stream Health in the Chesapeake Bay Watershed, 2018 – 2023 Update

December 19, 2025

Non-tidal stream health in the Chesapeake Bay watershed is evaluated every six years to accommodate rotational sampling schedules of the major stream monitoring programs. A 2024 Data Call brought in stream macroinvertebrate data and related water quality and habitat data collected by eighteen monitoring programs in the watershed between 2018 and 2023, the fourth 6-year interval. The percentage of stream miles with healthy macroinvertebrate communities increased an estimated 2,026 miles in this interval, for a 1.4% improvement over the previous interval. High flows in 2018 and 2019 and the COVID19 pandemic in 2020 disrupted monitoring activities, and stream evaluations in the interval are based on less data. Since 2000 – 2005, the first 6-year interval, stream miles with healthy macroinvertebrate communities have increased 10.3% overall, and an estimated two-thirds (66.7%) of stream miles in the watershed are now thought to be healthy. Improvement is unevenly distributed in the watershed. Stream health appears to be improving in eight more rural western and southern bioregions; it is not improving in two central, heavily populated bioregions, and is degrading in two eastern, highly agricultural bioregions. Additional data and new criteria for separating streams from rivers have slightly changed the percentages of healthy streams for previous intervals but not the overall findings. The average rate of improvement is 3.4% over a 6-year interval, which suggests the revised Stream Health goal can be achieved in 2024 – 2029, the next 6-year interval, as ongoing efforts to restore and protect the Bay watershed continue.

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A Larval Striped Bass Habitat Index for Chesapeake Bay Tributaries

January 19, 2022

A Larval Striped Bass Habitat Index (LSBHI) was developed to assess the suitability of annual environmental conditions in three Chesapeake Bay tributaries with the respect to recruitment of striped bass larvae. LSBHI values were calculated for the Potomac, Patuxent and Choptank Rivers for each year from 1986 through 2000, using Maryland water quality and zooplankton monitoring data from the Chesapeake Bay Program. The LSBHI is intended to provide an indicator of the quality and potential of each tributary’s striped bass spawning and nursery habitat for each year of available data. The LSBHI incorporates components of the Individual Based Model (IBM) of Rose and Cowan (1993) to score annual conditions with regard to initial spawning success, early survival of eggs and pre-feeding larvae, and feeding larval growth and production as a function of zooplankton availability. The annual LSBHI for a tributary is composed of three sub-indices, each calculated for regional semi-monthly cohorts: 1) a spawning sub-index, 2) an early survival sub-index, and 3) a zooplankton (food) availability sub-index. The spawning sub-index uses temperature and salinity data, as well as literature based relationships of these parameters to striped bass spawning to determine the suitability of the habitat for a successful spawn. The early survival sub-index uses temperature to assess the habitat with respect to egg and early larval survival. The zooplankton availability sub-index is based on a comparison of the available zooplankton (a function of zooplankton abundance and predator searching ability given relative sizes of predator versus prey and water clarity), the minimum food requirement based upon metabolic relationships, and the maximum consumption possible. To independently verify the LSBHI, annual values were quantitatively compared with annual juvenile abundance measures from beach seine surveys conducted by Maryland Department of Natural Resources. Application of the LSBHI was successful for the Potomac River (R=0.62, p<0.01), Patuxent River(R=0.55, p=0.03), but did not correlate well with the beach seine data for the Choptank (R=-0.14, p=0.61). Possible reasons for this lack of fit and recommendations for expanding the LSBHI to include additional factors (e.g. flow, pH) are discussed.

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Linking Altered Flow Regimes to Biological Condition: an Example Using Benthic Macroinvertebrates in Small Streams of the Chesapeake Bay Watershed

March 12, 2021

Regionally scaled assessments of hydrologic alteration for small streams and its effects on freshwater taxa are often inhibited by a low number of stream gages. To overcome this limitation, we paired modeled estimates of hydrologic alteration to a benthic macroinvertebrate index of biotic integrity data for 4522 stream reaches across the Chesapeake Bay watershed. Using separate random-forest models, we predicted flow status (inflated, diminished, or indeterminant) for 12 published hydrologic metrics (HMs) that characterize the main components of flow regimes. We used these models to predict each HM status for each stream reach in the watershed, and linked predictions to macroinvertebrate condition samples collected from streams with drainage areas less than 200 km2. Flow alteration was calculated as the number of HMs with inflated or diminished status and ranged from 0 (no HM inflated or diminished) to 12 (all 12 HMs inflated or diminished). When focused solely on the stream condition and flow-alteration relationship, degraded macroinvertebrate condition was, depending on the number of HMs used, 3.8–4.7 times more likely in a flow-altered site; this likelihood was over twofold higher in the urban-focused dataset (8.7–10.8), and was never significant in the agriculture-focused dataset. Logistic regression analysis using the entire dataset showed for every unit increase in flow-alteration intensity, the odds of a degraded condition increased 3.7%. Our results provide an indication of whether altered streamflow is a possible driver of degraded biological conditions, information that could help managers prioritize management actions and lead to more effective restoration efforts.

The report has been published in Environmental Management.

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The Influence of Jennings Randolph Lake and Dam Operations on River Flow and Water Quality in the North Branch Potomac River

December 20, 2019

A multi-year study began in 2018 to determine if an update of the Army Corps’ 1997 Water Control Plan for Jennings Randolph Lake is needed. Watershed and river conditions have improved significantly since the turn of the century, an outcome of regulatory enforcement, mine runoff mitigation, wastewater treatment,  infrastructure improvements, forest regrowth and the abatement of acid rain. The Commission, in partnership with the Corps, has produced a draft Scoping Study report that reviews the dam’s long-running operational objectives and procedures, and assesses the current importance of these procedures in achieving the four mandates. It also reviews various modeling approaches that incorporate modern science and technology for better future management. Learn more…

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Creating a stream health baseline for the Chesapeake basin from monitoring and model data

January 11, 2019

This report describes how monitoring and model data were analyzed and combined to generate a preliminary estimate of acceptable stream health in the Chesapeake Bay basin for the 2006 – 2011 baseline period. Streams in about 73% of the basin’s 64,020 sq. miles of drainage area were evaluated with monitoring results, and output from a predictive model was used to estimate stream health in the remaining 27%. Stream health was measured with the bioregion, family-level version of the “Chessie BIBI,” a multi-metric index for stream macroinvertebrate communities. Index scores are normally expressed as one of five index ratings: Excellent or Good (well-functioning), Fair (considered satisfactory), and Poor or Very Poor (stressed or poorly-functioning). Four versions of the predictive model were developed and tested, and the selected version outputs results as three-ratings: Excellent/Good, Fair, and Poor/Very Poor. The five ratings in the monitoring data were re-grouped to match the three ratings of the selected predictive model. The monitoring- and model-based ratings were then area-weighted to reduce bias caused by uneven sample densities and aggregated to the Chesapeake basin scale, with monitoring results given preference. The combined results suggest approximately 60% of the basin’s area had acceptable stream ratings (Excellent, Good, or Fair) during 2006 – 2011. This estimate is a preliminary baseline for the Chesapeake Bay Program’s stream health goal. A final baseline estimate will be produced after a higher resolution stream layer becomes available and acceptable stream health can be estimated as a percent of the basin’s stream miles.

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Biological Surveys of Three Potomac River Mainstem Reaches (2012-2014) with Considerations for Large River Sampling

December 5, 2018

The Interstate Commission on the Potomac River Basin (ICPRB) conducted a study to describe the biological composition of three under-represented reaches in the mainstem Potomac River Basin and determine the effort required to accurately assess large river sites for freshwater mussel and benthic macroinvertebrate populations. Located at Knoxville (MD), Carderock (MD), and Little Falls (MD), these reaches were selected because they are difficult to sample and represent gaps in spatial coverage of the mainstem in the otherwise comprehensive Maryland Department of Natural Resources (MD-DNR) Core Trend Monitoring Program. Data from the Knoxville reach will improve our understanding of the mixing zones below the confluence of the Shenandoah and Potomac rivers and the relative importance of each river at the Potomac water supply intakes downstream. The Carderock and Little Falls reaches are important in identifying stresses on the river’s biological communities that could relate to upstream consumptive losses and water supply withdrawals during severe droughts. The Little Falls reach is in the only stretch of the Potomac River with a minimum flow-by requirement.

Surveys of freshwater mussel and benthic macroinvertebrate populations were conducted during late-summer low-flow periods of 2012, 2013, and 2014. The three years of the study had moderate flows overall and did not experience extreme drought or floods, so managers and researchers should view the results as a characterization of biological communities unaffected by flow extremes. In addition to recording mainstem Potomac species distributions, biological collections underwent post-collection analyses that provided an informed baseline for the collection effort required to achieve sufficiently accurate data in the future.

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