Altered flow affects the biological health of streams in the Chesapeake Bay watershed

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Issue: The natural cycle of water flow, known as the flow regime, is one of the primary habitat conditions needed for healthy biological communities in streams. However, human activities have drastically altered the natural flow regime of most of the world’s rivers and streams, including those in the Chesapeake watershed, which has resulted in changes not only to the natural habitat but also associated organisms. The Chesapeake Bay Program (CBP) has an outcome to restore stream health throughout the watershed. Information is needed on the factors degrading the health of streams, including altered flows, to help identify potential places and management actions to improve their condition.

USGS Study: The USGS, in partnership with the Interstate Commission on the Potomac River Basin (ICPRB), examined the role of altered flows on biological condition of small streams (< 200 km2 upstream drainage area) in the Chesapeake Bay watershed.  The investigators determined there was a lack of paired observed hydrologic and biological condition data available to conduct any analysis, a common issue especially for small streams. To overcome the lack of observed data, the investigators used an approach to pair modeled estimates of hydrologic alteration with the biological condition of streams. The steps included:

  • Predicting flow status (inflated, diminished, or indeterminant) for 12 published hydrologic metrics (HMs) that characterize the main components of flow regimes for all small reaches in the watershed.
  • Estimating flow alteration intensity for each stream reach as the number of HMs with inflated or diminished status, which ranged from 0 (no HM inflated or diminished) to 12 (all 12 HMs inflated or diminished).
  • Stream condition was based on the Chesapeake Bay Basin-wide Index of Biotic Integrity (Chessie BIBI), which was developed by the ICPRB. The Chessie BIBI uses benthic macroinvertebrate data, which are collected by multiple CBP partners, as the estimate of stream condition. 

To do the analysis, the investigators paired the modeled estimates of flow alteration intensity with observed Chessie BIBI data for 4,522 stream reaches, providing a sufficiently large data set to examine the altered-flow and stream condition relationship.

Major findings:

The range of altered flows across the Chesapeake watershed are shown on figure 1. Some of the major findings about the relation of degraded stream condition with altered flows include:

  • When focused solely on the altered-flow and stream condition relationship, degraded stream conditions were, depending on the number of HMs used to estimate altered-flows, 3.8—4.7 times more likely in stream reaches where flow was predicted to be altered, than in streams with unaltered flows. 
  • When focused on the subset of sites within urban watersheds the likelihood of degraded stream condition was almost two-fold higher (8.7—10.8). However, flow alternation impacts on steam conditions was not found to be significant when focused on the subset of sites within agricultural watersheds.
  • When the investigators examined addition stressor variables, such as reservoir storage, urban development, and freshwater withdrawal, the probability of degraded stream conditions still increased 3.7 percent with every unit increase in flow alternation intensity. A unit increase in flow intensity is defined as how many components of the flow regime were predicted to be altered by human activities, a higher score signifies more components of the flow regime have been altered.
Map of Chesapeake Bay watershed showing the flow altered intensity score for all small streams

Figure 1. Map of Chesapeake Bay watershed showing the flow altered intensity score for all small streams (< 200 km2 in upstream drainage) and a focus area with stream condition overlain. (Maloney and others, 2021)

Management Applications:

The flow-alteration intensity index that was developed by the study, when coupled with a measure of stream condition, can identify reaches where altered flow is, and is not, a potential major stressor driving stream degradation.  Some examples of potential applications:

  • In areas where flow alteration is identified as a stressor of interest (higher flow alteration scores on figure 1), managers could consider actions targeted to mitigate altered flows. These management actions also may be more fruitful in streams with a lower flow alteration intensity index score (lighter colors in figure 1) because fewer aspects of the hydrograph requiring remediating.
  • In reaches where degraded conditions exist but are not due to altered flow (areas with zero flow alterations scores and degraded biological conditions on figure 1) other stressors are likely driving degraded conditions.  This could facilitate managers in not only identifying potential restoration sites but could also assist in identifying the most effective restoration practices (i.e., if no flow alteration is evident do not prescribe a management practice aimed at this stressor). 

For more information: The results of the study have been published in Environmental Management (with open access) at Linking Altered Flow Regimes to Biological Condition: an Example Using Benthic Macroinvertebrates in Small Streams of the Chesapeake Bay Watershed

Accompanying data release: Modeled estimates of altered hydrologic metrics for all NHDPlus v21 reaches in the Chesapeake Bay watershed.

The project team included Kelly Maloney, Daren Carlisle, Jennifer Rapp, Samuel Austin, Matthew Cashman and John Young of the USGS and Claire Buchanan of the ICPRB. Kelly Maloney ( is the primary contact for this project.

Posted on March 24, 2021


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