EPA and the Corps of Engineers have announced the release of the final peer review of EPA’s “connectivity” report — literally, Connectivity of Streams and Wetlands to Downstream Waters: A Review and Synthesis of the Scientific Evidence.
Here is the final peer review, posted on Endangered Species & Wetlands Report‘s website.
Here’s part of the letter from SAB Chair David T. Allen and Amanda D. Rodewald, chair of the panel that reviewed the report. (The full letter is at the beginning of the “final peer review” link in the preceding paragraph.)
The EPA Report is a thorough and technically accurate review of the literature on the connectivity of streams and wetlands to downstream waters. The SAB agrees with two of the three major conclusions in the Report. The SAB finds that the review of the scientific literature strongly supports the conclusions that streams and “bidirectional” floodplain wetlands are physically, chemically, and/or biologically connected to downstream navigable waters; however, these connections should be considered in terms of a connectivity gradient. The SAB recommends revisions to improve the clarity of the Report, better reflect the scientific evidence, expand the discussion of approaches to quantifying connectivity, and make the document more useful to decision-makers. The SAB disagrees with the conclusion that there is insufficient information available to generalize about the connectivity of wetlands in “unidirectional,” non-floodplain settings. In that case, the SAB finds that the scientific literature supports a more definitive statement that reflects how numerous functions of non-floodplain wetlands sustain the physical, chemical, and/or biological integrity of downstream waters, although the degree of connectivity can vary widely. The SAB’s major comments and recommendations are provided below.
- The Report often refers to connectivity as though it is a binary property (connected versus not connected) rather than as a gradient. In order to make the Report more technically accurate, the SAB recommends that the interpretation of connectivity be revised to reflect a gradient approach that recognizes variation in the frequency, duration, magnitude, predictability, and consequences of those connections. The SAB notes that relatively low levels of connectivity can be meaningful in terms of impacts on the chemical, physical, and biological integrity of downstream waters.
- The SAB recommends that the EPA consider expanding the brief overview of approaches to measuring connectivity. This expansion would be most useful if it provided examples of the dimensions of connectivity that could most appropriately be quantified, ways to construct connectivity metrics, and the methodological and technical advances that are most needed.
- The Report presents a conceptual framework that describes the hydrologic elements of a watershed and the types of connections that link them. The literature review supporting the framework is technically accurate and clearly presented. However, to strengthen and improve its usefulness, the SAB recommends that the framework be expressed as spatially continuous physical, hydrological (surface and subsurface), chemical, and biological flowpaths that connect watersheds. Layers of complexity should be included in the conceptual framework to represent important aspects of connectivity such as spatial and temporal scale. The water body classification system used in the Report (i.e., classification of waters according to landscape settings) should be integrated into the flowpath framework to show that continuous phenomena interact across landscape settings. In addition, the SAB recommends that each section of the Report be clearly linked to the conceptual framework.
- The SAB recommends that the Report more explicitly address the scientific literature on cumulative and aggregate effects of streams, groundwater systems, and wetlands on downstream waters. In particular, the Report should contain a discussion of the spatial and temporal scales at which streams, groundwater systems, and wetlands are functionally aggregated. The SAB also recommends that, throughout the Report, the EPA further discuss several important issues including the role of biological connectivity, biogeochemical transformation processes, and the effects of human alteration of connectivity.
- In the Report, the EPA has classified waters and wetlands as having the potential for either “bidirectional” or “unidirectional” hydrologic flows with rivers and lakes. The SAB finds that these terms do not adequately describe the four-dimensional (longitudinal, lateral, vertical, and temporal) nature of connectivity, and the SAB recommends that the Report use more commonly understood terms that are grounded in the peer-reviewed literature.
- The SAB commends the EPA for the comprehensive literature review in the Report, although additional citations have been suggested to strengthen it. To make the review process more transparent, the EPA should more clearly describe the approach used to screen, compile, and synthesize the information. The Report should also clearly indicate that the definitions used for rivers, streams, and wetlands are scientific, rather than legal or regulatory definitions, and may differ from those used in the Clean Water Act and associated regulations.
- The SAB finds that the review and synthesis of the literature describing connectivity of streams to downstream waters reflects the pertinent literature and is well grounded in current science. The literature review provides strong scientific support for the conclusion that ephemeral, intermittent, and perennial streams exert a strong influence on the character and functioning of downstream waters and that tributary streams are connected to downstream waters. However, the EPA should recognize that there is a gradient of connectivity. The SAB also recommends that the literature review more thoroughly address hydrologic exchange flows between main channels and off-channel areas, the influence of stream connectivity on downstream water temperature, and the movement of organisms throughout stream systems to use critical habitats.
- The SAB finds that the review and synthesis of the literature on the connectivity of waters and wetlands in floodplain settings is somewhat limited in scope (i.e., focused largely on headwater riparian wetlands) and should be expanded. However, the literature review does substantiate the conclusion that floodplains and waters and wetlands in floodplain settings support the physical, chemical, and biological integrity of downstream waters. The SAB recommends that the Report be reorganized to clarify the functional role of floodplain systems in maintaining the ecological integrity of streams and rivers and that the Report more fully reflect the literature on lateral exchange between floodplains and rivers.
- The SAB finds that, in general, the review and synthesis of the literature on the connectivity of non-floodplain (“unidirectional”) waters and wetlands is technically accurate. However, additional information on biological connections should be included. The SAB has provided numerous additional literature citations addressing the roles of multiple biological taxa in this regard, such as transporting propagules and nutrients and providing critical habitat.
- The SAB disagrees with the EPA’s conclusion that the literature reviewed did not provide sufficient information to evaluate or generalize about the degree of connectivity (absolute or relative) or the downstream effects of wetlands in “unidirectional” non-floodplain landscape settings. The SAB finds that the scientific literature supports a more definitive statement about the functions of “unidirectional” non-floodplain wetlands that sustain the physical, chemical and/or biological integrity of downstream waters. In this regard, the SAB recommends that the EPA revise the conclusion to better articulate: (1) what is supported by the scientific literature and (2) the issues that still need to be resolved.