Using Ecological Connectivity as a Basis for the Watershed Integrity of Western US Waters; Mark T. Murphy

Using Ecological Connectivity as a Basis for the Watershed Integrity of Western US Waters

Mark T. Murphy ¹

¹ NV5, Inc., Tucson, Arizona, mark.murphy@nv5.com

With the recently (11/21) proposed draft rule defining Waters of the US (WOTUS) offered by the US Environmental Protection Agency and the US Army Corps of Engineers (the Agencies), the “new” test of Clean Water Act (CWA) applicability has returned to the “old,” science-based significant nexus analysis (SNA) that was used prior to 2015. No doubt, the Agencies will be offering guidance documents on how to complete SNAs; however, in 2015, the Agencies published an extensive review[1] (the Connectivity Report) on how streams under the jurisdiction of the CWA ecologically depend upon their watershed, including those streams that only flow during rain events (aka, ephemeral waters) and those that only flow because of treated effluent, agricultural return flows or other discharged water (effluent-dependent waters, EDWs). Given the exhaustive amount of research described in the Connectivity Report, and the many research projects over the last six years adding to the report’s conclusions, the Connectivity Report will almost certainly be the core of this future guidance.

Over the last two years, NV5 has assisted clients in using the Connectivity Report to evaluate the health of two southern Arizona streams, an ephemeral stream (San Pedro River near Benson, Arizona) and an EDW (Santa Cruz River flowing through urban Tucson).  Specifically, we employed the flow pathway approach of the Connectivity Report to analyze the hydrological, hydrochemical, and hydrobiological, surface-water-mediated ecological connections, as individual and interactive transport mechanisms. We also examined “regional waters similarly situated,” as the SNA requires, focusing on ephemeral tributaries downstream of and within the studied reaches. Connections were defined by cause-and-effect couples that produced a potentially measurable individual or cumulative impact on the ecology of the studied reaches.

The exercise worked well, although the analysis benefitted from the subject of much peer-reviewed research and a long USGS stream-gage data archive. Applications in other watersheds may be limited where data is scarce. Nevertheless, the science and logic of ecological connectivity clearly seems to be the proper way to frame CWA applicability.