The Susquehanna River basin project demonstrates (1) the systematic organization of information sources to facilitate expert input on flow-ecology relationships and environmental flow recommendations, and (2) a novel expression of environmental flows for maintaining long-term hydrologic variability.

The 1972 Susquehanna River Basin Compact between New York, Pennsylvania, Maryland, and the Federal government established the interstate Susquehanna River Basin Commission (SRBC).

SRBC’s mission is to manage the basin’s water resources under comprehensive watershed management and planning principles, and it has authority to regulate water withdrawals within the three basin states. SRBC facilitated this science-based process to determine environmental flow needs.  Because the SRBC has interstate regulatory authority, the resulting recommendations are being used to revise water policy, inform basin planning, and improve water releases from reservoirs. 

The project was completed in 18 months.  Congress and the SRBC provided funding through the Water Resource Development Act (WRDA). 

Flow-ecology relationships and environmental flow recommendations emerged from a structured expert workshop process facilitated by The Nature Conservancy.  Agendas, materials, and outcomes of each step are archived here. A brief synopsis of the process follows:

Through consultations with experts, the technical team assembled a broad list of ecological indicators, including flow‐sensitive taxa groups, vegetation community types, and physical processes. Dependencies between these indicators and specific flow components were developed through a literature review.  Common habitat traits and microhabitat preferences were described for species groups. 

A basic 5-type habitat classification based on watershed size, temperature, and flow stability was developed for organizing and synthesizing information.
Workshop participants used this information to identify the most sensitive periods and life stages for each habitat type, and to formulate flow-ecology hypotheses. Following the first workshop, the technical team further compiled and synthesized diverse information, using the flow-ecology hypotheses to focus their research.

Ecosystem flow needs were then summarized graphically [Figure 1] by season in relation to high, seasonal, and low flows for each major habitat type. These graphs and supporting narratives describe the role of inter-annual as well as seasonal hydrologic variability in forming channels and floodplains; maintaining water quality; and supporting habitat.

Resulting flow recommendations [Report] are based on (a) existing literature and studies that described and/or quantified relationships between flow alteration and ecological response, (b) expert input, (c) the analysis of long-term flow variability at minimally-altered gages, and (d) results of water withdrawal scenarios that tested the sensitivity of various flow statistics.

Flow recommendations are expressed in terms of “traditional” metrics calculated by the Indicators of Hydrologic Alteration (IHA) program, as well as novel metrics based on seasonal flow duration curves to maintain long-term flow variability.
The team systematically documented the flow needs that each recommendation supports, and the literature and studies on which the recommendation is based. Structuring the flow recommendations in this way facilitated the review process and provides a framework for adding or refining flow needs, substituting flow statistics, revising flow recommendations, and documenting additional supporting information.

To further understand the sensitivity of each flow component and to help translate the flow recommendations into policy, the team analyzed a suite of future water withdrawal scenarios and compared alternative flow thresholds. In March 2012, SRBC released a draft Low Flow Protection Policy for public comment, based in part on the flow recommendations generated by this project.