Middle Potomac River Watershed Assessment: Potomac River Sustainable Flow and Water Resources Analysis April 2014 Middle Potomac River Watershed Assessment: Potomac River Sustainable Flow and Water Resources Analysis United States Army Corps of Engineers, Baltimore District The Natural Conservancy Interstate Commission on the Potomac River Basin Final Report April 2014 Recommended citation: U.S. Army Corps of Engineers, The Nature Conservancy, and Interstate Commission on the Potomac River Basin. 2014. Middle Potomac River Watershed Assessment: Potomac River Sustainable Flow and Water Resources Analysis. Final Report. 107 pp. and 11 appendices. Cover photograph: Great Falls on the Potomac River as seen from Virginia. Jim Palmer (ICPRB) Middle Potomac River Watershed Assessment Abstract The Middle Potomac River Watershed Assessment (MPRWA) was a collaborative effort to assess the relationship between streamflow alteration and ecological response in the Potomac River and its tributaries in a study area defined as the Middle Potomac. The assessment is comprised of five distinct components: (1) a large river environmental flow needs assessment, (2) a stream and small rivers environmental flow needs assessment, (3) a projection of future water uses, (4) a stakeholder engagement process, and (5) development of a concept or scope for a strategic comprehensive plan for watershed management. This information can be used to balance and mitigate water use conflicts and prevent ecological degradation. To assess flow needs in the Potomac watershed, the Ecologically Sustainable Water Management (ESWM) approach was used for large rivers and the Ecological Limits of Hydrologic Alteration (ELOHA) framework was adapted for streams and small rivers. In the large rivers included in this study, based on currently available information, there has been no discernible adverse ecological impact on focal species due to human modification of flows. As a precautionary measure, the team recommended that the current large river flow regime be maintained for the entire range of flows as defined by 20 flow statistics based on a 21-year period of record (1984-2005). In streams and small rivers, strong relationships were found between urbanization (impervious surface), hydrologic alteration, and ecological impacts. Land use was found to be a more significant cause of hydrologic alteration than water withdrawals and impoundments at the present time. Impervious surfaces move rainfall and snowmelt more rapidly into rivers and streams than surfaces with natural vegetation, causing increased flashiness (rate of change). Increased flashiness correlates with declines in the status of stream macroinvertebrate, the streamflow assessment’s indicator taxa. These taxa were not found to be sensitive to changes in low magnitude flows. Analyses of other taxa more sensitive to low magnitude flows, such as fish and mussels, could provide a more complete picture of biological response to flow alteration for water use decisions. Basin-wide datasets of these groups are not collected at present. The impact on flow of future changes in water use was assessed through six scenarios: a) three different forecasts of per capita domestic water use; b) climate change; c) hot and dry summer conditions; and d) conversion of power plants to closed cycle operation. There was no regional pattern of flow alteration that applied to all scenarios and, within scenarios, impacts on flow varied for each subwatershed’s unique combination of land and water uses. Stakeholder education and engagement are key elements in the process of transferring scientific findings to management and policy. Workshops, webinars, and technical meetings were convened in order to obtain advice on methods, interpret draft results, and share findings with government agencies at all levels and other parties with significant roles in water management. This dialogue and information sharing contributes to state water management decision making processes and to the ongoing dialogue among stakeholders about ensuring the sustainable use of stream flows for all purposes. Water use and basin development in each of the Potomac watershed’s jurisdictions affects the other jurisdictions. The final component of this study was to scope a basin-wide comprehensive plan process. Because there is no single agency with regulatory authority for water planning across the Potomac watershed, the concept for a future comprehensive plan is based on a collaborative, stakeholder driven model. Abstract page i Middle Potomac River Watershed Assessment Table of Contents Abstract ............................................................................................................................................................... i List of Tables ..................................................................................................................................................... iv List of Figures .................................................................................................................................................... iv Executive Summary .................................................................................................................................... ES-1 Chapter 1. Introduction .................................................................................................................................... 1 Chapter 2. Existing Conditions ........................................................................................................................ 6 2.1. Physiography ............................................................................................................................................... 6 2.2. Climate ......................................................................................................................................................... 7 2.3. Hydrology .................................................................................................................................................... 7 2.4. Impoundments ............................................................................................................................................ 8 2.5. Water Withdrawals and Discharges ....................................................................................................... 10 2.6. Land Uses .................................................................................................................................................. 10 2.7 Ecological Community ............................................................................................................................ 11 Chapter 3. Large River Environmental Flow Needs .................................................................................. 14 3.1. Introduction .............................................................................................................................................. 14 3.2. Large River Flow Needs Assessment Approach ................................................................................. 15 3.3. Findings of the Large River Flow Needs Assessment ........................................................................ 18 3.4. Conclusions for Large River Flow Needs Assessment ....................................................................... 20 3.5. Large River Information Gaps and Monitoring Needs ...................................................................... 21 Chapter 4. Future Water Use Projections .................................................................................................... 23 4.1. Methods for Future Water Use Projections ......................................................................................... 23 4.2. Findings for Future Water Use Projections .......................................................................................... 25 4.3. Comparison to Previous Studies ............................................................................................................ 27 4.4 Summary ..................................................................................................................................................... 27 Chapter 5. Stream Environmental Flow Needs .......................................................................................... 29 5.1. Ecological Limits of Hydrologic Alteration (ELOHA) ...................................................................... 30 5.2. Findings for Stream Environmental Flow Needs: Interpretation and Use of Relationships ....... 33 5.2.1. Magnitude ........................................................................................................................................... 34 5.2.2. Duration ............................................................................................................................................. 34 5.2.3. Frequency ........................................................................................................................................... 37 5.2.4. Rate of Change .................................................................................................................................. 37 5.2.5. Possible Mechanisms Underlying FA-E Relationships ............................................................... 39 5.2.6. Other Environmental Stressors ...................................................................................................... 41 5.2.7. Using the FA-E Curves ...................................................................................................................