1. The flow regime is a primary determinant of the structure and function of aquatic and riparian ecosystems for streams and rivers. Hydrologic alteration has impaired riverine ecosystems on a global scale, and the pace and intensity of human development greatly exceeds the ability of scientists to assess the effects on a river-by-river basis. Current scientific understanding of hydrologic controls on riverine ecosystems and experience gained from individual river studies support development of environmental flow standards at the regional scale. 2. This paper presents a consensus view from a group of international scientists on a new framework for assessing environmental flow needs for many streams and rivers simultaneously to foster development and implementation of environmental flow standards at the regional scale. This framework, the ecological limits of hydrologic alteration (ELOHA), is a synthesis of a number of existing hydrologic techniques and environmental flow methods that are currently being used to various degrees and that can support comprehensive regional flow management. The flexible approach allows

The vast majority of the world's rivers are now being tapped for their water supplies, yet only a tiny fraction of these rivers are protected by any sort of environmental flow standard. While important advances have been made in reducing the cost and time required to determine the environmental flow needs of both individual rivers and types of rivers in specific geographies, it is highly unlikely that such approaches will be applied to all, or even most, rivers within the forseeable future. As a result, the vast majority of the planet's rivers remain vulnerable to exploitation without limits. Clearly, there is great need for adoption of a "presumptive standard" that can fill this gap. In this paper we present such a presumptive standard, based on the Sustainability Boundary Approach of Richter (2009) which involves restricting hydrologic alterations to within a percentage-based range around natural or historic flow variability. We also discuss water management implications in applying our standard. Our presumptive standard is intended for application only where detailed scientific assessments of environmental flow needs cannot be undertaken in the near term.

[1] An extensive literature documents relations between reservoir storage capacity and water supply yield and the properties of instream flow needed to support downstream aquatic ecosystems. However, the literature that evaluates the impact of reservoir operating rules on instream flow properties is limited to a few site-specific studies, and as a result, few general conclusions can be drawn to date. This study adapts the existing generalized water evaluation and planning model (WEAP) to enable general explorations of relations between reservoir storage, instream flow, and water supply yield for a wide class of reservoirs and operating rules. Generalized relationships among these variables document the types of instream flow policies that when combined with drought management strategies, are likely to provide compromise solutions to the ecological and human negotiations for water for different sized reservoir systems. The concept of a seasonal ecodeficit/ecosurplus is introduced for evaluating the impact of reservoir regulation on ecological flow regimes.

ABSTRACT. Although hydropower is a source of low-carbon energy, without careful consideration and management, dams have the potential to degrade river ecosystems and the goods and services they provide to society. Today, a broad range of hydropower interests and stakeholders are seeking approaches to hydropower development and operation that are more environmentally and socially sustainable. The Penobscot River Restoration Project ('the Project') illustrates that basin-scale approaches can provide a broader set of solutions for balancing energy and riverine environmental resources than can be achieved at the scale of individual projects. The Penobscot basin is the largest in Maine and historically supported culturally and economically significant populations of migratory fish. These migratory fish populations declined dramatically following the construction of a series of hydropower dams on the main stem river and major tributaries in the early 20th century. The Project, negotiated between a power company (PPL Corporation) and a coalition including the Penobscot Indian Nation, resource agencies, and nongovernmental conservation organizations, features the removal of two main stem dams on the lower Penobscot and improved fish passage at the dams that remain. Because of various capacity and/or operational changes, power production will be increased at the remaining dams and total hydropower energy production from the basin will be maintained or increase slightly. The Project is expected to expand considerably the proportion of the basin accessible to migratory fish and contribute to significant increases in fish populations. The Project illustrates that a basin-scale approach can potentially yield more comprehensive solutions for sustainable hydropower than can be achieved at the project scale, and we recommend that such large-scale planning processes can improve the sustainability of both regulatory licensing of existing dams as well as the planning of future dams in regions undergoing the expansion of water-management infrastructure.

Balancing human and environmental water resource needs is critical to environmental sustainability. In this paper two concepts are advanced. First, a methodology is introduced to evaluate water management policies and their impacts on the characteristics of both instream flow and water supply reliability. The concept of an "ecodeficit" is introduced to quantify the impact of changes to the natural flow regime resulting from human withdrawals. This metric provides a numerical and graphical representation of the tradeoff between human and ecological needs for available water. Second, we evaluate an approach that involves both simulation and optimization of alternative reservoir release policies. We demonstrate that by refining the quantity and timing of reservoir releases the reliability of a water supply yield can be substantially maintained while improving the satisfaction of ecological flows requirements. These two concepts are early applications of a more comprehensive ecological water supply management approach currently under development.