Seminar Explores the Science of Natural Functional Flows to the Delta
Although the term “natural hydrograph” has been widely used in discussion about Delta flow objectives, a group of scientists recently debated whether that term adequately conveys the complex nature of today’s world. A hydrograph represents such river flow characteristics as frequency, duration, magnitude, timing and rate-of-change of the flow, but some questioned its practicality in a river system today that is highly modified by a large reservoir storage, hydropower supply upstream and flood control infrastructure downstream.
Chris Enright from the Delta Science Program introduced the concept of functional flow and used the term “hydro-geomorphograph” to emphasize the influence of the surrounding landscape on river flows during a January 18, 2013 seminar that explored how the hydrology and benefits of Delta inflows are affected by land-use changes, diversions and climate change. “Hydrology and landscape morphology (shape) are inseparable concepts, like two sides to the same coin,” he said.
“What is a Natural Hydrograph in Regulated Rivers? The Science of Natural Functional Flows to the Delta” was hosted by the UC Davis Center for Aquatic Biology and Aquaculture (CABA) January 18, 2013. The day-long CABA seminar, convened by Delta Science Program Lead Scientist Dr. Peter Goodwin and Chris Enright was organized to explore how to move beyond flow volume considerations in managing flows for multiple uses, and to propose approaches to reconcile sustainable ecosystem functions within managed rivers and landscapes.
Keynote speaker Professor Geoffrey Petts, Vice Chancellor of the University of Westminster in England, described the European Union Water Framework Directive for integrated river basin management in Europe and introduced the term ‘smart’ flow management where water supply demands are optimized with downstream ecosystem needs. Smart flow management mimics natural flow patterns and is evolutionary; allowing the regulated river flow regime to change with changing climate.
Petts explored the importance of river resilience and said the three dimensions of that resilience are: complexity of the stream network within a watershed (streams, rivers, lakes), seasonal flow variability (within a year, between years), and morphological complexity (braided channels, bypasses, meanders etc.).
The plenary talk was followed by the session “Landscape Drivers and their Significance to Altered Hydrographs.” Enright walked the participants through the differences between natural and functional flows, and presented two different methods of calculating unimpaired flow used for the Delta hydrology. The first approach calculates the unimpaired flow below the large dams of the Central Valley (rim dams) using site specific equations and does not include groundwater interaction. The second unimpaired flow calculation uses gauged discharges from specific watersheds and approximates flow to ungauged watersheds based on watershed size and precipitation-flow correlations; it includes groundwater interaction and surface water retention. None of these unimpaired flow calculations considers landscape or vegetation context.
A second talk under “Landscape Drivers” was given by Robin Grossinger of the San Francisco Estuary Institute, who discussed the importance of the water-surface interface. Historical ecology and historical research helps to illustrate natural connections between flow and landscape. A natural hydrograph may not produce the desired functions if landscape interactions are ignored. In the Delta, there is a residual topographic template of natural basins, levees and lakes still surprisingly intact.
Grossinger presented a concept of the landscape mediating the hydrologic regime; it is the landscape that provides resilience to flood and drought. That is why a complex of connected riverine and wetland landscapes needs to be restored.
The session on “Current Delta Flow Regime and Adaptive Potential” included a talk by Bruce Herbold of the U.S. Environmental Protection Agency, who compared how flow and landscape alterations change the impact of a variety of stressors, from toxic spills to invasive species. Herbold presented a conceptual model of flow and watersheds to demonstrate cascades of responses through aquatic ecosystems.
Sarah Yarnell, of the UC Davis Center for Watershed Sciences, discussed an example of how the life cycle of an aquatic-dependent species-the foothill yellow-legged frog-evolved in relation to the natural flow regime and the timing of the spring snowmelt.
Yarnell’s research shows that unregulated flow recession rates in all water year types create water depth decreases of less than 10 cm per week at frog breeding sites. This predictable flow recession rate provides frog eggs a 3-week period to hatch and grow into tadpoles large enough to follow the edge of the waterline as it recedes through the summer. Without the simultaneous occurrence of timing of egg laying and gradually decreasing spring snowmelt flows, eggs can be scoured by large increases in flow or desiccated by rapidly decreasing flows.
This information is useful for developing a functional flow regime where the goal is to maintain flows that provide key ecological cues for native species. Yarnell concluded that ecological functions observed in unregulated systems can be a guide to management of regulated systems, and that coordinating management of upstream reservoirs to better mimic natural climate signals has the potential to propagate more natural flow regimes to valley floodplains.
For the “Alternative Futures” portion of the seminar, Cliff Dahm of the University of New Mexico provided perspectives and examples of managed flow regimes nationally and internationally. Only a few countries, such as Australia, South Africa and the United Kingdom have integrated the concept of environmental flows (the amount of water needed in a watercourse to maintain healthy, natural, ecosystems) into water management.
In the Murray-Darling Basin of Australia, the new, independent Murray-Darling Basin Authority established by the Australian National Water Act of 2007 sets objectives for environmental sustainability, economic return and diversion limits based on science and the consideration of both surface water, groundwater and their interactions.
The new Constitution of South Africa recognizes basic human rights for access to sufficient food and water. General principles have been developed that include the need for modified flow regimes to mimic the natural ones; harvesting water during wet months; retaining the seasonal pattern of higher flows in the wet season; fully retaining the first flood of the wet season and it is better to retain certain floods at full magnitude than to mimic all floods at diminished levels (the high-flow event has the biggest channel forming impact).
Les Grober of the State Water Resources Control Board provided an overview of the Bay-Delta planning activities, including the status of proposed San Joaquin River flow and southern Delta water quality objectives.
The scientific basis for alternative San Joaquin River flow objectives acknowledges that more flow of a more natural pattern will benefit native fish. Key elements of the proposed San Joaquin River flow objectives are maintaining a percentage of unimpaired flow in the salmon-bearing tributaries to the San Joaquin River and the adaptive management process. The adaptive management proposed in the San Joaquin River flow objectives is intended to provide fishery protection and minimize water supply costs; it allows for an adaptive range of 25-45% of unimpaired flow, allows shifting in time to achieve higher flows, and provides parameters under which adaptive management may occur.
A lively question and answer session and panel discussion followed including real-time polling of opinions from the audience. Experts on the panel voiced opinions on important components of the natural hydrograph and land-use patterns, saying, “It is critical that natural flows are connected to the landscape.” Asked by the audience, the panel also gave examples of the importance of dry cycles in the river hydrograph and its potential beneficial effects on native populations.
“The seminar explored a wide range of fundamental principles that have been employed in other river systems with varying success,” said Delta Science Program Lead Scientist Peter Goodwin. “Our ‘working’ landscape will continue to experience change through climate change and watershed modifications, but within the concepts described by the speakers there is hope that California can find solutions for ensuring resilience of our river ecosystems.”
Goodwin summarized the presentations and conclusions and highlighted main points from the audience-speaker interaction:
Functional flows for regulated rivers and altered landscapes such as the Delta are designed to mimic key geomorphological and ecological processes that the natural flow regime provides, not necessarily to mimic the full natural flow regime.
Main features of a functional flow include: peak magnitude flows, wet season initiation flows (to mobilize channel-forming sediments), spring recession flows, and dry season low flows.
Functional flows should be considered in three dimensions: longitudinally (transport of sediment and nutrients downstream within the stream network), laterally (habitat and land interfaces) and vertically (groundwater and surface water interactions).
Resilience (ability to adjust and rebound) is a key feature especially considering climate change, and functional flows in a functional landscape must provide long-term resilience for biotic and abiotic components.
A publication will be produced from the seminar summarizing key terminology, principles and management opportunities for using the concept of functional natural flows to enhance ecological conditions of aquatic environments within the Delta.
For further information and to access the CABA seminar presentations, click here.