By Sarah Yarnell
Dams are no friend to biodiversity. Once impounded, a river answers first and foremost to human needs, be it water supply, energy production or flood protection. Releases are measured and timed to satisfy these demands.
As a result, the river downstream loses much of its natural variability in timing, volume and spread of flows. Dams also block passage of sediment that scours the stream channel and deposits fresh cobble bars. These activities create and maintain habitats for multiple species, contributing to biodiversity.
But dams don’t have to be death knells of biodiversity. Operators can manipulate flows in ways that restore some of their ecological functions that promote diverse riverine animal, plant and fish communities.
Releasing flows for environmental purposes is not new. California has long required dam owners to release enough flow “at all times” to keep fish “in good condition.” Further, some water and power suppliers are required under the federal Endangered Species Act to release flows at biologically important times for imperiled native fish.
These “environmental flows,” as water managers call them, may help fish survive, but they do not necessarily create habitat that promote high biodiversity. For that you need to implement a suite of well-timed flow patterns that move sediment and can access floodplains and over-bank areas.
My research colleagues and I recently identified five types of flows that are key to creating multiple habitats. We presented them recently at the annual meeting of the American Geophysical Union in San Francisco. [Update: On Aug. 5, 2015, Bioscience published the author’s paper describing in detail the functional-flows approach to managing heavily modified rivers. You can download the journal article here.]
We call these “functional flows,” as distinct from fish-saving “environmental” ones, because they provide certain geomorphic, ecological or biochemical functions that support breeding, migration, habitat diversity and, ultimately, biodiversity.
Five functional flow patterns key to creating and maintaining habitat for multiple species:
1. Wet season initiation flow
- Clears riverbed of organics, fine sediment
- Reconnects stream with riparian and over-bank areas
- Kick starts nutrient cycling
- Provides ecological cues for native species such as the delta smelt to migrate upstream
In many river systems, these actions can be accomplished simply by letting the first major, sediment-loaded storm runoff of the season – known as the “first flush” – pass through dams.
2. Peak flow
- Timed to coincide with the natural season of high flows and floods, ideally during big storms and other correlative weather conditions that native fish may respond to
- Should last long enough to scour out pools, form channel bars, activate floodplains and otherwise create diverse habitat
- Redistributes large amounts of sediment, creating geomorphic diversity
- Reduces extent of exotic species that are not adapted to these disturbances
- Keeps vegetation from encroaching on stream channels
- Resets the natural process of ecological succession
Water spilled from flooded reservoirs also is good for moving sediment. But these events happen only once every five to ten years. The annual peak flow helps maintain the form and structure of river channels, however these flows are often captured behind dams rather than passed downstream.
3. Spring recession flow
- Timed to coincide with the springtime transition between high and low flows
- Mimics the natural rate of decline in snowmelt flows, which is gradual
- Provides distinct annual cues for native aquatic species to reproduce and out-migrate
- Should last long enough to sustain habitats that species need to successfully reproduce and to redistribute sediment throughout the stream
As dam operators in the Sierra Nevada fill reservoirs, river levels can drop sharply, from the peak spring flows spilling over dams to the low, flat-lined summer flows. Gradually ramping down the spill flows can provide the in-stream conditions needed for survival of native species, such as the rare foothill yellow-legged frog, whose submerged eggs could get stranded and left to bake in the sun.
4. Dry season low flow
- Timed to occur during the warmest and driest part of the year (typically September in California)
- Should be low enough to disconnect the stream from its floodplains, to create a variety of ecological niches that promote a medley of riparian plants and trees
- Should maintain the natural ephemeral or perennial conditions
Releasing artificially high base flows often benefits non-native species that are not adapted to the biologically stressful low-flow periods.
5. Inter-annual flow variability
- Mimics the natural variability between years in magnitude, timing and duration of specific flow events
- Supports diversity in habitat and native species over the long term
Bigger, longer floods should be planned for years when water is plentiful, while smaller, shorter peak flows should occur in drier years.
Dam operators need to bring greater sophistication into the design and implementation of flows for multiple uses, including ecosystem services, water supply and flood control.
To maximize the limited allocations of water for ecosystem purposes, the focus of discussions should shift from flow volume to “functional flows” that support natural disturbances, promote certain physical dynamics and drive ecosystem functions.
When geomorphology and sediment processes are considered with flow magnitude, timing and duration, the creation and maintenance of habitats for multiple species can be sustained, and biodiversity is supported. A functional flows approach provides the best opportunity to encompass these ecosystem processes alongside human needs.
Sarah Yarnell is a senior researcher with the UC Davis Center for Watershed Sciences.
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