River Refugia

River Refugia

 
 
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It’s important to identify and manage cold water patches when discussing climate change adaptation strategies. Although, managing refugia is much more than just identifying the cold water habitats. Habitat shade from riparian vegetation, sediment loads, physical habitat complexity, biological community interactions, dissolved oxygen, flow dynamics, food sources, groundwater hydrology, disturbances (e.g. fire), and access to streams may be equally, or more important than the presence/absence of cold-water. Therefore any approach to synthesizing information about riverine refugia should also consider there other habitat characteristics.

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Refugia as “networks”: We need to think of refugia as networks, not just individual locations to be managed independently and in perpetuity. Many of the species of conservation concern that persist within these cold-water habitats have wide-ranging life cycles and migration patterns and would benefit increasingly if the entire network of cold-water patches within a river system or region are managed. Existing restoration strategies and the biology of how refugia help fish thermoregulate, point to the need for managing refugia as a network. However, many of the approaches currently considered for managing and studying refugia look at refugia as independent units.  Therefore, research and management agendas for cold-water refugia should be built around the concept of a heterogeneous thermal network.

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Geographic location and watershed context matters: Where refugia is located within a river system or a geographic region is important in determining the type of management actions and research that are necessary. For example, enhancing the riparian function of vegetation shading to maintain water temperature is a strategy that is more likely to be effective in small tributaries than mainstream portions of river systems. In mainstream areas, where shade has a minor role in influencing water temperature, efforts to maintain vertical, lateral and longitudinal connectivity may be more important in creating thermal diversity.

In addition, there are significant differences in the characteristics of refugia in wet verses dry regions. For example, in dry areas of eastern Oregon and Washington, cold-water refugia may be more at risk to fire and water use than in the wetter western parts of this region. However, existing research efforts are at the scale of a single watershed or species range that have locally adapted populations and don’t often encompass the wide variety of environments that exist in the NW region. Therefore, a regional-scale approach will allow us to:

1)     Better capture the diversity of management and research approaches needed to implement refugia as a tool for adapting to climate variability in the NW

2)     Provide a better understanding of the risks and opportunities that are likely to impact the resources managed

3)     Highlight common strategies and overarching themes that can help create cohesive management approaches

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Refugia in managed systems: Biota uses areas buffered from climate variability by more stable local conditions as refugia, to mitigate large scale change. However, human activities have accelerated changes in the condition of habitats by fragmenting the landscape. This in turn has reduced the number of cool-water refugia and their access to cool-water, and has changed their function and distribution as well. For example, high-head dams in the McKenzie River system have blocked access to the primary cold-water habitats used by the ESA-listed fish, although assisted migration is needed for fish to access spawning and rearing habitats. Under certain conditions, dams and other water control structures can be important management tools in altering flow regimes to increase the availability of cooler water or greater flow during crucial hot, dry periods. However, many approaches to identifying cold-water only focus on the natural drivers (e.g., climatology, topography, hydrology, etc.). Therefore, research on how management of resources affects distribution, function, and accessibility of refugia is needed (e.g. projection of drought and water demand conditions or instream flow analysis that balances needs of stakeholders and fish).

Contributing Management Areas (CMAs) are areas at a scale that corresponds approximately with stream segments. Strategies at this scale should be designed to support a climate resiliency strategy and habitat functionality for subordinate focal areas. CMAs could encompass un-fragmented tributary networks or large river reaches. An example CMA would be a segment-scale riparian reserve or floodplain that provides natural regimes of large woody debris recruitment, sediment transport, hydrology, and temperature buffering.  Estuarine areas are also good examples of CMAs. CMAs should also be managed to provide spatial continuity for dispersal of fish population between high quality patches or focal areas.

Freshwater Focal Areas (FFAs) is a scale that corresponds most closely with core anchor patches or stream reaches for portions of a fish population. They contain specific ecological-based features that will help fish populations maintain resilience to climate change stressors. FFAs designated as climate resilience assets include high tributary confluence densities, and important cold-water tributaries or source areas with associated subsurface features.  FFAs could also include specific estuarine zones such as intertidal freshwater marshes. Biologically they are characterized by high endemicity or species richness, and/or high fish reproduction potential.  In most cases FFAs would be considered high functioning with respect to ecosystem processes, but some areas may be identified that have high intrinsic climate resilience but current low function with respect to fish use.