One of the most important long-term threats to western fishes is climate change. Some of the major effects of climate warming in western North America include (1) Higher temperatures will result in more precipitation falling as rain rather than snow. (2) Snowpack will diminish and seasonal streamflow patterns will be altered. (3) Peak river flows will likely increase. (4) Water temperatures will continue to rise. Not all of these anticipated trends are necessarily harmful to aquatic habitat, and many pale in comparison to other anthropogenic factors, but they do have implications for native fishes. Climate change scenarios predict an increase in large flood events, wildfires, and forest pathogen outbreaks, all of which have some potential to actually improve habitat complexity as a result of flood plain reconnection and large wood recruitment. Many effects of climate warming, however, will have negative habitat consequences for aquatic organisms. A higher frequency of severe floods will result in increased egg mortality owing to gravel scour. Retreating winter snowpacks will run off earlier in the spring, potentially impacting species whose migration to the ocean is timed to coincide with plankton blooms. Summer base flows will be lower, and the network of perennially flowing streams in a drainage system will shrink during the summer dry period, forcing fish into smaller wetted channels and less diverse habitats. Warmer water temperatures will increase physiological stresses and lower growth rates, and summer peak temperatures may approach or exceed lethal levels for salmon and trout. Higher temperatures will also favor species that are better adapted to warmer water, including potential predators and competitors.
From a habitat resilience standpoint, maintaining as much water as possible in streams and lakes during periods of low flow will likely be the most effective way to combat the harmful effects of climate change, but other management actions could also produce long-term benefits. Increased flooding associated with higher peak discharge in winter may result in greater societal pressure to prevent damage to homes and infrastructure by isolating rivers from their flood plains; therefore, habitat managers would be well served to ask where flooding can be allowed in a watershed and in particular where flooding will reconnect the river with flood-plain habitats of direct importance to fish populations. Maintaining key flood plain connections will also act as a hydrologic safety valve that helps reduce the scouring effect of high flows on redds. Another management response to climate change involves restoring longitudinal connections throughout a drainage network, i.e., removing anthropogenic blockages to fish migrations up and down the watershed. With a constricted system of perennial stream channels in summer, it will be important for all potentially usable habitats to be available. Another management safeguard involves protecting and restoring riparian forests on valley floors and on alluvial terraces adjacent to stream channels. Riparian forests play an important role in the dynamics of the water table beneath and adjacent to streams, in moderating discharge during flow extremes, in controlling the concentration of soluble nutrients, in mediating the seasonal input of organic matter and terrestrial food items to aquatic ecosystems, and in regulating microclimate. Policies that explicitly maintain flows by protecting springs and seeps from water appropriation, limiting water withdrawals, enhancing flood plain connectivity by opening historically flooded areas where possible, removing anthropogenic barriers to fish movement, and protecting riparian forests will be needed to conserve habitat resilience in the face of climate change. Without such policies in place, aquatic habitats are likely to become increasingly isolated, simplified, and less likely to recover after significant disturbance events.