Climate Change, Aquatic Ecosystems, 

and Fishes in the Rocky Mountain West: 

Implications and Alternatives for Management

Bruce E. Rieman and Daniel J. Isaak

United States Department of Agriculture / Forest Service

Rocky Mountain Research Station

General Technical Report RMRS-GTR-250

November 2010
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Rieman, Bruce E.; Isaak, Daniel J. 2010. Climate change, aquatic ecosystems, 

and fishes in the Rocky Mountain West: implications and alternatives for 

management. Gen. Tech. Rep. RMRS-GTR-250. Fort Collins, CO: U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Research Station. 46 p.

Abstract

Anthropogenic climate change is rapidly altering aquatic ecosystems across the 

Rocky Mountain West and may detrimentally impact populations of sensitive species 

that are often the focus of conservation efforts. The objective of this report is to 

synthesize a growing literature on these topics to address the following questions: 

(1) What is changing in climate and related physical/hydrological processes that may 

influence aquatic species and their habitats? (2) What are the implications for fish 

populations, aquatic communities, and related conservation values? (3) What can 

we do about it? In many instances, proactive efforts may help populations adapt 

to climate change; but elsewhere, transitions of aquatic ecosystems to alternative 

states may need to be facilitated. The magnitude of the challenges posed by climate 

change makes collaborative efforts essential among resource disciplines, agencies, 

and the public.

Keywords: climate change, native fishes, fisheries, stream flow, temperature, 

management, prioritization, resilience, vulnerability

Authors

Bruce Rieman is a Research Fisheries Scientist (emeritus). He retired in 2007 from 

the Rocky Mountain Research Station Boise Aquatic Sciences Laboratory in Boise, 

Idaho, but remains active in research, management, and outreach. He holds B.S., 

M.S., and Ph.D. degrees from the University of Idaho. He has worked with issues in 

native fish conservation and fisheries management for the past 37 years.

Dan Isaak is a Research Fisheries Scientist with the Rocky Mountain Research Station 

Boise Aquatic Sciences Laboratory in Boise, Idaho. He holds a B.S. degree from 

South Dakota State University, an M.S. degree from the University of Idaho, and a 

Ph.D. from the University of Wyoming. He has worked in native fish conservation 

and fisheries management and research in the western United States for the past 18 

years, and his primary research interest is in understanding how climate change, 

disturbance, and biophysical interactions across spatial and temporal scales affect 

population dynamics and habitat in headwater streams.

Acknowledgments

Support for this project was provided through funding from the Rocky Mountain 

Research Station and the R1-R4-RMRS technology transfer program. Kerry Overton 

was instrumental in guiding the effort. Dona Horan provided support throughout 

preparation of the figures and document, and Sherry Wollrab and Laura Anglin 

provided editorial assistance. The final draft was improved through constructive 

comments provided by Kerry Overton, Kurt Fausch, James Roberts, Katherine Smith, 

and John Chatel.

Cover: Background: Photo courtesy of Boise National Forest. Inset images (top to 

bottom): Image courtesy of NASA Goodard Institute for Space Studies; Photo courtesy 

of Bart Gamett, USDA Forest Service; Image from Rieman and others (2007); Photo 

courtesy of Dan Isaak, USDA Forest Service.
Executive Summary

Little question exists that Earth’s climate is changing and that human 

causes are fundamentally important. Climate model projections indicate 

that the climate will continue to warm at rates equal to or faster than rates 

in recent decades until at least the middle of the twenty-first century. The 

effects of climate change could be particularly profound for native fishes 

and aquatic ecosystems of the Rocky Mountains because those systems often lack resilience and are strongly dependent on temperature and stream 

flow regimes that are already documented to be changing. The vulnerability of fish populations and communities to climate change will vary across 

the region based on local conditions and the amount of change that occurs. 

Because management budgets are limited, it will be important to prioritize 

limited resources based on some understanding of that vulnerability and 

the range of management alternatives. To consider management in the face 

of climate change, we have synthesized information on climate change 

and native fishes, stream habitats, and the observed and anticipated effects 

of climate change in the Rocky Mountain West. This report is organized 

around the following questions: What is changing, what are the implications for native fishes, and what can we do about it?

Important changes for fishes and their habitats will be driven by two 

factors that are the principle components of climate: air temperature and 

precipitation. Air temperatures across the Rocky Mountain West are warming faster than global averages and have increased by about 1 °C over 

the last century. Changes in precipitation are less consistent, but slightly 

drier summers and wetter winters are anticipated in the northern Rocky 

Mountains while the southern Rockies will probably experience generally drier conditions. The temporal and spatial patterns of change are not 

likely to be constant or linear but will vary with local trends and shorterterm climate cycles such as the Pacific Decadal Oscillation and the El 

Niño-Southern Oscillation. As climate change progresses, however, longterm warming trends and increasing variability will result in more frequent 

events (e.g., droughts, intense precipitation, and periods of unusually 

warm weather) that were considered extreme during the twentieth century, 

and the magnitude of these events may also exceed recent historical levels. 

Changes in stream environments will parallel trends in the climate system, 

with streams becoming warmer, more variable in flow timing and amount, 

and subject to more frequent extreme events that could be synchronized 

across broader areas through regional flooding, droughts, and wildfires. 

Climate change is also likely to influence channel structure and forest and 

riparian communities through altered patterns and severity or intensity of 

wildfire, inputs of sediment and large wood, and disturbances such as debris flows. Although stream changes have been and are anticipated to be 

widespread, changes will not necessarily occur at the same rates across the 

Rocky Mountain West. Some stream systems are changing more rapidly 

than others, and some even show trends opposite to general expectations. 

The characteristics of watersheds and streams that may either aggravate or 

confer some resistance to the general effects of climate change will be an 

important question for further research.
The implications of climate change for native fishes are challenging 

to anticipate because of a general lack of long-term monitoring data for 

Rocky Mountain populations and the complexity of interactions between 

biological and physical processes that are involved. In general, however, 

suitable habitats that are defined by temperature, flow, and other physical 

or biotic conditions must shift in location (generally to a higher elevation 

or latitude). Some species, populations, and communities may be able to 

track these changes and simply “relocate” but upstream limits of available 

habitat and barriers to dispersal and migration will limit many others. In 

other cases, the interaction of climate change, heterogeneous landscape 

responses, shifting species distributions, and new biotic and physical 

interactions will create novel environments, trophic cascades, and communities that have no natural precedent. These complex interactions will 

make prediction and management even more difficult. Natural selection 

and phenotypic plasticity could help many species and populations adapt 

to new environments, but the speed and capacity for adaptation of most 

species are not well known and may be outpaced by the rate of climate 

change.

What we do about climate change will depend on available information, management resources, and policy direction. The basic decision, 

however, is to either work to conserve existing species, populations, and 

communities through adaptation strategies or to facilitate their transitions to new conditions that are most desirable and feasible in the future. 

Our recommendations to guide management responses fall in five general areas. First, if the goal is adaptation and conservation of existing 

species or communities, efforts to enhance resistance and resilience of 

existing populations will be key. Important steps will include efforts to reduce non-climate stresses that may influence survival, growth, and habitat 

capacities; conserve and expand critical habitats; reconnect streams; and 

conserve genetic and phenotypic diversity. Second, because the potential 

threats of climate change and the feasibility of successful adaptation will 

vary widely across managed landscapes, it will be important to prioritize 

limited resources. Steps to that end include efforts to: clarify goals and 

values; focus on populations as fundamental units of conservation; consider the relative vulnerability and relative value among populations and 

habitats; and favor actions robust to uncertainty. The latter step has also 

been called a “no regrets” strategy, where the focus is on actions that will 

be useful whether the climate changes as anticipated or not. Third, if it is 

not likely that existing populations or communities can be maintained in 

the face of climate change, managers may consider efforts to facilitate 

transitions to new conditions. Facilitation might occur passively through 

the simple removal of existing barriers, allowing native or non-native species to move into new areas. It might be active through the intentional 

introduction of species to new environments. The latter option can be particularly controversial and requires clear articulation of the conservation 

values at stake. Fourth, the effectiveness of any action associated with 

the first three recommendations depends on the quality of available information to guide decisions. For that reason, it is important to develop 

local information. Important steps toward that end include efforts to: 

understand context based on existing climate projections and models of 

hydrologic, temperature, or biological changes near or encompassing the 

areas of interest; synthesize existing information for the area of concern 

ii
such as inventory and monitoring data or local climatic trends that could 

be used to understand the status, distribution, and vulnerability of important species, habitats, or watersheds; model to fill gaps where useful data 

are limited or future projections are needed; and monitor to document 

trends and validate models. Finally, because the issues associated with 

climate change will be socially and ecologically complex and because new 

information and tools are developing quickly, it will be important to coordinate efforts across disciplines, across agencies and jurisdictions, and 

with the public.

The resources available to managers and biologists dealing with climate 

change will always be limited. The challenges associated with climate 

change are substantial, perhaps as important as the habitat losses already 

imposed on aquatic ecosystems by past human actions. The urgency is 

confounded by large uncertainties associated with the Earth’s climate trajectory and poor understanding of how broad climate trends will translate 

to local effects on streams and aquatic communities. Where do we start? 

Some have argued that public education on aquatic issues and the tradeoffs we make with other values may be the single most important thing 

aquatic managers and biologists can do for the long-term conservation 

of aquatic ecosystems, biological diversity, fishes, and fisheries. Without 

a fundamental change in understanding and support of aquatic conservation, not much may change. We believe that local monitoring with climate 

change in mind is one of the most important steps biologists and managers can take in response to climate change. Monitoring data will help test 

hypotheses relevant to stream system responses to climate change, will 

provide managers with important insight to the relative utility of management actions, and may also provide a basis for making difficult decisions. 

Partnerships between research and management and continued monitoring 

and education through both local and broader collaborative efforts could 

promote effective adaptation to a changing climate.

iii
Contents

Executive Summary...................................................................................................... i

Introduction................................................................................................................ 1

What Is Changing? ...................................................................................................... 3

Climate ................................................................................................................ 3

Stream Environments............................................................................................ 6

Implications for Native Fishes................................................................................... 12

Temperature and Hydrology as Primary Controls................................................ 12

Summary............................................................................................................ 17

What Can We Do About It? ...................................................................................... 18

Enhance Resistance and Resilience .................................................................... 18

Prioritize Limited Management Resources.......................................................... 23

Facilitate Transitions to New Conditions............................................................. 28

Develop Local Information................................................................................. 28

Coordinate Efforts............................................................................................... 33

Epilogue .................................................................................................................... 35

References ................................................................................................................ 36

iv
USDA Forest Service RMRS-GTR-250. 2010. 1

There is little question that Earth’s climate is changing and that human causes are fundamentally important 

(figure 1). Periodic assessments of the International 

Panel on Climate Change (IPCC 2007) synthesize the 

most comprehensive evidence on the nature of global 

climate change and its causes, scientific uncertainty, 

and implications for human and natural systems. A 

rapidly expanding literature (e.g., Abatzoglou and 

Redmond 2007; Barnett and others 2008; Joyce and others 2008), instrumental record (e.g., Luce and Holden 

2009; Stewart and others 2005), and other resources 

(e.g., http://cses.washington.edu/cig/fpt/cloutlook.shtml; http://www.fs.fed.us/ccrc/) document the nature of 

changes in the western United States as well as potential effects on natural resources and aquatic ecosystems 

(e.g., Bisson 2008; ISAB 2007; Mantua and others 

2009; Nelitz and others 2009).

Effects of climate change could be particularly 

profound for aquatic ecosystems. Warming air temperatures and changing precipitation translate to increasing 

water temperatures; alteration of stream hydrology; 

and changes in the frequency, magnitude, and extent 

of extreme climate events such as floods, droughts, and 

wildfires (Hamlet and Lettenmaier 2007; Jentsch and 

others 2007; McKenzie and others 2004). The biology 

of aquatic organisms is largely dependent on temperature and flow and most have evolved in a dynamic 

environment defined by hydrologic and geomorphic 

processes (Bisson and others 2009; Brannon and others 

2004; Waples and others 2009). However, fundamental 

changes in climate could lead to fundamental changes in physiology, behavior, and growth of individuals 

(Jager and others 1999; Neuheimer and Taggart 2007; 

Pörtner and Farrell 2008); phenology, growth, dynamics, and distribution of populations (Hari and others 

2006; Rieman and others 2007; Robards and Quinn 

2002); persistence of species and structure of communities (Finney and others 2002; Hilborn and others 2003); 

and functioning of whole ecosystems (Moore and others 2009).

In relation to environmental variation over evolutionary and ecological time scales that shaped existing 

species of fishes and phenotypic diversity within those 

species (approximately 103 to 106 years, respectively; 

Lichatowich 1999; Waples and others 2009; Wood and 

others 2008), contemporary changes in climate may 

be relatively minor. Indeed, many species such as the 

Pacific salmon, trouts, and chars that are now a focus 

of intensive conservation management have recovered, 

diversified, and expanded in distributions following 

continental glaciation, volcanism, and the cataclysmic processes that shape mountain streams and rivers 

(Chatters and others 1995; Montgomery 2000; Wood 

and others 2008). The effects of contemporary climate 

change, in contrast to the degradation and loss of aquatic habitats associated with human development over the 

Figure 1. Instrumental 

record of average 

global air 

temperatures from 

1880 to 2008 (from 

NASA Goddard 

Institute for Space 

Studies).

Introduction
2 USDA Forest Service RMRS-GTR-250. 2010.

last two centuries, might represent a relatively minor 

loss of historical habitat capacity or productivity. The 

problem is that modern climate change is occurring 

especially quickly at the end of an already warm period (IPCC 2007; Lüthi and others 2008), indicating a 

potential for new conditions with no natural precedent 

(Williams and others 2007), and in the wake of already 

extensive habitat and aquatic community disruption 

(Wood and others 2008). Many species and populations 

no longer have the networks and diversity of habitats 

and refugia, genetic diversity, or evolutionary potential 

that allowed them to resist or rebound in the face of 

past environmental disturbance and change. In some 

cases, anticipated changes may outpace the remaining 

capacity for adaptation and dispersal (Crozier and others 2008).

Vulnerability of aquatic species, populations, and 

communities to climate change will depend on a context defined by the characteristics of those species and 

local environments, past habitat disruption, fragmentation and loss, and the nature of the change that occurs. 

As a result, that vulnerability may vary dramatically 

across populations, species, and landscapes that are a 

focus of land and natural resource management (e.g., 

Rieman and others 2007). In most cases, capacity for 

conservation management is constrained by limited 

budgets, time, and other resources. It is also constrained 

by current understanding of the implications of climate 

change and management actions or alternatives that 

might effectively influence results. It will be important 

to prioritize limited resources and to guide management 

based on some understanding of the vulnerability of 

species, populations, and ecosystems of interest.

The objectives of this report are to synthesize information on native fishes, stream habitats, and the 

anticipated effects of climate change in the Rocky 

Mountain West. This region may be particularly sensitive to climate change given the rate at which it is 

warming and because many of its ecosystems are constrained by water availability (Brown and others 2008; 

Saunders and others 2008). It is not our intent to provide 

an exhaustive review of the climate-aquatic-fisheries 

literature; several good ones already exist (Bisson 

2008; Ficke and others 2007; Francis and Sibley 1991; 

Furniss and others 2010; Hauer and others 1997; Heino 

and others 2009; ISAB 2007; Poff and others 2002; 

Rahel and others 2008; Schindler and others 2008). 

Our intent is to provide an overview of important information as context for management that might begin to 

address the implications of climate change. The report 

is organized around the following questions:

• What is changing in climate and related physical/

hydrologic processes that may influence aquatic 

species and their habitats?

• What are the implications for fish populations, aquatic 

communities, and related conservation values?

• What can we do about it?
USDA Forest Service RMRS-GTR-250. 2010. 3

Climate

Air temperature and precipitation are the principle 

components that constitute “climate.” Short-term variability in these factors is often considered to be the 

“weather” and seasonal cycling, whereas long-term 

variation (i.e., decadal to millennial timescales) constitutes climatic regimes (figures 1 and 2). Interactions 

among climate, geology, and topography have formed 

the distinctive physiographic regions into which species 

of the Rocky Mountains have dispersed and evolved 

over millennial timescales (Hessburg and others 2005; 

McPhail and Lindsey 1986; Schumm and Lichty 1965; 

Wolock and others 2004). The expansive and varied 

regions across the Rocky Mountain West are characterized by a diversity of climatic conditions, ranging from 

greater than 2000 mm of annual precipitation and mean 

air temperatures of approximately 0 °C at the highest 

elevations in the north to less than 100 mm of annual 

precipitation and annual temperatures exceeding 15 °C 

in parts of the southern Rockies (Hijmans and others 

2005). Annual temperature cycles create strong seasonality in most northern areas, and precipitation during 

winter often occurs as snow and accumulates at higher elevations until warming spring temperatures drive 

snowmelt and broadly synchronized patterns of stream 

runoff (Barnett and others 2008; Stewart and others 

2005). This general pattern holds only at high elevations in the southern interior due to latitudinal clines 

toward warmer temperatures and decreases in snow 

accumulation. At lower elevations, seasonal cycles are 

less pronounced and stream flows are more variable, 

with many smaller streams exhibiting periods of drying 

interspersed with periodic flooding from summer monsoons and convective thunderstorms (Minckley and 

Deacon 1991; Propst and others 2008).

Climate has been changing across the Rocky 

Mountains in association with global patterns. However, 

instrumental records suggest that mean annual air temperatures during the twentieth century increased by 

approximately 1 °C (Saunders and others 2008), which 

is considerably more than the 0.6 °C increase in global 

temperatures for the same period (IPCC 2007). The 

larger increase is due to warming rates that are faster 

over land masses than over the oceans (the global average includes both), but the Rocky Mountain West also 

has been warming more rapidly than other areas of the 

coterminous United States (Saunders and others 2008). 

This spatial heterogeneity is also apparent at finer 

scales across the interior, where a four-fold variation 

in warming rate has been observed and a few locations even exhibit cooling trends (figure 3a; Mote and 

others 2005). Adding further complexity is seasonal 

variation in warming where trends have generally been 

most pronounced in the spring, early summer, and winter and have been subdued in the late summer and fall 

(Abatzoglou and Redmond 2007; Knowles and others 

2006; Pederson and others 2009).

What Is Changing?

Figure 2. Stream temperature, flow, and other 

conditions influenced by climate vary 

from year to year and over longer periods 

associated with periodic phenomena like 

the Pacific Decadal Oscillation and the 

El Niño-Southern Oscillation. The overall 

trends associated with climate change are 

evident in long-term monitoring data, but 

trends over shorter periods (e.g., the black 

arrow in the figure) may be more strongly 

positive, neutral, or even negative and 

obscure the longer-term patterns for periods 

of time.
4 USDA Forest Service RMRS-GTR-250. 2010.

Trends in total annual precipitation during the 

twentieth century are less consistent than those of air 

temperatures, although small increases in precipitation have been observed in many areas of the Rocky 

Mountains (figure 3b; Hamlet and others 2007; Mote 

and others 2005), and changes in warm season/cool season proportions are apparent in some regions (Hamlet 

and others 2007). More importantly, perhaps, is that a 

larger proportion of precipitation now falls in extreme 

events (Wehner 2005) and as rain rather than snow 

due to the interaction with warmer air temperatures 

(Knowles and others 2006).

As substantial as the trends in twentieth century climate have already been, Global Climate Models (GCM) 

project continuation and even acceleration of warming 

through at least the middle of the twenty-first century 

(figure 4; IPCC 2007). Current projections for the western United States suggest mean annual air temperatures 

will increase by another 1 °C to 3 °C by mid-century 

(Mote and others 2008; GCRP 2009), and early indications put the Earth’s climate trajectory toward the 

higher end of this range (Pittock 2006; Raupach and 

others 2007). Even the most conservative estimates suggest a warming rate in future decades that is twice that 

Figure 3. Observed spatial variability 

in (a) air temperature and (b) 

precipitation trends across the 

western United States from 1930 

to 1997 (from Mote and others 

2005). Red (blue) circles indicate 

warming (cooling) air temperatures 

or decreasing (increasing) 

precipitation.

Figure 4. Global climate change scenarios based on 

different assumptions regarding greenhouse gas 

emissions and trends in human societies (from IPCC 

2007). Note that most scenarios predict similar 

amounts of warming in the middle of the twentyfirst century, but large differences emerge later in 

the century due to unknowns about greenhouse gas 

emissions in the next few decades.
USDA Forest Service RMRS-GTR-250. 2010. 5

observed during the twentieth century. 

Moreover, these increases in temperature 

are expected to be accompanied by greater overall variability. Thus, it may still be 

possible to continue exceeding historical 

records for cold temperatures even as record highs become increasingly frequent 

(Meehl and others 2009). More importantly, the combination of a shift toward 

higher mean temperatures and enhanced 

variability implies that the frequency of 

weather conditions now considered “extreme” could increase rapidly (figure 5; 

IPCC 2007; Jentsch and others 2007).

Projections regarding changes in precipitation are generally less certain than 

those for air temperatures. Most GCMs 

forecast small precipitation increases for 

the northern interior, accompanied by a 

seasonal shift toward drier summers and 

wetter winters (Mote and others 2008; 

GCRP 2009). In the south, however, significant annual precipitation decreases 

on the order of 15 to 40 percent are projected, and this area is one of the few for 

which GCM projections have a high level of agreement (figure 6; Hoerling and 

Eischeid 2007; GCRP 2009).

Superimposed on climate warming trends are cycles associated with 

the Pacific Decadal Oscillation (PDO), Atlantic 

Multidecadal Oscillation (AMO), and the El NiñoSouthern Oscillation (ENSO) that periodically enhance 

and dampen long-term trends (figures 2 and 7). All 

Figure 5. Changes in the probability of 

extreme weather events (red and blue 

distribution tails) associated with a 

shift in the mean climate and increased 

variability (from Jentsch and others 2007).

Figure 6. Distribution of Palmer Drought Severity Index scores based on 

historical conditions and future projections (from Hoerling and Eischeid 

2007).

such climate cycles are derivatives of atmosphericoceanic heat exchanges and manifest as multi-year 

periods with above (or below) average temperatures 

and precipitation across the western United States 

(McCabe and others 2004; McPhaden and others 1998; 
6 USDA Forest Service RMRS-GTR-250. 2010.

Mote and others 2003). The length, consistency, and intensity of cycling depend on how the phases (cool and 

wet or warm and dry) are aligned and vary with periodicities of the oscillations (ENSO is approximately 2 

to 7 years; PDO and AMO are approximately 20 to 30 

years; Mantua and others 1997; McPhaden and others 

1998). There is some evidence for more frequent and 

intense cycling with especially warm climatic periods 

in the Earth’s distant past (Wara and others 2005), and 

similar changes could occur in the future (Yeh and others 2009).

Although considerable uncertainty exists regarding 

future climate, it is safe to assume that managers in the 

Rocky Mountains will have to adjust to a warmer and 

sometimes drier world. Continued warming is a virtual 

certainty given the greenhouse gases already emitted, 

inertia in the global climate system, and human energy demand. But just how different the future will be is 

difficult to predict. A best case scenario is temperature 

gains comparable to those already experienced during 

the twentieth century but compressed into approximately half the time with a transition to a new climate 

equilibrium sometime in the mid to late twenty-first 

century. Other (potentially more likely) scenarios include greater warming over a longer period, with higher 

temperatures exacerbated in some areas by decreases 

in precipitation and growing human populations with 

water supply needs. Moreover, the long life of greenhouse gases in the atmosphere means that even after 

emissions are curbed, it may take several millennia for 

reversion to a cooler climate that would approximate 

recent historic conditions (Solomon and others 2009). 

Future environmental changes, therefore, might be 

considered more or less permanent with regard to any 

practical management horizons.

Stream Environments

Channel and riparian conditions. Climate change 

during the twentieth century resulted in a series of 

related environmental trends across western North 

America that could translate to wide-ranging effects 

on channel structure and riparian conditions (van 

Mantgem and others 2009; Yarnell and others 2010). 

Warmer air temperatures, for example, have decreased 

winter snow accumulations by increasing the rainfall 

fraction, advancing spring snowmelt, and altering the 

pattern of stream flow recession (Hamlet and others 

2005; Mote and others 2005; Yarnell and others 2010). 

Smaller snowpacks that melt sooner have translated to 

increasing drought frequency and severity (Hoerling 

and Eischeid 2007) and more extensive wildfire activity (Littell and others 2009; Morgan and others 2008; 

Westerling and others 2006). These stresses are in turn 

effecting changes in the composition of the region’s 

forest, rangeland, and riparian plant communities, with 

more xeric, drought-tolerant species becoming more 

common (Breshears and others 2009; van Mantgem and 

others 2009). Changes are occurring gradually through 

direct climate effects on demographic processes (van 

Mantgem and others 2009), during dramatic regional 

die-offs associated with extreme droughts and wildfires (Breshears and others 2005; van Mantgem and 

Stephenson 2007), and by making plant communities 

more susceptible to invasions by non-native plants 

and outbreaks of pests like mountain pine beetles 

(Dendroctonus ponderosae; Logan and Powell 2001; 

Pettit and Naiman 2007).

Altered forest and riparian communities, combined 

with wildfire activity, will change inputs of sediment 

and large wood, and these basic channel constituents 

will be routed differently by hydrologic regimes that are 

also evolving (Barnett and others 2008; Miller and others 2003). In steep topographies, post-fire debris flows 

could become more common in small channels and 

sometimes cause extirpations of local fish populations 

(e.g., Bozek and Young 1994; Brown and others 2001). 

Debris flows may also simplify habitats in small, steep 

channels through scour and removal of local alluvium, 

bank soils, and woody debris, while paradoxically creating more diverse habitats in downstream channels 

where sediment and wood deposition occur (Miller and 

Figure 7. Monthly values of the 

Pacific Decadal Oscillation 

from 1900 to 2008 (N. Mantua, 

unpublished data; http://jisao.

washington.edu/pdo/).
USDA Forest Service RMRS-GTR-250. 2010. 7

others 2003; Reeves and others 1995). At present, few 

studies have linked channel evolution to rapid, anthropogenic climate change in mountainous areas, but see a 

review by Blum and Tornqvist (2000) for information 

on longer-term (i.e., 102 to 104 years) channel responses 

that could prove relevant. With the exception of debris 

flows, however, time lags between alteration of hillslope processes and channel response mean that aquatic 

ecosystems and management options are more likely to 

be constrained by rapidly changing hydrologic and temperature regimes.

Hydrologic regimes. The interaction between 

warming temperatures and precipitation has been associated with a variety of hydrologic alterations during 

the twentieth century. These changes have been well 

documented by a monitoring network that consists of 

several hundred flow gages with long-term records 

across the western United States (e.g., Regonda and 

others 2005; Stewart and others 2005). Data from these 

sites suggest that the timing of stream runoff steadily 

advanced during the latter half of the twentieth century 

and now occurs 1 to 3 weeks earlier (figure 8; Regonda 

and others 2005; Stewart and others 2005), due largely 

to concurrent decreases in snowpack and earlier spring 

melt (Mote and others 2005; Stewart and others 2005). 

These changes also diminish recharge of subsurface 

aquifers that support summer baseflows (Hamlet and 

others 2005), and flow declines during this period are 

also apparent across many Rocky Mountain streams 

(Rood and others 2008; Stewart and others 2005). In 

watersheds with densely forested vegetation, these 

declines may be exacerbated as a warmer climate increases water loss through evapotranspiration (Hamlet 

and Lettenmaier 1999; Hamlet and others 2007). In one 

striking example from the Pacific Northwest, Luce and 

Holden (2009) found that three-fourths of the 43 gage 

records they examined exhibited statistically significant 

declines in summer low flows (25th percentile or one-infour year low flow) of 29 to 47 percent during the latter 

half of the twentieth century (figure 9). That decline has 

been accompanied by longer inter-annual correlation, 

indicating that when extreme droughts occur, they are 

more likely to persist across multiple years (McCabe 

and others 2004; Pagano and Garen 2005).

Mid-winter flood frequency and extent also appear to 

be affected by warming air temperatures, and streams in 

watersheds with temperatures that are near freezing are 

especially sensitive. Streams in these watersheds often 

exhibit “transitional” hydrologies that are a mix of rain 

Figure 8. Trends in the timing 

of spring runoff initiation 

at flow gages across the 

western United States 

from 1948 to 2002 (from 

Stewart and others 2005). 

Large circles indicate gages 

with statistically significant 

changes in runoff timing.
8 USDA Forest Service RMRS-GTR-250. 2010.

and snowmelt runoff. Any additional warming in these 

watersheds translates directly to an increase in the proportion of precipitation falling as rain and increases the 

potential for rain-on-snow linked to mid-winter floods 

(Hamlet and Lettenmaier 2007; Marks and others 1998). 

Streams at lower, warmer elevations where hydrology 

is already dominated by rainfall, and streams at higher 

elevations, where mid-winter temperatures will remain 

well below freezing for the foreseeable future, will be 

less sensitive (figure 10; Hamlet and Lettenmaier 2007).

Although climate change is driving systematic 

changes in stream hydrology, important local variability 

cannot be discounted. Trends toward decreasing summer flows and advances in runoff, for example, may be 

weaker in watersheds where local precipitation rates 

are trending higher, where snow is a minor source of 

stream flow, or where warming rates are slower (figure 

3). Underlying geologies may also buffer some streams 

better than others against changes by providing greater 

subsurface aquifer storage and contributions to summer 

Figure 9. Trends in 

the 25th percentile 

low flows at gaging 

stations across the 

Pacific Northwest 

from 1948 to 2006 

(from Luce and 

Holden 2009).

Figure 10. Relative changes in 20-year flood probability from 

1915 to 2003 west of the Continental Divide modeled 

using the Variable Infiltration Capacity hydrologic model. 

Watersheds in blue had increased probabilities while those 

in brown or red had reduced probabilities (from Hamlet 

and Lettenmaier 2007).
USDA Forest Service RMRS-GTR-250. 2010. 9

baseflows. Work by Tague and others (2008) in central 

Oregon streams suggests this buffering effect may be 

substantial, with summer flows in some streams projected to decline one-third as much as others (-15 percent 

versus -45 percent) under different climate change scenarios. Note, however, that this study occurred in an 

area with pronounced differences in geology that may 

not be indicative of other areas that are comparable in 

size. Regardless, it is clear that significant variation 

exists across the region (figure 11; Santhi and others 

2008).

Temperature regimes. Long-term temperature records from lakes and streams are relatively rare in 

contrast to flow records. Where long-term data are 

available, trends generally tracked air temperature 

increases in the latter half of the twentieth century 

(figure 12; e.g., Bartholow 2005; Morrison and others 

2002; Peterson and Kitchell 2001; Robards and Quinn 

2002). In one recent assessment, Kaushal and others 

(2010) examined river temperature trends across the 

United States and found that 14 of 16 western rivers 

had warmed during the last 30 years (10 of the warming 

trends were statistically significant). The average rate 

of increase was 0.17 °C/decade, with the warming trend 

exacerbated in some places by urbanization and water 

development.

To compensate for a lack of long-term temperature 

data from lakes, Schneider and others (2009) used 

time-series of satellite imagery calibrated to a few years 

of lake surface temperature data to reconstruct previous thermal regimes. This study suggests that from 

1992 to 2008, all of the lakes examined in California 

and Nevada showed statistically significant warming 

trends. Interestingly, the surface temperatures of the 

lakes warmed at approximately twice the rate of local air temperature increases, although it was unclear 

whether lake heat gains were integrated throughout the 

full depth profile.

Temperature data for small headwater streams are 

abundant, but like lakes, long-term time-series are 

relatively rare because inexpensive digital temperature 

sensors became widely available only in the 1990s (figure 13; Dunham and others 2005). To compensate for 

this lack of data, Isaak and others (2010) compiled a 

spatially extensive temperature database from multiple 

Figure 11. Baseflow index calculated as the ratio of baseflow to peakflow across the U.S. (from Santhi and others 2008). 

Higher values of the index are an indication of flow stability and potentially greater groundwater inputs.
10 USDA Forest Service RMRS-GTR-250. 2010.

resource agencies across a large, central Idaho river 

network and used the data to build statistical models 

with climate predictors so that long-term trends could 

be reconstructed and examined. Results suggest that 

climate-related trends in air temperatures and stream 

flow had increased mean summer stream temperature 

by +0.38°C across the network from 1993 to 2006, 

which translated to a warming rate of 0.27 °C/decade 

(figure 14; Isaak and others 2010).

Another limitation of temperature data from headwater streams is that sampling has often occurred only 

during summer months because of difficulties with site 

access or maintenance of sites with large annual floods. 

Unfortunately, therefore, few details are known regarding how full annual thermal regimes may be changing 

(e.g., timing of spring onset, changes in growing season 

length, and differences in seasonal trends) or the potential consequences for aquatic biotas (Olden and Naimen 

2009; Danehy and others 2010). Isaak and others (2010) 

recently developed a simple protocol that involves gluing temperature sensors directly to large boulders that 

facilitates the collection of more annual stream temperature data, but it may take several years to accumulate 

the data necessary for assessing annual patterns.

Aside from the systematic changes driven by climate 

change, there are a variety of factors that will impart local 

variability in stream warming rates. This local variability is apparent among streams, indicated by differences 

in regression slopes with warming trends in figure 13. 

It is also illustrated by inter-annual differences at sites. 

Figure 12. Trend in total annual 

degree days for the Columbia 

River at Bonneville Dam from 

1938 to 1998 (Robards and 

Quinn 2002).

Figure 13. Trends in mean summer stream 

temperature from 1993 to 2006 for five 

headwater streams in central Idaho. 

Similarity among stream trends indicates a 

response to a common climatic driver, but 

differences in slopes of the relationships 

fit by linear regression indicate streams 

are varying in their response to climate 

change (B. Rieman and D. Isaak, 

unpublished data). If monitoring has 

been conducted for at least a decade on 

small streams, warming trends are often 

apparent.
USDA Forest Service RMRS-GTR-250. 2010. 11

In the Boise River basin, for example, where the same 

33 sites were sampled in 2007 and 2008 (figure 15), 

the average change in mean summer stream temperature was -1.59 °C, but a range of temperature responses 

was observed from -0.49 °C at the least sensitive site 

to -2.41 °C at the most sensitive site (the average sitelevel deviation from the systematic shift between years 

was -0.38 °C). This variable response could have been 

caused by variation in local microclimatic forcing 

(Daly and others 2009) or the sensitivity of streams to 

a given level of forcing (Hari and others 2006). Over 

longer time or broader spatial scales, differences in 

stream temperature response could also result from spatial heterogeneity in climate trends, changes in riparian 

vegetation conditions related to wildfire or drought, and 

evolving hydrologic regimes. The potential effects of 

wildfires on stream temperatures are well documented 

(Dunham and others 2007; Hitt 2003; Isaak and others 

2010), but broad differences in hydrology may also be 

important (Mohseni and others 1999; Morrill and others 2005). In a national-scale assessment, Mohseni and 

others (1999) found that streams dominated by snowmelt runoff warmed only 0.44 °C for every 1 °C air 

temperature increase, in contrast to the 0.67 °C average across the United States. This result was attributed 

to buffering by snowmelt groundwater and implies that 

many streams across the Rocky Mountains may be currently less sensitive to warming. If this is true, it also 

implies that future sensitivity may increase as snowpacks decline.

Figure 14. (a) Increases in mean summer stream temperature across the Boise River stream network from 1993 to 2006 due to 

wildfires and long-term trends in air temperatures and decreasing summer flows. (b) Relative influence of factors on stream 

temperature increases. Total stream temperature increase and decadal warming rates for the entire network and for areas 

within wildfire perimeters are shown above the bars (from Isaak and others 2010).

Figure 15. Variation in mean summer stream temperature 

changes between 2007 and 2008 at 33 sites in the Boise 

River network. The mean change between years was 

-1.59 oC (represented by the systematic change across 

all sites), but the average site-level deviation from the 

mean change was 0.38 oC (represented by the differences 

in slopes among all sites). These patterns indicate that 

although streams across broad areas may respond similarly 

to climate change, some streams will be more (or less) 

sensitive than others.
12 USDA Forest Service RMRS-GTR-250. 2010.

The influence of climate change on stream environments, patterns of disturbance, hydrologic and 

geomorphic process holds important implications for native fishes and aquatic communities in Rocky Mountain 

streams. There is growing evidence that terrestrial 

ecosystems are responding to climate change through 

shifting distributions, invasions, local extinctions, and 

reorganization of whole communities and food webs 

(CCSP 2009; Parmesan and Yohe 2003; Root and others 

2003). There is less documentation of effects in aquatic ecosystems (Heino and others 2009), but important 

changes have been observed in species distributions 

(Hari and others 2006), phenology and expression of life 

histories (Hilborn and others 2003; Quinn and Adams 

1996; Quinn and others 1997), and basic hydrologic and 

limnological processes that ultimately structure populations and communities (Isaak and others 2010; Mantua 

and others 2009; Poff and others 2001; Schindler and 

others 2008; Wrona and others 2006). A slower pace 

of documentation in aquatic systems is not necessarily cause for optimism but perhaps a reflection of the 

relative difficulties of observation; complex linkages 

among climate, terrestrial, and aquatic systems; and 

more limited long-term monitoring in aquatic environments (Heino and others 2009; Isaak and others 2010). 

Because stream temperature and flow represent fundamental constraints on physical and biological processes 

in aquatic systems, climate driven changes seem almost inevitable (Crozier and others 2008; Jonsson and 

Jonsson 2009; Pörtner and Farrell 2008). The direct constraints imposed by thermal tolerances and stream flows 

are relatively easy to anticipate, and most efforts to consider the implications of climate change have focused 

on these effects. Interactions of physical, biological, and 

ecological processes will add complexity (table 1) and 

will lead to indirect effects that are also likely to be important but more difficult to anticipate (Jager and others 

1999; Yarnell and others 2010).

Temperature and Hydrology as Primary 

Controls

Temperature and flow are not only very sensitive to 

climate forcing, but they also directly influence many 

of the physical and biological processes that are basic to 

aquatic organisms and communities (Olden and Naiman 

2009; Poff and others 2010). Temperature controls solute 

chemistry for oxygen, CO2, primary nutrients, trace elements, and toxic substances. Temperature and flow 

interact to influence mixing of water masses and create 

important thermal and solute gradients (e.g., Tiffan and 

others 2009). Temperature directly controls the rates of 

biochemical and physiological processes such as protein synthesis and disruption, photosynthesis, energy 

conversion, and respiration (Coutant 1999; Jonsson and 

Jonsson 2009; McCullough and others 2009). For cold 

blooded organisms, including most aquatic species and 

especially fish, temperature controls capacity for, and 

efficiencies of, activity, food consumption, metabolism, 

and growth (Coutant 1999; Pörtner and Farrell 2008). 

Ultimately, temperature defines the gradients of performance and absolute bounds for life for most aquatic 

organisms as well as rates of growth and timing of key 

life history events or transitions.

The influences of stream hydrology may be less 

constraining than temperature (many aquatic organisms, including fish, can survive in standing water), but 

they are still fundamentally important. Flow influences 

temperature (Isaak and others 2010; Meier and others 

2003) and acts as a primary control on the supply and 

exchange of materials such as oxygen, nutrients, organic matter and food, and metabolic wastes (Harvey and 

others 2006; Wrona and others 2006). Flow controls 

the volume of habitat in streams and, through interactions with temperature and channel characteristics, the 

structure, distribution, and spatiotemporal availability 

of distinct resources such as spawning, foraging, migration, and refuge habitats that are critical to completion 

of life cycles (Geist and others 2008; Northcote 1997; 

Yarnell and others 2010). Multiple authors argue, for 

example, that the distribution and success of stream 

salmonids is fundamentally shaped by the interaction of hydrologic and temperature regimes across the 

range of habitats required to complete life histories and 

the capacity for adaptation in life history necessary 

to persist in that context (Brannon and others 2004; 

Crozier and others 2008; McClure and others 2008). 

Ultimately, temperature and flow interact with geology, 

geomorphology, and terrestrial and riparian communities to create the mosaic of habitats and environmental 

conditions that represent the template for species occurrences and dynamics and the expression and evolution 

of aquatic biological diversity (Bisson and others 2009; 

McClure and others 2008; Poff and others 2001).

Implications for Native Fishes
USDA Forest Service RMRS-GTR-250. 2010. 13

Direct effects. Direct effects of climate will be those 

that impose limits on the distribution or performance of 

organisms and, by their nature, should lead to the most 

obvious responses. Temperature, for example, can impose clear limits through lethal conditions but can also 

influence the performance of individuals through scope 

for activity and growth (Coutant 1999), embryonic development, and physiological rates that are key to life 

history transitions (e.g., Jonsson and Jonsson 2009). The 

thermal tolerances of individual organisms reflect these 

basic processes. Thermal preferences, or the selection 

of thermal environments by individuals, often reflect 

conditions that are optimal for growth and relative performance or success across thermal gradients. Thermal 

tolerances, or the physiological responses to temperature, have been quantified for many fishes through 

laboratory trials (e.g., Richter and Kolmes 2005; Selong 

and others 2001), but they might also be inferred from 

observations of behavior, life history timing, and distribution along thermal gradients (e.g., Eaton and others 

1995; Isaak and others 2010). Important differences 

exist in thermal tolerances and efficiencies within and 

among families, genera, and species (Crozier and others 

2008; McCullough and others 2009; Selong and others 2001), although differences are generally smaller 

within than among taxonomic groups (McCullough 

and others 2009). Thermal tolerances help explain the 

broad patterns for species occurrences and persistence, 

and there are predictable patterns in species geographic 

ranges and longitudinal distributions within riverine 

networks and along thermal gradients tied to latitude 

and elevation (Nakano and others 1996; Paul and Post 

2001; Shuter and Post 1990). In general, salmonids and 

some sculpins are associated with colder (and higher 

Table 1. Examples of aquatic biological and ecological conditions and processes important in potential responses to 

climate change at different levels of biological organization. Although important effects are likely at all levels of biological 

organization, higher levels of organization are increasingly complex, leading to limited capacity to predict the direction or 

magnitude of changes with any certainty.

Level of organization

Conditions or processes likely to change

Molecular

Solute chemistry, rates of biochemical processes, structural constraints on proteins

Physiological

Metabolism (respiration, consumption, excretion, energy conversion)

Scope for growth and activity

Capacity for osmotic regulation

Organism

Changing rates of embryo development and timing of emergence relative to food and disturbance

Length, seasonal timing, and suitability of periods of growth or stress

Realized growth and growth-dependent life history, age of maturation, fecundity, and migration. Potential changes 

in size and timing for migration, growth for overwinter survival, development of novel combinations of conditions 

with unpredictable results

Migration success linked to physiological exhaustion with high temperature or low water migration barriers

Increasing probability of acute stresses and lethal temperature or flow conditions for some coldwater species, 

increasing opportunity for others

Population

Changing rates of mortality, reproduction, and recruitment and inherent population growth rates

Changing frequency of catastrophic mortalities

Shifting distribution, creation, and loss of suitable habitats; changing access to new habitats

Changing size, geometry, and connection of existing habitats

Community

Altered rates of predation, competition, parasitism, and disease

Changing forage availability

Changing abundance, productivity, and structure of communities

Ecosystem

Interactions of changing disturbance and habitat size/geometry

Changing structure of communities with novel landscapes

Complex feedbacks with human adaptation such as water development with expanding agricultural growing 

seasons or demands linked to population growth and increasing storage needs

Counterintuitive results and surprise, trophic cascades
14 USDA Forest Service RMRS-GTR-250. 2010.

elevation or latitude) waters, while minnows, suckers, 

and sculpins are often found in warmer waters and mid 

to lower elevations, although all of these species can 

range widely through a system depending on season 

(Crozier and others 2008; Reeves and others 1998). 

Many of the introduced species that include basses, 

sunfishes, perches, and catfishes are commonly found 

in the lowest elevation and warmest waters across the 

western United States (Lee and others 1997).

Temperatures that commonly exceed physiological 

thresholds or lethal limits will presumably represent 

relatively hard limits to species occurrence, although 

variation in life history and behavior, such as the increasing exploitation of thermal refugia, may mitigate 

hard constraints (e.g., Crozier and others 2008; Keefer 

and others 2009). Organisms and populations that occur near, or more frequently encounter, their thermal 

limits, however, seem more likely to fair poorly in 

a warming world. Chronic warming should lead to 

increased mortalities and shifting habitat distribution or 

range limits (presumably northward or upstream; e.g., 

Jonsson and Jonsson 2009; Rieman and others 2007; 

Shuter and Post 1990). Isaak and others (2010) confirmed that shifts in thermal habitats were occurring but 

also found that effects on species can differ dramatically within the same river network. For a species like bull 

trout, which is often confined to the coldest headwater 

streams, warming trends may cause a net loss of habitat (estimated to be 8 percent to 16 percent per decade; 

Isaak and others 2010) because areas are not available 

farther upstream to replace those that are becoming unsuitably warm (figure 16). But for species like rainbow 

trout, which are constrained by temperatures that are 

too cold in upstream areas, warming trends may merely 

shift habitats toward higher elevations without causing 

a net gain or loss in the total amount of habitat (figure 16). Recent episodes of mortality in adult salmon 

linked to unusually high temperatures and low flows 

Figure 16. Shifts in the distribution of thermally suitable 

habitat for (a) rainbow trout and (b) bull trout 

spawning and early juvenile rearing in the Boise 

River basin from 1993 to 2006 due to long-term 

climatic trends. Mean summer stream temperatures 

greater than 11 oC and less than 14 oC were used 

to delineate suitable habitats for rainbow trout; 

whereas temperatures less than 10 oC were used for 

bull trout (from Isaak and others 2010).
USDA Forest Service RMRS-GTR-250. 2010. 15

in the Klamath (Bartholow 2005) and Columbia rivers 

(Coutant 1999; Keefer and others 2007), are evidence 

that climate change is starting to cause acute thermal 

stress near downstream distribution limits and be emblematic of future changes in many populations.

At finer spatial scales, thermal tolerances may be imprecise predictors of species occurrences or abundances. 

For example, the probability of finding juvenile bull 

trout often declines dramatically in streams that exceed 

summer mean temperatures of 10 °C to 12 °C (Dunham 

and others 2003; Isaak and others 2010), but individuals 

still sometimes occur in much warmer water. In other 

cases, species may persist in streams that commonly 

exceed their perceived thermal limits (Tiffan and others 2009; Zoellick 1999) perhaps because of increased 

availability of food, lack of competition with other species, or adaptations that better exploit thermal refugia 

or shift timing of life history transitions (Crozier and 

others 2008; Jonsson and Jonsson 2009). Temperature 

clearly has an important influence on general limits of 

distributions, but the complexity of interactions with 

other biological and physical processes are also important to consider.

Flow also may directly constrain the distribution and 

performance of fishes based on timing and volume of 

runoff, water velocities, and their effects on suitability 

of habitats for incubation, holding, foraging, and migration. Depending on life history stage, the frequency of 

extreme events associated with stream drying (Labbe 

and Fausch 2000) or the timing and distribution of 

flooding and scour events can be important (Fausch and 

others 2001; Jonsson and Jonsson 2009; Montgomery 

and others 1999).

Interactions and indirect effects. Important, but less 

direct, climate effects emerge through the interaction of 

biological and ecological processes that are influenced 

by temperature and flow at the scale of individual organisms, populations, habitats, and streams. For example, 

temperature fundamentally controls potential growth, 

but because the responses are nonlinear and depend 

on the amount of food, activity, and past conditions, 

warming may lead to increases or reductions in growth 

(Coutant 1999; Jonsson and Jonsson 2009; Richter and 

Kolmes 2005). Changes in growth are in turn linked to 

changes in age and timing of maturation, fecundity, and 

reproductive potential (Holtby 1988). Temperature can 

influence reproductive success through the rates of embryo development and timing of emergence, dispersal, 

and migration in relation to seasonal flooding or scour 

(Fausch 2008; Fausch and others 2001); the availability of food following emergence; the extent of seasonal 

growth; and the timing of migrations or critical life 

history transitions (Brannon and others 2004; Coutant 

1999; Crozier and others 2008; Jonsson and Jonsson 

2009; Yarnell and others 2010).

Because the demographic growth rate of a population 

is ultimately an integration of reproduction and survival 

of individuals, stream temperatures can constrain the 

occurrence and distribution of populations well within 

their absolute physiological limits (see Beever and others [2010] for a relevant discussion regarding a small 

mammal), but the level of that constraint will vary. The 

observance of broad thermal tolerances has commonly 

led to predictions of local population declines and the 

general expectation of contracting distributions for native salmonids across large regions (Flebbe and others 

2006; Jonsson and Jonsson 2009; Rahel and others 1996; 

Rieman and others 2007). In reality, the local responses 

will depend on both species and environmental context. 

The interaction of temperature and flow directly influence conditions in migratory corridors for both juvenile 

and adult salmonids, but the capacity of individuals to 

continue to exploit existing or new food resources or 

thermal refugia and for populations to compensate for 

increased mortality can exacerbate or ameliorate the 

immediate effects (Crozier and others 2010). Salmon 

biologists have long argued that deteriorating flow and 

temperature conditions have negatively influenced travel rates and survival of migrating smolts (ISAB 2007), 

but some populations have fared better than others in 

response (Connor and others 2005; Williams and others 

2008). Some salmonids, such as cutthroat trout in highelevation streams, that are commonly limited by low 

water temperatures or short growing seasons (Coleman 

and Fausch 2007; Harig and Fausch 2002) could 

benefit from warming. Rainbow trout and steelhead 

near the species’ southern limit in California appear to 

be highly vulnerable to climate change (McCarthy and 

others 2009) while rainbow trout in central Idaho may 

be influenced relatively little (Isaak and others 2010). 

Highly productive populations or those that can find, 

gain access to, and exploit alternative habitats or refugia to seasonal climate constraints might also fare better 

than anticipated.

Community and ecosystem effects. Climate change 

will also influence native and introduced fishes through 

more complex interactions at population, community, 

and ecosystem levels of organization. Interactions 

among species and between species and changing 

landscapes, and even adaptation within species and 

populations, could become important (Crozier and 

others 2008; Williams and Jackson 2008; Yarnell and 
16 USDA Forest Service RMRS-GTR-250. 2010.

others 2010). Some changes in growth, productivity, 

abundance, and distribution associated with warming 

are likely to occur with many of the organisms in any 

community. Because those changes will vary among 

species, relative abundances and even the composition 

of communities in streams may shift (e.g., Hilborn and 

others 2003). The availability of species serving as forage and the numbers, consumption rates, and activity 

levels of those acting as predators or competitors may 

change as well. Petersen and Kitchell (2001) used bioenergetic models to show that warming of 1 °C to 2 

°C in the Columbia River could result in a 26 to 96 

percent increase in consumption of juvenile salmon by 

northern pikeminnow. Others suggest that longer growing seasons and warmer waters will allow the effective 

expansion in distribution, abundance, and influence 

of other potential competitors, disease, and predators (Hari and others 2006; ISAB 2007). Warming in 

the Columbia River has been associated with a rapid 

expansion in populations of American shad, for example, that are anticipated to consume zooplankton prey, 

which are important for growth and survival of migrating juvenile fall Chinook. Northern pikeminnow that 

may also act as important competitors with juvenile salmonids in smaller tributary streams might be expanding 

further into upstream salmonid rearing areas (Reese and 

Harvey 2002). Introduced parasites and diseases may 

expand with warming temperatures and could play an 

increasingly important role as additional constraints 

on the distribution of some species (Franco and Budy 

2004; Hari and others 2006).

The interaction of temperature with flow and other 

physical processes may lead to new patterns of disturbance that will influence the resilience and persistence 

of broader habitat and population networks. Changing 

flow regimes, more extreme weather events, and increasing frequency of large wildfires suggest important 

changes in the extent, frequency, and magnitude of 

disturbance for aquatic environments (McKenzie and 

others 2004; Isaak and others 2010). The aquatic ecosystems associated with streams and rivers of the Rocky 

Mountains evolved in highly dynamic landscapes. 

Wildfire, flood, drought, mass erosion, glaciation, and 

volcanism have been important in shaping these systems over ecological and evolutionary time scales 

(Bisson and others 2009; Kirchner and others 2001; 

McPhail and Lindsay 1986; Waples and others 2008). 

In an ecological and evolutionary sense, disturbance 

and even radical changes in climate are nothing new. 

But in many cases, the fundamental mechanisms that 

are contributing to the resilience and adaptive capacity 

of native populations (i.e., connectivity among diverse 

habitats and populations, local adaptations, and the 

broad expression of genetic and phenotypic diversity) 

have been eroded or lost through the myriad effects of 

human development, habitat loss and fragmentation, 

and the introduction and continuing invasions of new 

species (Bisson and others 2009; Rieman and Dunham 

2000; McClure and others 2008). For some species and 

populations, climate change could exacerbate fragmentation (Fagan 2002; Rieman and others 2007), leading 

to smaller and more isolated habitats and, simultaneously, to larger more frequent disturbances (Isaak and 

others 2010). In fact, Jentsch and others (2007) argue 

that uncharacteristic disturbance will play the dominant role in structuring ecosystem response to climate 

change as some species and populations that once flourished in a complex and highly dynamic world may no 

longer have the necessary resilience.

Biological adaptation may help. Most of the efforts 

to consider the implications of climate change assume 

that species and populations will continue to use and 

respond to the environment as they have in the recent 

past. In some instances, biological adaptation to changing environments could mitigate some of the challenges 

organisms face. Change in landscapes, watersheds, and 

ecological systems that support native fishes is nothing new. In some cases, these changes have been rapid 

and dramatic such as those linked to large wildfires. In 

others, they have been more gradual as climates have 

shifted in the past. Large-scale dispersal from post-glacial refugia (McPhail and Lindsey 1986) and evolution 

through natural selection have played central roles 

in the adaptation of native fishes and communities to 

changing environments and climate in the past (Waples 

and others 2008). The pace of change and the capacity for resilience in many populations is arguably much 

different than it was through most of the natural history 

of the species and communities we manage today, so 

the capacity for rapid adaptation has become an important consideration of ongoing research (e.g., Williams 

and others 2008). There is growing evidence that many 

fishes can adapt relatively quick to changing conditions 

through behavioral or phenotypic plasticity and rapid 

evolution (Crozier and others 2008), although evolutionary scope varies among different traits (McCullough 

and others 2009).

Many salmonids can colonize or exploit new habitats 

almost as they become available (Isaak and others 2006, 

2007; Milner and others 1987, 2000, 2008). Species 

like bull trout that exhibit extensive movements linked 

to spawning, foraging, and overwintering appear to be 

flexible in their use of distinct environments (Brenkman 
USDA Forest Service RMRS-GTR-250. 2010. 17

and others 2007; Volk and others 2010). If recent patterns of ranging or foraging are no longer profitable, 

they may have the capacity to shift patterns of movement until they find ones that are. Chinook salmon have 

been observed to use new spawning habitats created 

by debris flows that are not in immediate proximity 

to traditional spawning sites (Isaak and Thurow 2006; 

R. Thurow, personal communication of unpublished 

data). Similarly, bull trout that are blocked from past 

spawning streams can exploit new streams (Neraas and 

Spruell 2001). Even though climate change may reorganize the distribution and availability of critical habitats, 

some species and populations may be able to effectively 

exploit what emerges, as long as suitable habitats still 

exist and are accessible.

Fishes also have some capacity to adapt to change in 

place. Often, multiple life histories exist within a population or among several closely allied populations. Some 

may be favored by new conditions while others are not. 

Hilborn and others (2003) summarized long-term variation in relative success of three distinctly different life 

histories for sockeye salmon in a large lake system in 

response to climate variability. The expression of diversity in life history stabilized overall sockeye production 

even as individual stocks responded to changing conditions. Plasticity or flexibility in life history can also 

emerge within individual populations over short time 

scales (e.g., within a single generation). Because growth 

and other population responses are linked, changing 

growth rates can influence population dynamics and 

resilience. For example, rainbow trout growth and maturation rates increase substantially in warmer post-fire 

streams, which is associated with earlier maturation and 

could increase population growth rates and recovery in 

response to an initial disturbance (J. Dunham, personal 

communication of unpublished data). Heck (2007) and 

Kennedy and others (2003) observed similar responses 

in other salmonids. Distinct life histories could be lost 

with changing climates (Beechie and others 2006), but 

if new environments provide conditions that are within 

the range of possibilities for a species, the phenotypic 

plasticity that allows populations to adapt quickly may 

still exist even if it has not been important in the recent 

past (e.g., Healey and Prince 1995).

Finally, evolution through natural selection could be 

important as well. There is concern that rates of climate 

change will overwhelm capacity for evolution through 

natural selection in remnant populations (e.g., Beever 

and others 2010; Crozier and others 2008). But there 

is also growing evidence that important evolution can 

occur in fishes within 10 to 20 generations (40 to 80 

years for species with a 4-year generation time; Hendry 

and others 2000; Stockwell and others 2003; Waples 

and others 2007). Changes in spawning and migration 

timing and the expression of distinctive life histories 

required to exploit different environments may be particularly responsive to natural selection (Crozier and 

others 2008; Quinn 2005). Recent work suggests that 

fall Chinook salmon in the Snake River, for example, 

are evolving novel rearing and migration timing in response to changes in flow and temperature caused by 

water development over the last 40 years (Williams 

and others 2008). There is also some evidence that local adaptation may be possible in thermal tolerances or 

growth efficiencies for some salmonids, but the capacity for rapid evolution in these traits is unclear and may 

be more limited than with others (McCullough and others 2009).

Summary

The bottom line is that change is happening and will 

continue, though it will not always be intuitive or easily predicted. For many fish species, suitable habitats 

defined by temperature, flow, and other physical or 

biotic conditions may simply shift in location (e.g., to 

higher elevations or latitudes). Some species, populations, and communities may be able to track these 

changes and simply “relocate,” but barriers to dispersal and migration will limit many others. Interactions 

between biological and physical process may also lead 

to results that are difficult to anticipate, even for wellstudied species. Jager and others (1999), for example, 

used individual-based bioenergetic and population 

models to show that changes in foraging, growth, and 

scour could lead to counterintuitive results in the distributions of co-occurring brown trout and rainbow trout 

in streams of the Sierra-Nevada mountains. In other 

cases, the interaction of climate change, heterogenous 

landscape responses, shifting species distributions, and 

the new suite of potential biotic and physical interactions will lead to novel environments, trophic cascades, 

and communities with no natural precedent and little 

foundation for prediction (Williams and Jackson 2008; 

Williams and others 2007). Adaptation to changing environments through natural selection and plasticity is 

possible, but the speed and capacities for adaptation 

are not well known and may be outpaced by the rate of 

climate-driven environmental change.
18 USDA Forest Service RMRS-GTR-250. 2010.

Already, there is considerable literature that offers 

guidance on how natural resource managers might respond to climate change (e.g., Furniss and others 2010; 

Hodgson and others 2009; Millar and others 2007; 

Nelitz and others 2009; Noss 2001; Welch 2005). Some 

documents are focused on particular ecosystem issues 

such as coral reef bleaching (West and Salm 2003), 

reorganization in montane forests (Millar and others 

2007), and salmon conservation (Bisson 2008; ISAB 

2007; Schindler and others 2008). But common concepts and elements of conservation and restoration 

management emerge repeatedly in much of the discussion (Mawdsley and others 2009).

Two common themes in management response to climate change are “adaptation” and “mitigation” (Millar 

and others 2007; USDA Forest Service 2008). The 

Forest Service Strategic Framework for Responding to 

Climate Change (USDA Forest Service 2008) defined 

adaptation as “actions to adjust to and reduce negative 

impacts of climate change on ecological, economic and 

social systems.” The Framework discussion considered 

actions that may support conservation of existing ecological elements as well as transition to fundamentally 

new conditions. We follow others in distinguishing facilitation as action to move the system to a new state 

and adaptation as action to conserve an existing state. 

In this context, we define adaptation as management 

to help populations, species, communities, and ecosystems absorb climate change with limited alteration of 

the structures, functions, or services we value. The terminology can be confusing, but from the perspective of 

native fish conservation, we suggest that adaptation is 

essentially an effort to conserve species, populations, 

communities, or fisheries as, and where, they currently exist. We might focus, for example, on actions that 

would help ensure persistence of a particular species, 

population, or fishery by enhancing resistance and resilience to changing climate and its effects.

Mitigation in this context reflects the efforts that 

governments, agencies, and the public make to reverse 

or slow the causes of climate change such as energy 

conservation, greenhouse gas reductions, and carbon 

sequestration. Although land and forest managers are 

heavily engaged in mitigation efforts (e.g., wildfire 

management, reforestation, and stand management 

for carbon sequestration) that can directly influence 

aquatic ecosystems, this synthesis focuses primarily on 

adaptation strategies for aquatic ecosystems. Rieman 

and others (2010) consider some issues in coordinated 

management of terrestrial and aquatic systems that may 

emerge in response to management that is focused on 

mitigation.

In the remainder of this section, we outline the general guidance for adaptation and facilitation in response 

to climate change in aquatic systems in five topic areas: (1) enhance resistance and resilience; (2) prioritize 

limited management resources; (3) facilitate transitions 

to new populations or communities when appropriate; 

(4) develop local information; and (5) coordinate management among entities, resources, and actions (table 

2). We consider each of these in turn.

Enhance Resistance and Resilience

Resistance and resilience are ecological concepts 

that reflect the capacity of natural systems to absorb 

or recover from environmental change or disturbance 

and thus to persist into the future. There is considerable literature devoted to the characteristics of resistant 

and resilient biological systems (e.g., Harrison 1979; 

Holling and Meffe 1996) and management efforts to 

conserve or restore those characteristics in the face of 

consumptive exploitation (e.g., Healey 2009), habitat 

loss and disruption (e.g., Bisson and others 2009), and 

changing environments (e.g., Healey and Prince 1995; 

Hodgson and others 2009). The terms are often linked 

and are sometimes used interchangeably but are, neverthe less, the essence of managed adaptation to climate 

change.

In our perspective, resistance represents the capacity of important habitats, populations, or communities 

to absorb an environmental shift or disturbance with 

limited or negligible deflection in abundance, structure, or function (figure 17; e.g., West and Salm 

2003). For example, some streams may show limited 

response to increasing air temperatures because they 

are well buffered by the influence of groundwater or 

snowmelt (Boxall and others 2008; Mohseni and others 1999). Alternatively, some populations may not 

change in response to streams that warm substantially 

What Can We Do About It?
USDA Forest Service RMRS-GTR-250. 2010. 19

Table 2. Examples of management options to support adaptation of salmonid fish populations and stream communities to the 

effects of climate change (see also Bisson 2008; Furniss and others 2010).

Management option Rationale or anticipated effect

Enhance resilience and resistance

Reduce non-climate stresses

Maintain or restore instream flows and Maximize available habitat, increases terrestrial interactions, and buffers

natural hydrologic regimes streams against temperature increases and the loss of habitats during low 

flow events.

Maintain forest and vegetative cover to Mitigate loss of snow pack storage, earlier runoff, and reduced summer low

reduce rain-on-snow flooding and delay flows. Conserves forest, wetland, and riparian areas that tend to store 

snow melt water for later summer base flows.

Maintain or restore riparian, floodplain, Maximize stream shading, bank stability, terrestrial food inputs, and

and wetland conditions and connections recruitment of woody debris that helps form diverse habitat; enhance

with streams; reintroduce beaver water storage for delayed summer discharge during warm, low flow 

periods.

Protect and restore critical or unique Ensure that nodes connecting seasonal or complimentary habitats or

habitats that buffer survival during refugia do not become bottlenecks to production. Off-channel habitats,

vulnerable periods seasonally or in the spring brooks, and seeps important as early rearing environments; flood

life history or thermal refugia and stream segments important as connections; 

broader expanses of habitat are examples.

Disconnect roads from the drainage network, Buffer the effects of peak flow events.

and remove roads and dikes that constrain 

or disconnect channels and flood plains

Limit or stop introduction and expansion of Reduce potential competitors, predators, diseases, and hybridization that

non-native species may constrain habitat capacity, individual growth rates, and survival.

Eliminate or control pollutants or Reduce stresses associated with eutrophication, toxic materials, or other 

contaminants effects on growth, productivity, and survival.

Conserve and expand the size of habitat networks and migratory connections

Remove or modify barriers to fish Allow individuals to move freely and track suitable habitat distributions

movements or re-colonize disturbed areas. Allow full expression of alternative life 

histories and increased productivity of migratory forms.

Maintain or reconnect large networks of Larger habitats support larger populations that are less susceptible to

habitat extreme events and loss of genetic diversity; they are also more diverse 

and capable of supporting other needs outlined above and below.

Conserve genotypic/phenotypic diversity

Conserve or restore a diverse Provide biological resilience and increase odds that some populations or

representation of habitats across river individuals will be adapted to future conditions or have the capacity to

basins evolve.

Conserve or restore large networks Maintain large population sizes to minimize loss of genetic variability and

(see above) adaptive potential.

Prioritize

Clarify goals and values Minimize confusion and conflict among disciplines and agencies, increasing 

the chances of recognizing conflict and opportunity before the fact.

Focus on populations as units of Ensure that some populations are as resilient or complete as possible rather

conservation than moving all incrementally and leaving all vulnerable to extreme events.

Weigh vulnerability Focus on the greatest benefits for the least cost; avoid lost causes.

Consider relative values and balance Ensure maintenance of some populations and as much diversity as possible

resilience, representation, and in the face of extensive change or large, catastrophic events.

redundancy

Take no action Conserve limited conservation resources so they can be expended in the 

most beneficial areas.
20 USDA Forest Service RMRS-GTR-250. 2010.

Table 2. Continued.

Management option Rationale or anticipated effect

Facilitate transition to new states

Human-assisted migration Transport individuals to existing but otherwise inaccessible habitats or 

refugia to maintain gene flow, establish or re-establish self-sustaining 

populations, and buffer potential for catastrophic losses.

Remove barriers to invasion Where native species can clearly no longer persist, allow colonization by 

new species that may be better suited to new environments and still 

provide some ecological function and value.

Introduce new species See “Remove barriers to invasion.”

Develop local information

Understand context Recognize regional and local trends in climate that provide a context for 

the past and future changes that are relevant to the local systems of 

interest.

Synthesize existing information Understand changes or trends in habitats or populations that have already 

occurred in local or nearby representative systems. Indentify important 

gaps in information for further work.

Model to fill gaps Extrapolate likely or potential changes based on current models. Use the 

models as a basis for thinking about how change may occur but recognize 

limitations and uncertainty.

Monitor and document trends Strengthen local knowledge; test, validate, or reject models, predictions, and 

hypotheses. Review, revise, and refine management.

Coordinate efforts

Across disciplines and resource values Recognize potential conflicts and opportunities to focus high-resolution 

analysis where actually needed; mitigate activities and recognize 

opportunities to leverage common values and supporting work.

Across agencies and jurisdictions Maximize environmental range of suitable habitats used by native species 

and decrease chances of hybridization in some instances.

With the public Build understanding of ecological values in aquatic systems and support for 

the actions and tradeoffs that inevitably must be addressed.

Figure 17. Resistance and resilience reflect 

the capacity of a population to absorb 

and recover from disturbance.
USDA Forest Service RMRS-GTR-250. 2010. 21

if temperatures remain within the range of thermal 

tolerances. Some fish populations may show limited response in overall number to increased mortality caused 

by environmental change that is substantial because of 

strong density-dependent compensation in juvenile recruitment (Crozier and others 2010).

Resilience can be viewed as capacity for, or rate of, 

biological and ecological recovery from change or disturbance that causes substantial reduction in abundance. 

Resilient populations, for example, retain the capacity 

to recover from overexploitation or short-term disturbances such as wildfires or floods (e.g., Dunham and 

others 2003; Rieman and Clayton 1997) even though 

population size or structure might have been depressed 

to only a fraction of pre-disturbance levels. Resilience 

might also be reflected in the capacity of populations 

to adapt to longer-term changes in environments by altering behavior, life history, or other characteristics in 

ways that allow them to recover and persist even though 

conditions remain fundamentally different than in the 

past. As suggested earlier, biological adaptation that 

contributes to the resilience of a population or species 

might occur either through behavioral or phenotypic 

plasticity or through natural selection and rapid evolution (Healey 2009; Waples and others 2009).

Much has been written about the conditions and 

characteristics of resistant and resilient systems, particularly in the contexts of conserving and restoring 

commercial fisheries (e.g., Healey 2009), conserving 

biological diversity in response to managed landscapes 

(e.g., Bisson and others 2009; Duffy 2009; Holling and 

Meffe 1996), changing fire regimes (e.g., Bisson and 

others 2003; Dunham and others 2003; Gresswell 1999; 

Rieman and Clayton 1997), and changing climate (Noss 

2001). Based on that literature, adaptation to restore or 

conserve resistant and resilient native fish populations 

might be distilled to four key areas: reduce non-climate 

stresses, conserve and expand critical habitats, reconnect streams and habitats, and conserve genetic and 

phenotypic diversity. These are considered below.

Reduce non-climate stresses. Decreasing traditional 

stresses of aquatic systems that contribute to reduced 

growth or survival of individuals or reduced capacity 

of habitats can lead to increased potential growth rates 

in local populations and increased resistance and resilience to climate change effects (Wooldridge and Done 

2009). These are actions that fisheries biologists have 

focused on for decades. Populations that are productive and have the capacity to absorb fishing are likely 

to be resilient and resistant to changing environments 

that impose additional mortality or restrict habitat 

capacity. Efforts to conserve watersheds and habitats 

that already are productive and highly functional are 

likely to be the most effective and efficient steps, but 

restoration of watershed processes that support the 

creation and maintenance of complex and productive 

habitats can be important as well (Beechie and others 

2010). Habitat degradation has been a central problem 

in conservation and management of native fishes and 

fisheries, so opportunities to restore more productive 

environments are often widespread. Efforts to restore 

stream flows (Van Kirk and Benjamin 2001); remove or 

mitigate contaminants, pollutants, and other extraneous 

sources of mortality (e.g., Peterson and others 2010); 

reconnect streams with their floodplains; and restore 

riparian functions (especially those important to flow 

and temperature) are all examples of important work in 

the region. Rahel and others (2008) suggest that efforts 

to control invasive species that may act as predators 

and competitors is also an important tool, and substantial interest has focused on this issue in recent years 

(e.g., Eby and others 2006; Fausch and others 2009; 

Muhlfeld 2009; Peterson and others 2008; Shepard 

2002). Reducing the threats of invasion through direct 

reductions of source populations or through construction of barriers to dispersal may be more effective than 

control of widely established populations (Meyer and 

others 2006). There are, however, important tradeoffs 

to consider with use of barriers designed to limit the 

expansion of non-native species because these will also 

limit dispersal or expansion of native species (see “Reconnect streams” below).

Conserve and expand critical habitat. Larger habitat areas or capacities will help expand the size and 

diversity of populations and the diversity of communities (Hodgson and others 2009; Neville and others 

2009). A growing body of work shows that salmonids 

are more likely to persist indefinitely in larger or more 

complex habitat networks (Fausch and others 2006). 

Larger networks are more likely to provide complementary habitats required to complete life histories; internal 

complexity and area needed to absorb catastrophic 

disturbances without loss of the entire population; and 

greater genetic and phenotypic diversity that can facilitate adaptation (Fausch and others 2009; Hodgson and 

others 2009; Neville and others 2009). In some cases, 

climate change may result in the expansion of suitable 

habitats, but for many coldwater and headwater species 

and populations, changes will result in no net change 

or further shrinking of what are already small, highly 

fragmented habitat networks (Fausch and others 2006; 

Isaak and others 2010; Rieman and others 2007). For 
22 USDA Forest Service RMRS-GTR-250. 2010.

example, many native salmonids in Rocky Mountain 

streams are already constrained to colder headwater 

reaches and are commonly found in remnant stream 

networks that consist of a few kilometers at most (e.g., 

Brown and others 2001; Fausch and others 2006; Harig 

and Fausch 2002). For a species such as bull trout, 

which appears to be constrained at its upstream extent 

by stream gradient or size, many populations are essentially against the wall (Isaak and others 2010; Rieman 

and others 2007). Species that occupy shrinking habitats sensitive to warming or more extensive dewatering 

could change even more quickly than anticipated if 

disturbances linked to fire, large storms, and drought 

become more frequent or extensive (Brown and others 

2001; Isaak and others 2010; Jentsch and others 2007).

Conserving the size and extent of existing, high-quality habitats and habitat networks will be an important 

step wherever possible. But expanding those by removing constraints or restoring processes that create 

or maintain productive habitats adjacent to good condition areas could be important to gain areas large enough 

to absorb the effects of a changing climate. Important 

questions still exist about how much area is enough to 

conserve productive, resilient populations. Maintenance 

of genetic diversity and adaptive potential will require 

enough habitat to support hundreds of adults at a minimum (Rieman and Allendorf 2001), but persistence in 

varying environments may require considerably more. 

Some salmonids have persisted in habitats that are 

limited to a few kilometers of stream (Hastings 2005; 

Morita and Yamamoto 2002), while others may need 

an order of magnitude more (e.g., Dunham and others 

2002; Dunham and Rieman 1999; Fausch and others 

2006). There are few rules of thumb to guide decision 

making for all the species and communities important 

across the Rocky Mountains, but conserving and creating habitat networks larger than a few kilometers 

wherever and whenever possible will likely be a fundamental step in climate adaptation (e.g., Hodgson and 

others 2009).

Reconnect streams. Removal of barriers to fish 

movement can be an important step in the expansion 

of habitat networks, but it will also help re-establish 

the full expression of migratory life histories and connections among populations. Many fishes native to 

the Rocky Mountain West, particularly the salmonids, 

express a variety of life history strategies that include 

a broad range of movements among complementary 

habitats (Brenkman and others 2007; Northcote 1997; 

Rieman and Dunham 2000). Juvenile migration to productive rearing areas and adult migration and homing 

to natal areas for anadromous, fluvial, and adfluvial life 

histories have been widely documented and explored in 

the salmonid literature for decades (e.g., Quinn 2005). 

Expression of migratory behavior can contribute to resilience of populations through larger adult body size, 

fecundity, recruitment, and potential population growth 

rates (Peterson and others 2007; Rieman and Dunham 

2000). It can also represent a diversity of strategies that 

effectively hedge against disturbance (e.g., Rieman and 

Clayton 1997) and environmental variability (Hilborn 

and others 2003) likely to accompany climate change. 

Connection and migratory life histories are also key 

to linkage among populations (Dunham and Rieman 

1999). Metapopulation theory has received substantial 

attention in the ecological literature over the last two 

decades, and there is growing evidence that movement 

among populations, most likely through juvenile dispersal or straying of migrant adults, is important to gene 

flow, maintenance of genetic diversity, demographic 

support, and recolonization of disturbance-prone or recently available habitats (Dunham and Rieman 1999; 

Fausch and others 2006; Isaak and others 2007; Letcher 

and others 2007; Neville and others 2006, 2009).

There are concerns that removal of barriers will facilitate invasions of non-native species (Fausch and others 

2006, 2009). Non-native species are an important threat 

to persistence of many native fishes throughout the 

Rocky Mountain West (Eby and others 2003; Fausch 

and others 2006, 2009; Rahel and others 2008), and the 

intentional use of migration barriers may be an important strategy against non-native invasions that could be 

accelerated with changing climate (Jackson and Pringle 

2010; Rahel and others 2008). The tradeoffs with isolation, however, are clear as well and must be carefully 

considered by managers who are contemplating intentional isolation (Fausch and others 2006; Peterson and 

others 2008). If isolation is a serious option, the area 

and quality of the habitat to be isolated and the potential 

influences of climate change on those characteristics 

will be important constraints to consider. Peterson and 

others (2008) provide one framework for assessing the 

tradeoffs.

Conserve genetic and phenotypic diversity.

Maintaining biodiversity is important to ensure the greatest possible capacity for natural biological adaptation to 

variable and changing environments. As we discussed 

earlier, adaptation can occur through varied success of 

distinct life history strategies, short-term plasticity in 

phenotypic or behavioral characteristics, and natural 

selection and evolution. Regardless of the mechanism, 

conservation and restoration of biological diversity are 
USDA Forest Service RMRS-GTR-250. 2010. 23

fundamentally important strategies for managers faced 

with climate change and other natural threats (Hodgson 

and others 2009; Levin and Lubchenco 2008; McClure 

and others 2008). Because the precise nature of change 

and the conditions that favor different life histories are 

largely unknown, conservation or representation of 

adaptive potential and the fullest range of genetic and 

phenotypic diversity possible is prudent. Maintenance 

of genetic diversity will depend largely on the size of 

local populations and potential for gene flow among 

populations (e.g., Neville and others 2009; Rieman and 

Allendorf 2001), so the extent and connection of habitats considered above will be important here as well. 

Genetic and phenotypic diversity will also depend on 

representation of populations (and their habitats) across 

diverse environments that reflect as much of the full 

genetic variation, local adaptation, and differential phenotypic expression as possible (Allendorf and others 

1997; McClure and others 2008; Healey 2009). If details 

of population biology are not available, representation 

of populations across distinct gradients of productivity 

and growth (see McGrath and others 2008), hydrologic 

regime (Beechie and others 2006), disturbance history 

(Waples and others 2009), or spawning habitat (Hilborn 

and others 2003) could be useful starting points.

Prioritize Limited Management 

Resources

One of the most important problems for conservation 

and restoration in the context of climate change will be 

the allocation and effective prioritization of limited 

management resources. In general, the list of known or 

anticipated habitat and watershed problems is far larger 

than available funding or logistical or technical capacity 

can address. This is a widely acknowledged problem 

for conservation biology, where the concepts of triage and prioritization of limited resources have been a 

focus of research, management application, and some 

debate (e.g., Allendorf and others 1997; Bottrill and 

others 2009). It is a challenging problem for fisheries 

and aquatic managers in the West as well. The prioritization of some habitats, populations, or streams over 

others can be difficult because it may imply giving up 

on some to focus on others. Focusing financial resources 

rather than giving everyone a piece of the pie can also 

be subverted by political or professional provincialism. 

The alternative of spreading limited resources across 

as many habitats or populations as possible, however, 

may only guarantee that all remain compromised and 

vulnerable to the challenges of climate and other environmental change in the future (Bottrill and others 

2009; Frissell and others 1997; Reeves and others 1995; 

Rieman and McIntyre 1995).

There is an extensive literature in conservation 

biology and restoration ecology considering prioritization (e.g., Fausch and others 2006; Groves 2003) and 

a growing effort to place it in the context of climate 

change (Nelitz and others 2009; Noss 2001). A common element in many discussions has been the concept 

of risk (Fasuch and others 2007; Francis and Shotton 

1997) or vulnerability, which can be defined as the 

probability that something of significant value will be 

lost as a result of a potential change. Clarifying risk or 

vulnerability can help identify priorities, but requires 

some sense of the relative value in different watersheds, 

habitats, species, populations, or other targets of conservation or restoration management and the sensitivity 

or probability that they will be lost if change occurs (see 

the following Text Box).
24 USDA Forest Service RMRS-GTR-250. 2010.

Terms like vulnerability and risk have received 

considerable attention in the climate-adaptation literature. In the context of this report, we consider them 

to be essentially equivalent. Formally, risk has been 

considered the product of two conditions: the value

placed on a resource or condition of interest (i.e., the 

consequences of climate change) and the probability

of losing that resource if an event or change occurs 

(in this case, the effects of a changing climate; e.g., 

Wood and others 2008). Vulnerability has been defined as the product of sensitivity and exposure (e.g., 

Furniss and others 2010), where sensitivity reflects 

both value and the probability of change or departure 

in that value given change in climate, and exposure is 

the expectation or probability that a change of a given 

magnitude will occur. In this sense, risk and vulnerability might be used as shorthand for the chances 

of losing or substantially altering specific, important 

habitats or populations. If the probability for change, 

sensitivity to that change, or the value in question is 

low, risk or vulnerability will be low. If the probability, 

sensitivity, and value are high, vulnerability or risk will 

be high. Presumably, some knowledge of the natural 

resource values most sensitive to climate change can 

help managers prioritize limited funding and other management resources (see section on Prioritizing Limited 

Resources). Structured decision analyses or formal 

risk analyses provide quantitative frameworks to consider important tradeoffs. Actions that are anticipated 

to produce the largest benefit (reduction in vulnerability 

or risk) for the least cost (e.g., high benefit/cost relative 

to other alternatives) may be logical priorities.

Formal decision models are becoming more common in the natural resource literature with some 

examples now in regional aquatic or fish and wildlife 

issues (Marcot and others 2001; Peterson and others 

2008; Reckhow 1999). Models that quantify probabilities and the relationships among driving and dependent 

variables can require detailed information that may be 

unavailable or difficult or expensive to collect. A variety of approaches have been developed, however, 

that allow incorporation of data and established relationships as well as professional judgment (Peterson 

and others 2008). Whether decision models are better 

served by hard information or general experience can 

be debated, but an important point remains—any management decision is based on some model of how the 

world (or habitats and populations) works. That model 

may be a highly complex mathematical representation, 

a simpler argument of logic (e.g., if this, then this), or 

something in between. Regardless of the approach, 

all models start with some effort to articulate the basic logic and important relationships. That effort alone 

can be important as biologists and managers attempt 

to communicate their rationale, weigh tradeoffs, and 

identify critical uncertainties for further work. Influence 

diagrams are a useful tool in this process and can 

become a foundation for more quantitative models or 

discussion and refinement of logic that guides management actions.

In the figure below, we present an influence diagram that represents a “model” of persistence of a 

local fish population in response to climate. This model reflects our experience with native salmonids like 

cutthroat trout and bull trout in the Rocky Mountain 

West. We have outlined the interactions of natural and 

human-caused disturbances with habitat and population characteristics that we believe influence the 

resilience of populations in streams across the region 

(e.g., Dunham and others 2003; Dunham and Rieman 

1999; Fausch and others 2006; Isaak and others 2007, 

2010; Peterson and others 2008; Rieman and Dunham 

2000; Rieman and others 2003, 2007). The influence 

diagram focuses on population persistence or the 

probability that a local population of native salmonids 

will persist for some extended period given a set of environmental conditions and a particular climate. In our 

view, population persistence will be determined by essentially three conditions:

•  Population growth rate—reflects the capacity of 

the local population to absorb environmental stresses and resist or rebound following disturbance.

•  Adjacent populations—reflects the connectivity 

to surrounding populations that can be sources of 

gene flow and recolonization or rescue if the immediate population cannot maintain itself.

•  Exposure to a catastrophic event—reflects the 

chance that the population can be reduced to a very 

low level by a natural or human-caused disturbance, 

which increases as the population is restricted to a 

smaller area or size or as disturbances become larger or more frequent in relation to that area or size.

The conditions represented by each of these 

nodes will in turn be influenced by the conditions in 

the environment that define the extent and quality of 

available habitats; interaction with other species; and 

the frequency, magnitude, and extent of watershed 

disturbances that could threaten the persistence of a 

population confined to a limited area. By highlighting 

the nodes or conditions that can be directly influenced 

Considering Vulnerability
USDA Forest Service RMRS-GTR-250. 2010. 25

An influence diagram showing linkages among climate, environmental conditions, habitat, and the anticipated persistence 

of a local population of native salmonids. Vulnerability of a local population would be represented by the probability of 

persistence and could be combined with a measure of the “value” of that population to represent risk. The shaded nodes 

represent conditions that could be influenced by management and thus opportunities for action that support adaptation to a 

changing climate.

by management actions, it’s possible to see the underlying logic and assumptions that ultimately justify 

any adaptive actions. For example, removal of a migration barrier could lead to expansion or reconnection 

of the population with surrounding habitats and the reexpression of migratory life histories that could offset 

the effects of a changing climate on habitat quality or 

catastrophic disturbances.

In this example, we assume that the population is 

valuable for conservation purposes. Population persistence, however, could be modified by a utility function 

or measure of the value of the population to represent a measure of risk that could be used to compare 

across populations or management actions. A model 

like this, if fully specified through a series of underlying 

quantitative relationships implied by the arrows, could 

support a risk analysis or formal decision process 

where the user can modify inputs that reflect changes 

in climate or management and evaluate the relative 

change in risk that results (see Peterson and others 

2008 for an example). We don’t suggest that this model 

is the correct or only one, but rather that it is the logical 

framework underpinning much of the discussion about 

the alternatives for adaptation to climate change in this 

report. We suggest this as a starting point to stimulate 

thinking about the conditions and processes important 

to persistence of populations that biologists and managers work with across the region.
26 USDA Forest Service RMRS-GTR-250. 2010.

Clarifying risk also requires some sense of the feasibility of reducing that probability with the resources 

at hand. Issues of scale (how much is enough; Fausch 

and others 2006; Peterson and others 2008; Rieman and 

others 2007), spatial structure (where should efforts be 

in relation to each other and how they should be connected; Frissell and others 1997; Hodgson and others 

2009; Rieman and Dunham 2000), critical processes 

that constrain habitats or populations (what do you actually restore; Beechie and others 2010), and decisions in 

the face of uncertainty (Lawler and others 2009; Ludwig 

and others 1993; Pielke 2009) are also part of the broader 

discussion. There is no simple generalization for prioritization that lends itself to all situations (e.g., Hodgson 

and others 2009); and any strategy must reflect local 

knowledge, constraints, and conditions. However, there 

are common themes and concepts in much of the discussion that might help guide managers and biologists. We 

suggest the following points for consideration.

Clarify goals and associated values. Goals represent a vision of future natural resource conditions that 

reflect values society and managers place on those 

conditions (Beechie and others 2008). The values that 

guide management goals and objectives for aquatic systems and inland fisheries are diverse, but Fausch and 

others (2006) and Rieman and others (2010) suggested 

that they can be classified as evolutionary, ecological, 

and economic (see also Angermeier and others 1993; 

Beechie and others 2008). Evolutionary values may 

focus on evolutionary legacies and native biological 

diversity such as that represented by genetically pure 

populations of native cutthroat trout. Ecological values may focus on maintenance of ecological function 

and ecosystem services and capacities for resilience 

and adaptation to environmental change. Economic 

values may focus on economic return linked to tourism or sport fishing opportunity. These values clearly 

are not independent, but in managed systems, they are 

also not always simultaneously possible (Fausch and 

others 2006). For example, conservation of genetically 

pure cutthroat trout may require intentional isolation 

of a population threatened by invading brook trout or 

hybridizing rainbow trout (Fausch and others 2006; 

Rahel and others 2008). Alternatively, a focus on ecological function or economic contribution might view a 

hybridized population or a brook trout population that 

retains some resilience to disturbance or a fishery by 

virtue of larger network connections and the presence 

of migratory life histories as an acceptable result (see 

Fausch and others 2006, 2009; Peterson and others 

2008; Rieman and others 2010 for extended discussion 

of these issues). The point is not that one set of values is 

primary, but that different objectives that are seemingly 

allied under more general goals (e.g., conservation of 

aquatic ecosystems) can be in conflict (e.g., intentional 

isolation or transition to a new species better suited for 

the local environment) and may require different responses to conservation and restoration in the context 

of climate change. It is important to be clear about conservation goals and why they are important.

Populations as fundamental units of conservation. Management actions and restoration projects, in 

particular, are often relatively limited in extent (e.g., an 

individual habitat, stream, or road segment). But those 

actions will be biologically meaningful only to the extent that they contribute to increased size, resilience, or 

resistance of an entire population that may depend on 

several kilometers of stream or even an entire network 

of streams. A life history perspective might consider 

whether the habitats influenced by management actions 

are likely to be limiting to a local population where individuals must link spawning, rearing, foraging, and 

refuge environments to complete their life cycles (Bilby 

and others 2003; Lake and others 2007; Northcote 

and others 1997). A population perspective might ask 

whether the network of habitats represent a capacity 

that is large and internally complex enough to support 

populations that will be resilient and retain the capacity to adapt (e.g., Isaak and others 2010; Neville and 

others 2009; Rieman and others 2007). A population 

perspective also suggests that, to the extent possible, 

logical priorities might focus first on securing one network or population before moving on to others (Frissell 

and others 1997). Linked population and habitat models (Fullerton and others 2009; Honea and others 2009; 

Jorgensen and others 2009) are being explored as tools 

for prioritizing habitat conservation or restoration in a 

population and full life history context. The important 

point, however, is that to be effective, conservation and 

restoration will need to provide enough of the right 

habitat for complete populations to persist and hopefully flourish in the face of climate change. Essentially, 

it may be better to secure one or a few populations 

completely than to do a little work in many (Levin and 

Lubchenco 2008).

Consider relative vulnerability among populations 

and habitats. An important point from monitoring and 

model predictions is that some habitats and populations 

will be more vulnerable to climate change than others. 

Populations that occur on the margins of suitable habitat or in restricted habitat networks, for example, are 
USDA Forest Service RMRS-GTR-250. 2010. 27

probably more vulnerable than those in large networks 

that are well within suitable conditions or on the leading 

edge of habitats likely to improve in suitability. Streams 

that are buffered by ground water or hyporheic exchange may warm more slowly than those that are not. 

Streams now in transitional (rain-snow) hydrology are 

more likely to see a dramatic change in winter flooding 

than those that are well within current rain- or snowdominated systems (Beechie and others 2006; Hamlet 

and Lettenmeier 2007). There is considerable effort to 

refine climate-hydrologic-temperature predictions to 

help managers consider the variability in sensitivity to 

climate change (e.g., Isaak and others 2010; Mantua 

and others 2009; Nelitz and others 2009; Wenger and 

others 2010). But even without those models, managers 

and biologists often have a good sense of the sensitivity of their respective streams and populations. Systems 

that have fared well in unusually warm-dry years or that 

remain productive despite the climate challenges of the 

last decade may be particularly important to secure and 

expand as future cores most resistant to future change. 

Logical priorities are systems that are least vulnerable 

or where vulnerability can easily be reduced (e.g., removal of migration barriers) because they represent the 

best chances with the least investment.

Consider relative value among conservation units.

Deciding where to focus first has been a central topic 

in conservation biology that depends on values and the 

conditions that support resilience. A common shorthand 

includes the concepts of resilience, representation, and 

redundancy (Fausch and others 2006; Groves 2003; 

Scott and Csuti 1997). As we discussed, resilience 

implies something about the capacity to absorb or rebound from disturbance or change. We might focus on 

the most resilient or resistant habitats and populations 

first because they will have the best chance against future change and because they may also require a limited 

investment to secure (i.e., greatest benefit for the least 

cost). Representation refers to the distribution of biological and ecological diversity among populations or 

conservation units if there is opportunity to focus on 

more than one. We’ve considered the role of diversity in 

hedging, plasticity, and adaptation with disturbance and 

change. In this case, we might favor habitats or populations that would reflect distinctly different life history 

types or populations that exist in distinctly different environments under the assumption that they will respond 

in different ways to a changing environment that we 

cannot readily anticipate (e.g., Gibson and others 2009; 

Hilborn and others 2003; Lesica and Allendorf 1995). 

Redundancy represents the replication of conservation 

efforts, units, or populations to minimize the probability 

that all will be lost in a single catastrophic event. There 

are obvious issues and tradeoffs between the number, 

size, and spatial distribution of replicates that are needed (e.g., many small versus a few large). Tradeoffs will 

depend largely on local conditions such as the frequency and extent of the dominant disturbance regime (e.g., 

the typical size of the stream networks influenced by 

fire and/or the length of channel segments influenced 

by debris flow); the dispersal distances possible for a 

given species; and the geographic gradients of diversity 

in genes, life history expression, and habitat potential. 

The general concepts, however, are that larger more 

productive populations have a better chance of surviving a given disturbance; multiple populations spread 

the risk that not all will be lost simultaneously; and 

diversity among those populations helps hedge against 

uncertainty and enhance the potential for adaptation to 

new conditions. A simple rule of thumb may be to seek 

a balance of all three concepts among the conservation 

units that are possible (Levin and Lubchenco 2008).

Favor actions robust to uncertainty. Even as we 

learn more about the nature of climate change, substantial uncertainty remains. The world is warming, and 

patterns of precipitation and stream flow are changing 

in ways that we can anticipate but not always predict 

with precision. The changes have been and will become more dramatic and influential in some places than 

others. We will be surprised. The rate of change and 

the ultimate magnitude and variability of change will 

depend on the effectiveness of mitigation and myriad 

smaller scale controls and interactions that we can only 

guess at, while the conditions that influence those controls and interactions are variable and changing (Pielke 

2009). This is not a new problem in the management 

of complex ecological systems. Management that depends on prediction of a sustainable harvest level or a 

threshold of acceptable habitat disruption simply has 

not proven very effective (Ludwig and others 1993; 

Poole and others 2004). The alternative is to accept that 

uncertainty and favor options that least depend on the 

precision of our knowledge or predictions (Lawler and 

others 2009; Ludwig and others 1993; Pielke 2009). 

Actions that generally contribute to the resilience 

of populations (e.g., removing barriers, expanding 

high-quality habitats, reducing non-climate stresses), 

regardless of the threat, might be favored over those that 

attempt to mitigate or control the influence of a specific 

threat (fire suppression or intentional barriers to protect 

small remnant populations; Fausch and others 2006). 

A corollary to this idea is to favor the conservation or 
28 USDA Forest Service RMRS-GTR-250. 2010.

restoration of natural process over the restoration of 

structure (Beechie and others 2008, 2010) or the control of processes that require a continuous investment 

of time, energy, and money to maintain (Rieman and 

others 2010). Joyce and others (2009) term this strategy 

one of “no regrets,” implying that the choices are likely 

to provide important benefits regardless of the ultimate 

effects of climate change.

The bottom line is that there are no strict rules for 

prioritization but it remains important, if not critical. 

Random actions are more likely to waste time and limited resources and to be less effective than those focused 

through a specific and strategic process that is relevant 

to the system at hand. The considerations outlined 

above may provide some foundation, but ultimately, 

managers and biologists will need to devise a strategy 

that makes sense for the systems they know. Further discussion and some examples are available in other work 

(e.g., Beechie and others 2006; Fausch and others 2006; 

Frissell and others 1997; Groves 2003; Roni and others 

2002). For the most part, the work we can do is not new. 

The tools of watershed, habitat, species, and population 

conservation and restoration are well known to most biologists working throughout the Rocky Mountain West. 

In the context of climate change, however, we have an 

even greater need to implement those tools as effectively and efficiently as possible.

Facilitate Transitions to New Conditions

It seems likely, given current and anticipated trends 

in climate change, that it simply will not be possible to 

conserve all existing species’ populations or the structure of aquatic communities across the streams and 

habitats where they currently occur. In some cases, the 

existing habitats suitable for some species and communities we hope to conserve will be lost locally or 

they may be shifted upstream or north. Many terrestrial 

organisms may be able to move to other suitable habitats or track the changing distribution of habitats. But 

for others, and especially for fishes confined to stream 

networks constrained by natural and anthropogenic 

barriers, that may be impossible. As a result, there has 

been considerable discussion and debate in the conservation literature on “facilitated dispersal” or “assisted 

migration” (e.g., Ricciardi and Simberloff 2009; Sax 

and others 2009; Vitt and others 2009). Concerns exist because introductions of non-native species can 

create a suite of ecological and environmental problems (Ricciardi and Simberloff 2009). Others argue 

that facilitation should only mimic transitions or dispersals that would have occurred naturally but are now 

impossible or too slow because of anthropogenic fragmentation of existing habitats (Vitt and others 2009). 

In reality, fisheries biologists and managers have long 

facilitated dispersal and migration of fishes through 

hatchery supplementation programs, installation of passage structures at natural or human-caused barriers, trap 

and haul programs (e.g., Columbia River salmon), and 

intentional species introductions. There are even significant efforts to move fishes like Gila trout to artificial 

refugia (i.e., hatcheries) and back to historical or new 

habitats in response to wildfire-driven catastrophic disturbances (Brooks 2006). In each case, managers must 

weigh the costs and potential risks both of attempting 

to control ecological processes that no longer occur 

naturally and of establishing non-native communities 

that may function without further substantial investment (Fausch and others 2006). We do not argue for any 

particular solution but suggest that those decisions be 

placed in the context of climate change. In streams and 

habitats where it seems unlikely that native species and 

communities will persist in the future, managers may 

choose to facilitate the transition to a new community 

actively (e.g., through introduction of new species to 

the existing habitat or of the old species to a new habitat) or passively (e.g., through removal of barriers that 

exclude invasions). Consideration of the sensitivity of 

habitats, populations, and communities outlined in the 

previous section could provide important context here 

as well, although the implied options are different. As 

suggested above, it will be important to consider the 

ecological, evolutionary, and social values involved 

since they directly define the tradeoffs.

Develop Local Information

Developing and implementing efficient adaptation 

and facilitation strategies for climate change requires 

information specific to local climatic trends and integration of this knowledge with current and future status 

of local landscapes, streams, and aquatic resources of 

concern. Global climate models (GCM), though capable of providing climatic predictions for all areas of the 

Earth, have minimum resolutions of hundreds to thousands of square kilometers and were not designed to 

provide the sort of site-specific information typically required for ecological assessments (Wiens and Bachelet 

2009). Moreover, GCM outputs usually consist only 

of air temperature and precipitation, which must be 

translated to relevant habitat features for aquatic biotas. 

Fortunately, new analytical techniques for streams; increased availability of large, geo-referenced databases 

of aquatic attributes; online summaries of local climate 
USDA Forest Service RMRS-GTR-250. 2010. 29

information; and advances in spatial technologies like 

geographic information systems (GIS) and remote 

sensing can provide managers a potential wealth of sitespecific information and integration tools. Linking this 

information with local fish and habitat surveys provides 

a powerful means of understanding climate-related 

trends and risks to individual populations or other 

aquatic resources. In this section, we highlight factors, 

techniques, and data sources to consider when developing local climatic assessments.

Understand context. At the outset of any local climatic assessment, it is important to understand the broader 

context associated with regional climate trends, stream 

responses, and species’ distributions. This knowledge 

provides context that may help managers narrow their 

focus to resources that are most at risk (Hurd and others 

1999; Wiens and Bachelet 2009). Numerous studies that 

document both observed and projected trends across the 

western United States are useful for understanding this 

context. For example, research that links mid-winter 

air temperatures to changing hydrology and flood risks 

(figure 10; Hamlet and Lettenmaier 2007) can highlight 

specific watersheds or forests where increases in winter 

flooding might be anticipated and others where flooding 

may decrease. Similarly, earlier and smaller snowmelt 

runoff, when accompanied by reductions in annual precipitation, means that summer low flows are decreasing 

rapidly in some areas (Luce and Holden 2009) but 

slowly in others (Regonda and others 2005; Stewart and 

others 2005). Managers may be relatively certain about 

changes where historical trends agree with future climate model projections, but in other areas, especially in 

the Southwest, projections sometimes differ from recent 

historical observations and additional uncertainty exists 

(Hoerling and Eischeid 2007; Mote and others 2005; 

GCRP 2009). Finally, a growing number of regional 

bioclimatic and conservation assessments now exist 

that provide population inventories and projections of 

future climate effects across significant portions of species’ ranges (e.g., Battin and others 2007; Keleher and 

Rahel 1996; Rieman and others 2007; Young 2008). 

These are useful for identifying the relative position 

of local populations within the range of future climate 

changes.

Synthesize existing information. Many types of data 

exist that can be compiled into a local climatic assessment. Examples of useful information are: fish survey 

data, stream temperature records, and information on 

watershed disruption or habitat loss. The latter includes 

any activity that potentially impairs riparian, watershed, 

or stream integrity. Of particular interest in many areas 

will be inventories of road culverts and water diversion structures that create fish barriers or warm stream 

reaches with degraded banks or riparian areas, as well 

as channels that are prone to debris flows or subject 

to chronic road sedimentation. These features or locations often provide some of the best opportunities for 

reducing existing stresses (Fausch and others 2006; 

Hendrickson and others 2008).

Useful fish data include spawn timing, dates of fry 

emergence, and migration times of adults or juveniles 

past weirs or other structures where they are counted 

(e.g., Crozier and others 2008; Juanes and others 2004). 

If these surveys are repeated over time, they may reveal 

trends that show how fish populations are responding to environmental change (Elliot and others 2000). 

Even a few years of data from contrasting climate 

years, however, could provide valuable clues regarding 

longer-term responses given the plasticity of phenotypic 

traits. Spatially distributed fish population surveys and 

stream temperature data are often abundant and useful 

for building bioclimatic models that facilitate inference 

beyond sample dates and locations and can be used to 

understand landscape, stream, and climatic factors that 

regulate species distributions and aquatic thermal regimes. When compiling these data, potential sources 

in other Federal or State agencies should be considered 

and may provide useful supplements to data collected 

on National Forests. Any data sources require careful 

checks for methodological consistency, but regional databases that are compiled using standardized protocols 

and geo-referenced survey sites are becoming more 

common (e.g., Meyer and others 2009; Pont and others 

2009). Stream temperature data are now routinely collected by many resource agencies and municipalities, 

and digital recording devices provide consistent data 

quality (Dunham and others 2005). Increasingly, these 

data are being archived and made available through 

corporate databases like the Aquatic Surveys module 

in the National Resource Information System (http://

www.fs.fed.us/emc/nris/), the Pacfish-Infish Biological 

Opinion monitoring group (http://www.fs.fed.us/biology/fishecology/emp/index.html), and the National 

Water Information System (http://waterdata.usgs.gov/

nwis).

Other useful information such as hydrography datasets and digital terrain models can be obtained at 

differing resolutions and can be used in a GIS environment to represent a study area and describe important 

environmental gradients. The National Hydrography 

Dataset–Plus (http://www.horizon-systems.com/nhdplus/), available at a scale of 1:100,000, provides a suite 
30 USDA Forest Service RMRS-GTR-250. 2010.

of applications and attributes calculated for individual 

stream segments from the National Elevation Dataset, 

the National Land Cover Dataset, and the Watershed 

Boundary Dataset. Climatic attributes such as interpolated air temperatures and flow values based on 

nearby stream gaging sites are included as stream attributes but represent only static values averaged over 

time. Understanding and calculating local climate 

trends requires obtaining historical climate data for air 

temperature and precipitation from weather stations 

maintained by the National Oceanic and Atmospheric 

Administration (http://www.ncdc.noaa.gov/oa/ncdc.

html) or the Snowpack Telemetry sensor network 

(http://www.wcc.nrcs.usda.gov/snow/) and stream flow 

data from U.S. Geological Survey gage sites (http://

waterdata.usgs.gov/nwis/; Falcone and others 2010). 

Summary metrics that describe annual timing, magnitude, and variance in these factors are easily calculated 

from the raw data for stations maintained in or near the 

area of interest and can be plotted relative to time to 

provide site-specific descriptions of historic climatic 

trends. Descriptions of how these trends translate to elevations necessary for maintaining a given temperature 

isopleth may be calculated using a web tool called the 

North American Freezing Level Tracker (http://www.

wrcc.dri.edu/cwd/products/). Extrapolating historical 

trends into the future may indicate when and by how 

much local climatic conditions could change, although 

this simple approach can be biased given the length of 

the time series and changes in the rates of climate responses (IPCC 2007). Regional climate centers like the 

Western Regional Climate Center (http://www.wrcc.

dri.edu/) and the Climate Impacts Group in Seattle, 

Washington, (http://cses.washington.edu/cig/) also provide many valuable climate summaries and tools for 

analyzing historical and future trends at local and regional scales.

Model to fill gaps. Organizing existing data inventories often reveals a wealth of useful information but 

also highlights many data gaps and limitations associated with raw data summaries. Where gaps exist, new 

data may be collected, but in many instances, additional 

data collections may be too costly or take too long, and 

models built from existing databases can be used to 

make predictions in unsampled locations. These models can be used to interpolate among sites and to predict 

future habitat and fish distributions under warmer climates. An entire subdiscipline focused on “bioclimatic” 

models now exists within the scope of climate-related 

ecological work (Dormann and others 2007; Guisan and 

Thuiller 2005; Latimer and others 2006). Bioclimatic 

models are similar to traditional species distribution 

or habitat models but integrate climate variables to the 

suite of predictive covariates. This establishes a linkage 

between current species distributions or habitat conditions and climatic factors that can be used to predict 

future conditions by changing the climate variable input 

values. This approach works well as long as the historical correlation between climatic predictors and a 

response variable remains consistent, but these relationships may also change through time due to unforeseen 

events (e.g., invasion by an exotic species) or an incomplete understanding of the system being modeled (i.e., 

omission of important predictor variables). Another 

major source of uncertainty is selecting the correct 

parameter values to represent future conditions, as significant unknowns exist regarding the Earth’s climate 

trajectory. Still, bioclimatic models provide a means for 

assessing a range of possible futures. This ability is important for assessing relative risk across populations or 

other valued resources and can help distinguish among 

potential lost causes, core areas that are more resistant 

and resilient, and areas intermediate to the extremes 

where management interventions might play a decisive 

role in the outcome.

Examples of bioclimatic models developed for 

salmonids in the Rocky Mountains are Keleher and 

Rahel’s (1996) work on projecting changes in thermally suitable habitat for a suite of trout species, and 

a more recent study by Rieman and others (2007) 

that examined potential changes in bull trout distributions across the interior Columbia River basin (figure 

18). Characteristic of bioclimatic studies, both were 

of broad spatial extent and limited local precision—a 

compromise that has often been necessary given previous analytical capabilities and a forced reliance on 

crude approximations of local climatic conditions (e.g., 

elevation and air temperature as surrogates for stream 

temperature). Previous bioclimatic models, therefore, 

were generally best used to provide strategic overviews 

of potential regional changes, with guidance at smaller 

spatial scales generally lacking.

These limitations are beginning to disappear with the 

compilation of large stream databases, accurate spatial 

referencing from global positioning systems, and improvements in GIS and remote sensing that facilitate 

better measurements of environmental characteristics. 

Synthetic stream hydrographs can now be generated for 

historic and future scenarios throughout river networks 

using a variety of hydrologic models when linked to 

climate model projections and digital representations 

of landscapes (Battin and others 2007; Hamlet and 

Lettenmaier 2007). The Variable Infiltration Capacity 
USDA Forest Service RMRS-GTR-250. 2010. 31

(VIC) model (Hamlet and Lettenmaier 2007; Liang and 

Lettenmaier 1994), in particular, has been widely applied across much of the western United States to model 

climate change scenarios (e.g., Christensen and others 

2004; Hamlet and Lettenmaier 2007). Recently, Wenger 

and others (2010) adapted the VIC model to work in 

small headwater streams and derived a series of ecologically relevant flow metrics for use in bioclimatic 

models. Improved statistical models that account for 

the spatial structure of streams, either by treating them 

as networks (Peterson and others 2007; Ver Hoef and 

Peterson 2010) or hierarchical systems (Cressie and others 2009), have also been developed and substantially 

improve predictive accuracy by accommodating spatial 

autocorrelation among sample locations (e.g., Isaak and 

others 2010; Rieman and others 2006). Using these new 

statistical techniques in conjunction with a large stream 

temperature database (n = 780) compiled from several 

resource agencies, Isaak and others (2010) built stream 

temperature models that explained 93 percent of the 

variation in temperatures over a 13-year period across 

an extensive river network. Similar results have been 

achieved using spatial stream techniques with other 

water quality attributes (Gardner and McGlynn 2009; 

Peterson and Urquhart 2006) and fish population attributes (Peterson and Ver Hoef 2010), and those results 

suggest a future wherein bioclimatic relationships in 

streams can be examined in great detail (McIntire and 

Fajardo 2009).

As our ability to model aquatic ecosystems improves, 

distributions of fish species and important habitat 

features will be predicted at resolutions that enable 

managers to make useful comparisons among different 

management options (figure 19). Some choices regarding resource allocations may be obvious, especially if 

a habitat or population appears likely to disappear (or 

Figure 18. Changes in thermally suitable habitat under different air temperature increases for bull trout spawning and juvenile 

rearing within the Interior Columbia River basin predicted by a bioclimatic model. All shades of blue indicate historically 

suitable habitats; whereas lighter shades of blue indicate habitats that could be lost with future air temperature warming of 

1.6 oC to 5 oC (from Rieman and others 2007).
32 USDA Forest Service RMRS-GTR-250. 2010.

persist) under most future climate scenarios. In many 

cases, however, choices will be less obvious due to 

complex interactions among multiple factors. Streams 

or populations that fall into this grey area are where 

management interventions may be instrumental in tipping the balance toward desired outcomes.

Monitor to document trends. Regardless of our ultimate ability to predict the biophysical attributes of 

streams, differences will invariably arise between the 

predicted and observed future states of these systems. 

Monitoring programs that are targeted at key biophysical attributes will be essential for validating and refining 

model projections and determining real rates of change. 

Due to costs, extensive monitoring programs may not 

be feasible for all attributes of potential interest; but 

stream temperatures, stream discharge, and fish distribution are likely to be common priorities.

Stream temperature measurements are easily and 

reliably obtained and are relatively inexpensive using 

modern digital sensors (Dunham and others 2005; Isaak 

and others 2010). Temperature sensor networks can be 

deployed in a few days or weeks to provide inference 

ranging in scale from individual streams to river networks or entire basins. Basic recommendations for the 

design of temperature sensor networks in streams are 

available (Isaak and others 2009; http://www.fs.fed.us/

rm/boise/AWAE/projects/stream_temperature.shtml), 

although site-specific applications will vary with local objectives. A few years of temperature data from a 

sensor network is sufficient to start identifying streams 

that may be more (or less) sensitive to heating based on 

inter-annual differences in climatic conditions. Several 

years of annual stream temperature from a site may be 

enough to reconstruct the long-term thermal history at 

that site (or future projections) based on relationships 

with air temperatures measured at nearby weather stations (Mohseni and others 2003; Mohseni and Stephan 

1999). Such reconstructions may help compensate for 

the short length of stream temperature monitoring in 

many areas.

Stream discharge has long been monitored by the 

U.S. Geological Survey, but most gage sites are located 

on larger streams and rivers that provide major water 

supplies. The relative dearth of flow measurements on 

smaller streams has, in part, been limited by cost, but 

Figure 19. Hypothesized scenario 

depicting landscape-level effects of 

climate change on headwater habitats 

for a sensitive fish species. Black areas 

depict currently suitable habitats that 

will become thermally unsuitable 

by mid-century; red lines indicate 

channels susceptible to debris flows 

if a wildfire occurs; and yellow bars 

represent road culverts that block fish 

passage. Which culvert barriers should 

be given highest priority for improving 

fish passage if sufficient resources are 

not available to modify all barriers?
USDA Forest Service RMRS-GTR-250. 2010. 33

new instruments based on pressure transducers are reducing logistical and financial costs and could open a 

new era of flow monitoring. Similar to stream temperatures, historical flow reconstructions may be possible 

with a few years of data by relying on correlations with 

long-term records at nearby gages. Even longer perspectives (i.e., several hundred to a thousand years) 

are possible from dendrochronology that uses tree ring 

width as a proxy for precipitation and stream flow (e.g., 

Graumlich and others 2003). Although these reconstructions sacrifice temporal resolution for extended 

length, they provide an important historical context for 

contemporary patterns.

Data on fish populations may be relatively abundant, 

but most often consist of spatially distributed information from reach-based surveys rather than long-term 

monitoring at individual sites. An exception may occur 

where weirs, screw-traps, or other means of counting 

fish during seasonal migrations have been maintained 

through time. Indeed, most of the direct evidence of 

climate change on fish populations is derived from 

trends in the timing of anadromous spawning migrations during the latter half of the twentieth century 

(e.g., Crozier and others 2008; Quinn and Adams 1996; 

Robards and Quinn 2002). Case histories that document climate-induced shifts in spatial distributions 

are almost non-existent, despite these shifts being the 

most common prediction generated by bioclimatic assessments (e.g., Keleher and Rahel 1996; Rieman and 

others 2007). Hari and others (2006) provide one of 

the few examples that shows brown trout populations 

in Switzerland have shifted upstream in recent decades associated with steadily increasing temperatures 

and frequency of disease. Monitoring to detect similar 

shifts in populations and communities of western fishes 

will be crucial to understand actual rates of habitat loss. 

Weak populations in thermally marginal areas near a 

species’ downstream or upstream distributional extent 

are expected to be the most sensitive to climate effects 

and could be specifically targeted to provide this information (e.g., Isaak and others 2009; Rieman and others 

2006).

Depending on the area of interest and available resources, it may or may not be possible for a single entity 

to develop and maintain adequate monitoring programs. 

The Pacfish-Infish Biological Opinion Monitoring 

Program and Environmental Monitoring and 

Assessment Program (Herlihy and others 2000; Roper 

and others 2010) are examples of single agency initiated efforts that have proven their utility through time. 

Between National Forests and State and Tribal agencies, the potential exists for significant redundancies. 

Such inefficiencies are problematic when faced with the 

scale and resolution of monitoring that climate change 

may necessitate. Interagency (or inter-forest) efforts 

to coordinate monitoring, therefore, will likely be important (see below). Standardization of data collection 

techniques; coordination of sample locations; and use 

of centralized, integrated databases could provide significant leverage for local monitoring efforts, decrease 

costs, and improve the ability to track environmental 

change.

Coordinate Efforts

Climate change represents a major challenge to 

aquatic biologists and managers. Managers of other 

natural resources face similar issues. Because aquatic 

systems are directly influenced by the conditions and 

management of other resources and by values and actions of society at large, aquatic management responses 

to climate change are not likely to be very effective if 

they are not coordinated with other efforts to understand and adapt. There are at least three areas where 

that coordination seems particularly important: across 

natural resource disciplines, particularly those focused 

on terrestrial and aquatic systems; across agencies and 

organizations; and between natural resource specialists 

and the general public.

Across disciplines. The history of land use management has often been one of conflict among disciplines 

or distinct natural resource objectives (e.g., timber versus fish). Biologists and managers who are focused on 

aquatic or terrestrial issues linked to wildfire, for example, may view the other’s efforts as a constraint on their 

own progress (e.g., Rhodes and Baker 2008; Rieman 

and others 2003; Rieman and others 2010). Conflict 

is accentuated by multiple jurisdictions, differences 

in mandates and subsequent goals or objectives, mismatch in temporal and spatial scales of issues, limited 

or ineffective communication, and limited management 

resources (Naiman and others 1998; Noss and others 2006; Rieman and others 2003). The results have 

often been attempts to reconcile differences through 

piecemeal negotiation or regulatory processes that occur project by project (Baron and others 2002), with 

patchwork integration of individual resource management plans after the fact or with political solutions that 

favor one option or the other depending on the current 

administrative rules and direction (Rieman and others 

2003). The inextricable linkages between terrestrial and 

aquatic systems and the simultaneous disruption of both 

through past land use activities means that opportunities 
34 USDA Forest Service RMRS-GTR-250. 2010.

for more effective coordination of terrestrial and aquatic restoration and conservation could be widespread 

(Rieman and others 2000; Rieman and others 2010). 

More successful collaboration might be built on broader perspectives that examine potential opportunities and 

conflict across entire landscapes (Noss and others 2006; 

Rieman and others 2010); active communication based 

on clear definition of values, goals, and objectives early 

in the process (Naiman and others 1998; Firth 1998); 

and a focus on the long-term restoration of ecological 

processes that contribute to the resilience of both terrestrial and aquatic systems (Rieman and others 2010).

Across agencies or jurisdictions. The conservation 

and management of natural resources dependent on, or 

directly influencing, aquatic systems fall under many 

different jurisdictions and across agencies and authorities with different missions, histories, and capacities. 

A broad collection of non-governmental organizations 

often work to facilitate or restrict different elements 

of that mix, as well. A common thread is that much, if 

not all, of the natural resource values and services important to each will be influenced by climate change in 

some way. Basic climate science has expanded rapidly 

in recent years, and substantial efforts are now focused 

on refining and downscaling GCMs to better understand 

the effects at finer resolutions approaching the scale of 

local management decisions. Researchers are actively 

developing linked hydrologic, biological, and ecological models to explore species and ecosystem responses 

(e.g., Isaak and others 2010; Wenger and others 2010). 

New tools and multiple syntheses are being developed 

to help managers sort through the complexity and potential implications and weigh alternatives.

In short, the science and information potentially 

useful to managers confronted with climate change is 

rapidly expanding. It is difficult for any one person to 

keep up with the literature and development in a single 

field such as aquatic ecology and even more challenging (if not impossible) in the broader interdisciplinary 

suite that is important to integrated landscape management. Fortunately, there are initiatives to summarize, 

synthesize, and serve information as it develops (e.g., 

The Forest Service Climate Change Resource Center; 

http://www.fs.fed.us/ccrc/). Interdisciplinary and interagency consortia are being discussed to extend capacity 

for modeling and analysis, leverage funding, minimize 

duplication, and potentially extend the communication 

across resource disciplines considered above. Biologists 

and managers struggling with climate change might find 

these efforts helpful, but they might also create other 

opportunities to extend limited resources and capacity. 

The Forest Service, State fish and game agencies, and 

State water quality biologists and managers, for example, might all benefit from high-resolution downscaled 

hydrologic predictions or regional stream temperature 

monitoring networks. Sharing the cost to develop and 

serve such key information with appropriate local resolution could be useful. Designation of a “climate lead” 

in each agency, work group, management office, or 

region might help facilitate discussion, track and communicate developments, and facilitate new linkages 

within and across agencies.

With the public. Ultimately, management of natural 

resources and any response to climate change must be 

responsive to the values society places on those resources and the tradeoffs required in the process. The effects 

of climate change on aquatic systems will probably be 

aggravated by continued development and land conversion and by growing demands for increasingly limited 

water. More frequent or more extreme disturbances in 

the form of wildfire, flooding, and drought could lead 

to more extensive attempts to mitigate effects through 

more extensive forest thinning and harvest, diking, and 

water storage, respectively. Where human lives, property, and livelihoods are concerned, natural resource 

issues will be considered only secondarily, if at all, 

unless they can be clearly linked to more pragmatic 

benefits (such as water storage and flood attenuation) 

that healthy, functioning watersheds provide in addition to maintenance of biological diversity, fishes, and 

fisheries. Paul Angermeier (2007) has argued (convincingly, in our view) that public education on aquatic 

issues and the tradeoffs we make with other values may 

be the single most important thing aquatic managers 

and biologists can do for the long-term conservation 

of aquatic ecosystems, biological diversity, fishes, and 

fisheries. Biologists and managers working in and with 

local communities may have the best opportunity and 

understanding to do that effectively. The realities of 

climate change and the broader political and economic 

challenges could make that even more important.

The bottom line is that climate change will influence virtually all natural resource management efforts 

and values in important ways. Few, if any, managers 

and biologists will have the capacity to understand or 

respond without reaching out to, guiding, and learning 

from others who influence their activities and decisions, 

the availability of critical information, and their opportunities to move forward.
USDA Forest Service RMRS-GTR-250. 2010. 35

Epilogue

Understanding and responding to climate change requires an ongoing dialogue between management and 

research that is informed by observed environmental 

trends. There has been a flood of new information to 

consider based on climate models and early downscaling with projections to a host of natural resource and 

social issues, including conservation of native fishes. 

Those models will be refined and expanded and they 

will provide important insight. Models must be tested 

against reality, however, so one of the most important 

foundations for a continuing dialogue will be syntheses 

and summaries of trends in monitoring data. Most of 

what is known about climate change came from longterm monitoring that was largely in place before climate 

became a central issue. That understanding will be 

extended by efforts to form cross-disciplinary and interagency collaborations and more complex infrastructure, 

including the databases and continued and refined monitoring, modeling, and decision support tools needed to 

consider the potential alternatives for adaptation across 

regions and river basins.

Those broad initiatives will depend on the integration 

of regional and local information. Like many others, our 

initial interest in climate change came from a growing 

discussion in the general literature. That interest became focused as we considered systems and species we 

worked with in the context of climate change and the results from our own monitoring and long-term research. 

As we developed our data, the trends and projections 

that emerged made climate change real to us. That led to 

new questions and more focused work. From that experience, we believe local monitoring efforts with climate 

change in mind, even if limited to a single temperature 

sensor maintained in a single stream, will be among the 

most important steps biologists and managers can take 

in response to climate change. As information develops, biologists and managers will help test hypotheses 

relevant to the systems they know. That will point out 

important strengths and weaknesses and will help refine projections of how aquatic systems will or won’t 

change in the future. Biologists and managers will gain 

better insight to the utility of some management actions 

and the futility of others. Empirical observations may 

also provide a basis for making difficult decisions. For 

example, redirecting conservation efforts away from 

marginal populations of a sensitive species based solely 

on climate model projections would, at present, be difficult to justify. If, however, model projections were 

accompanied by data from rigorous monitoring that had 

documented previous climate-related population losses, 

such decisions might become more acceptable.

Early model projections, despite their utility, are 

fraught with uncertainties, should be viewed critically, 

and must be revised periodically as new information 

becomes available. The single largest source of uncertainty is simply how much and how fast the Earth’s 

climate will warm. Other important uncertainties, however, include the specifics of how future warming will 

affect freshwater ecosystems, western streams, and 

aquatic communities. Researchers and managers can 

generate much of the needed information independently, but partnerships between research and management 

could be particularly useful.

Research is needed to develop models that translate 

both current and future climate effects to aquatic ecosystems, but data to build and validate such models will 

be required across broad spatial and temporal scales. 

Research likely will never have the resources to do that 

work independently but could harness data from routine 

monitoring programs. Monitoring of key biophysical 

attributes, therefore, may provide not only trend data 

but also much of the information necessary to develop 

models, test hypothesized mechanisms, or stimulate 

new research by documenting unanticipated responses. 

A collaborative, adaptive system that is characterized 

by iterations of model development/predictions and 

subsequent calibration/refinement against monitoring 

data and a record of management results could systematically reduce uncertainties.

As new information regarding the Earth’s climate 

trajectory and its effects on aquatic systems develops, 

model projections and management implications will 

evolve. Model projections will become more accurate 

as understanding of key processes improves and supporting databases expand. Spatially explicit forecasts 

of habitat conditions, species distributions, and probabilities of persistence for the latest climate scenarios 

will be possible, much the way that harvest quotas are 

forecast for commercial fisheries. Instead of revising forecasts over intervals of weeks and months that 

are relevant to a fishery, however, forecasts of climate 

change effects might be updated over longer intervals 

(e.g., 5 to 10 years). Management decisions will have 

to be informed of, and open to, the evolving state of 

knowledge. Partnerships between research and management and continued monitoring through both local 

and broader collaborative efforts could help that process 

and promote effective adaptation to a changing climate.
36 USDA Forest Service RMRS-GTR-250. 2010.

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