Causal Analysis/Diagnosis Decision Information System (CADDIS)

• Common Candidate Causes
• Interactive Conceptual Models

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Figure CC.5-1. Stream temperatures are influenced by many atmospheric and hydrological processes.

Temperature is the concentration of thermal energy in a substance, for example, water. The phrase “thermal regime“ is used when emphasizing the temporal and spatial distribution of temperature. Temperatures in streams and rivers are influenced by many atmospheric and hydrologic processes that influence the movement of heat (Figure CC.5-1, Section CC.5.1.2 ). In turn, temperature plays a fundamental role in shaping the structure and function of aquatic systems (Table CC.5-1), and is frequently used as a basis for classifying streams (e.g., cold-water, warm-water).

Advice for deciding whether to include temperature in your list of candidate causes is provided on the Section CC.5.1. Ways to measure temperature and thermal regime modification are discussed in Section CC.5.2. You may go directly to a specific section of interest by clicking on the topic outline in the right navigation bar.

CC.5.1. What to Consider When Determining if Temperature Should be Included as a Candidate Cause

Figure CC.5-2. This simplified generic conceptual model traces causal pathways from sources to impairments for temperature. Click on the diagram to go to a larger temperature figure, accompanying narrative, and links to other related diagrams.

Temperature should be listed as a candidate cause when potential or observed human source and activities, site observations, or observed effects support portions of the source-to-impairment pathways in the conceptual model for temperature (Figure CC.5-2). The more the thermal regime departs from the natural regime, the more likely is it to cause undesirable biological effects. Although increased temperature is most frequently thought of as the stressor of concern, undesirable effects can also be associated with decreases in temperature, increases in temperature ranges, and increased rates of temperature change.

CC.5.1.1. Checklist of Sources, Site Evidence, and Biological Effects

A checklist is provided below to help you identify key data and information useful for determining whether to include temperature in your list of candidate causes; the title for each column is linked to more detailed descriptions. The list is intended to guide you in collecting evidence to eliminate or enhance support for temperature as a candidate cause. You also may be aware of other situations when temperature should be eliminated or included as a candidate cause; please send us your insights using the “contact us” button.

Consider contributing, modifying, and related factors as candidate causes when selecting temperature as a candidate cause:

• Dissolved oxygen
• Ionic strength
• Flow alteration
• Sediments

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CC.5.1.2. Sources and Activities that Suggest Listing Temperature as a Candidate Cause

Many human activities can change the temperature or modify the thermal regime of waterbodies and, subsequently, the structure and function of aquatic ecosystems (Poole and Berman, 2001; Galli and Dubose, 1990). Mechanisms underlying thermal modifications range from simple, direct relationships (e.g., discharge of heated cooling water) to complex, indirect relationships (e.g., bank erosion leading to increased channel width, increased solar radiation, reduced stream flow, and greater conductive heating).

Climate change may also affect water temperature via several interrelated causal steps (e.g., changes in air temperature, humidity, cloud cover, precipitation quantity and intensity, vegetative composition and cover). However, a discussion of climate change impacts on aquatic ecosystems is beyond the scope of this module, because of its complexity and broad spatial and temporal scale.

Sources of altered thermal regimes are best evaluated by considering them in context of the atmospheric and hydrologic processes that influence temperature in streams and rivers (Figure CC.5-1). Some sources may affect certain stream temperature processes but not others.

Dominant sources of heat flux into and out of streams include solar radiation, groundwater input, upstream temperature and flow, atmospheric exchange (via convection and evaporation), streambed conduction, and longwave radiation. Additional heat exchange processes not depicted in Figure CC.5-1 may be important in specific streams or situations (e.g., friction generated by rapidly moving water in steep-sloped streams).

Importantly, many naturally occurring environmental conditions can substantially modify the impact of these heat flux processes on stream temperature. For example, riparian cover can greatly reduce the amount of solar radiation that reaches the stream surface, particularly in small streams. Modifying factors associated with the underlying geology include stream flow, hyporheic exchange, channel morphology and complexity, and upland vegetation. Other modifying factors include relative humidity and wind speed.

Some common anthropogenic sources and activities that modify temperature are described below.

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Image CC.5-1. A cooling tower lowers the temperature of power plant cooling water to avoid a heated discharge.

Discharge of heated water can directly modify downstream temperatures, as well as indirectly affect temperature by altering water volume and velocity (Image CC.5-1).

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Image CC.5-2. Riparian devegetation may change water temperature.

Removal of riparian vegetation (Image CC.5-2) allows increased solar radiation to reach the stream surface, directly increasing water temperature. Removal of trees also decreases evapotranspiration, which reduces evaporative cooling. Vegetation removal also may reduce the insulating capacity of these buffers, due to increased longwave radiation from surface water to atmosphere, decreased longwave radiation from canopy to surface water, or increased wind speed; each of these factors may contribute to lower minimum temperatures. In addition, destabilization of stream banks can lead to increased channel widths, decreased depths, and associated temperature changes (Moore et al., 2005).

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Image CC.5-3. Upland devegetation indirectly increases water temperatures.

Removal of upland vegetation (Image CC.5-3) can result in increased surface runoff with elevated temperatures due to increased heat absorption across the landscape. Higher surface soil temperatures, increased sedimentation and channel alteration, and increased or decreased groundwater inputs also may affect stream temperatures (Moore and Wondzell, 2005; Johnson and Jones, 2000).

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Image CC.5-4. Impervious surfaces can heat or cool stormwater runoff.

Impervious surfaces (Image CC.5-4) can rapidly delivery of large volumes of warm or cold stormwater runoff, which can increase water temperatures in the summer and decrease them in the winter (Galli and Dubose, 1990).

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Image CC.5-5. Channel alterations may include straightening and widening the channel.

Channel alteration (Image CC.5-5) can change flow regimes, reduce stream-floodplain connectivity, and reduce groundwater discharge. Changes in groundwater discharge affect stream temperature by reducing the capacity of streams to buffer changes in heat loads from other sources (Paul and Meyer, 2001; Poole and Berman 2001; LeBlanc et al., 1997; Klein, 1979).

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Image CC.5-6. Temperature changes caused by impoundments vary based on design.

Impoundments or dams can directly increase or decrease downstream temperatures, depending on the type (i.e., surface or bottom) and timing of water releases (Image CC.5-6). Upstream of impoundments, associated hydrologic changes may alter temperatures, due to greater stream widths and retention times. In general, discharges from shallow impoundments that do not stratify will increase the maximum summer temperature of the water. Alteration of upstream and downstream sediment loads also may lead to channel alteration and subsequent temperature changes (Webb and Walling, 1996; Webb and Walling, 1993).

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Image CC.5-7. Water withdrawal for irrigation.

Removal of water from surface or groundwaters for agricultural (Image CC.5-7), industrial, or municipal uses can affect stream thermal regimes. For example, surface water withdrawals can reduce stream flow, leading to accelerated heating and higher temperatures; reduced baseflow due to groundwater withdrawals may lead to greater diurnal variation in stream temperatures (Poole and Berman, 2001; LeBlanc et al., 1997).

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Sources and activities that can influence temperature can also increase other stressors, which should also be included on your list of candidate causes. For example, removal of riparian vegetation and increased impervious surfaces can result in increased sediments, alter flow and increase ionic strength. Increased water temperature reduces the amount of oxygen that water can hold and increases the solubility of many ions; for these reasons, consider including reduced dissolved oxygen and increased ionic strength on your list of candidate causes whenever you include increased temperature.

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CC.5.1.3. Site Evidence that Suggests Listing Temperature as a Candidate Cause

Except for phase changes of water (freezing or vaporization), alterations in water temperature are not directly observable under typical field settings and observers usually must measure water temperature to detect a change. Site observations indicative of temperature change are therefore indirect and largely restricted to observing the presence of one or more sources of thermal modification. For example, because incident solar radiation often has a strong effect on stream temperature, observations of reduced riparian cover over a stream relative to reference conditions may suggest changes in thermal regime. Congregation of coldwater fish near groundwater inputs to streams during summer may indicate suboptimal temperatures outside these zones, or congregation of fish near industrial discharges during winter may indicate the presence of heated discharges.

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CC.5.1.4. Biological Effects that Suggest Listing Temperature as a Candidate Cause

The effects of modified thermal regimes can occur at various levels of biological organization and involve numerous endpoints. For example, biochemical changes related to temperature include the rates of enzymatic reactions, metabolic processes, and protein synthesis (e.g., heat shock proteins). At the organism level, modified thermal regimes can affect survival, growth rate, gamete production, swimming speed, disease susceptibility, migratory behavior, timing of metamorphosis, and other traits. At the population, community, and ecosystem levels, modified thermal regimes can alter attributes such as population density, age- or size-class structure, predator/prey dynamics, temporal dynamics of populations and communities, species richness, rates of microbial decomposition, and ecosystem productivity. For reviews of the thermal effects literature, see U.S. EPA (2003), U.S. EPA (2001), Beitinger et al. (2000), Galli and Dubose (1990), Cravens and Harrelson (1987), Ward and Stanford (1982), and Vannote and Sweeney (1980).

Biotic responses to altered stream temperatures are often linked to different spatial or temporal attributes of a stream's thermal regime. For example, the timing of fish migration and spawning or the emergence of benthic insects may be initiated by the gradual warming of stream temperatures during spring or the cooling of temperatures in the fall. However, increases in maximum temperatures during summer or decreases in minimum temperatures during winter may be stressful enough to cause acute lethality. The availability of thermal refugia, or small stream areas with thermally preferred habitat, may ameliorate the effects of otherwise lethal thermal conditions, at least over short time periods. Longer-term increases in stream temperatures may make organisms more susceptible to disease.

Aquatic organisms are believed to be adapted to specific thermal regimes or "thermal niches" to maximize their competitive advantage (Johnson and Kelsch, 1998; Beitinger and Fitzpatrick, 1979; Magnuson et al., 1979; Hokanson, 1977). These thermal niches are reflected when scientists classify organisms in terms of thermal preference (e.g., cold water or warm water). However, because water temperatures often fluctuate extensively over different temporal and spatial scales (e.g., diurnally, seasonally, laterally, vertically, and longitudinally), organisms in aquatic ecosystems have developed behavioral and physiological mechanisms to acclimate to or avoid suboptimal temperatures. This can make it difficult to interpret observations of species’ presence and absence. In addition, prior exposure to fluctuating temperatures [i.e., an organism's "thermal history" (Beitinger et al., 2000)] influences an organism's ability to tolerate suboptimal temperatures. This suggests that an organism's thermal history may influence its response to temperature and makes it to difficult to make generalizations.

Although numerous and diverse, the biological effects of modified thermal regimes generally are not sufficiently specific to diagnose thermal modification as the primary causative agent, or to rule out other candidate causes. There are, however, a number of readily observed biological effects that may suggest modified thermal regimes, including:

• Absence of coldwater taxa (e.g., salmonids, stoneflies) in streams where these taxa are known or expected to occur naturally,
• Presence of warmwater taxa in streams where coldwater taxa are known or expected to occur naturally,
• Changes in the onset of certain reproductive or developmental events cued to temperature (e.g., earlier insect emergence or fish migration in warmer waters), and
• Behavioral changes, such as congregation of fish near cold- or warmwater inputs.

Consider including temperature as a candidate cause when you see changes in aquatic community structure or acute biotic effects as described above. Please note, however, that observations of these effects do not confirm a causal relationship, because they may also be caused by other stressors, or by a combination of factors.

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CC.5.1.5. Site Evidence that Supports Excluding Temperature as a Candidate Cause

There are no site observations that specifically provide evidence of the absence of thermal regime changes. General reasons for excluding a candidate from the list are described in Step 2.2 of the Step-by-Step guide and in Tips for Listing Candidate Causes.

We strongly caution against using benchmarks of effects (e.g., water quality criteria) as evidence for excluding temperature from your initial list of candidate causes, because different species have different temperature requirements and different sites have different naturally occurring levels of temperature.

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CC.5.2. Ways to Measure Temperature

When considering temperature as a candidate cause of impairment, measurements of stream temperature should incorporate spatial and temporal variability of the thermal regime to the extent feasible. These considerations are particularly important in terms of seasonal and inter-annual variation in stream temperatures. Various automated temperature monitoring devices can be used to incorporate temporal variability in water temperature measurements (e.g., min/max thermometers and automated temperature loggers). One can measure spatial differences in temperature over an area of water surface using airborne and satellite remote sensing techniques (Torgersen et al., 2001). One-time measurements of stream temperature (called “spot”, or point measurements), can be unreliable indicators of thermal modification and are best reserved for situations where the source is highly localized (e.g., point source discharges of heated cooling water). If spot measurements are the only available technique for measuring stream temperatures, care should be taken to control for diurnal variation: for example, measurements should be taken at the same time of day at reference and impaired sites. Measurements also should be made in a variety of habitats within the stream to capture within-channel spatial variability. Some common metrics for stream thermal regimes, and their relationship to biological effects, are provided in Table CC.5-2.

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Beitinger, TL; Bennett, WA; McCauley, RW. (2000) Temperature tolerances of North American freshwater fishes exposed to dynamic changes in temperature. Environ Bio Fishes 58:237-275.

Beitinger, TL; Fitzpatrick, LC. (1979) Physiological and ecological correlates of preferred temperature in fish. Am Zoologist 19:319-332.

Cravens, JB; Harrelson, ME. (1987) Thermal effects. J Water Poll Cont Fed 59:531-539.

Galli, J; Dubose, R. (1990) Water temperature and freshwater stream biota: an overview, appendix C. In: Thermal impacts associated with urbanization and stormwater management best management practices. Washington DC: Dept. of Environmental Programs, Metropolitan Washington Council of Governments.

Hokanson, KEF. (1977) Temperature requirement of some Percids and adaptations to the seasonal temperature cycle. J Fish Res Board Can 34:1524-1550.

Johnson, JA; Kelsch, SW. (1998) Effects of evolutionary thermal environment on temperature: preference relationships in fishes. Environ Bio Fishes 53:447-458.

Johnson, SL; Jones, JA. (2000) Stream temperature responses to forest harvest and debris flows in Western Cascades, Oregon. Can J Fish Aquat Sci 57(Suppl. 2):30-39.

Klein, R. (1979) Urbanization and stream quality impairment. Water Res Bulletin 15:948-963.

LeBlanc, RT; Brown, RD; FitzGibbon, JE. (1997) Modeling the effects of land use change on the water temperature in unregulated urban streams. J Environ Manage 49:445-469.

Magnuson, JJ; Crowder, LB; Medvick, PA. (1979) Temperature as an ecological resource. Am Zoologist 19:331-343.

Moore, RD; Spittlehouse, DL; Story A. (2005) Riparian microclimate and stream temperature response to forest harvesting: a review. J Amer Water Res Assoc 41:813-834.

Moore, RD; Wondzell, SM. (2005) Physical hydrology and the effects of forest harvesting in the Pacific Northwest: a review. J Amer Water Res Assoc 41:763-784.

Paul, MJ; Meyer, JL. (2001) Streams in the urban landscape. Annu Rev Ecol Syst 32:333-365.

Poole, GC; Berman, CH. (2001) An ecological perspective on in-stream temperature: natural heat dynamics and mechanisms of human-caused thermal degradation. Environ Manage 27(6):787-802.

Torgersen, CE; Faux, RN; McIntosh, BA; et al. (2001) Airborne thermal remote sensing for water temperature assessment in rivers and streams. Remote Sens Environ 76:386-398.

US EPA (Environmental Protection Agency). (1985c) Quality criteria for water. U.S. Environmental Protection Agency, Office of Water, Washington, DC; EPA/440/5-86-001.

US EPA (Environmental Protection Agency). (2001) Issue paper 5 technical summary of literature examining the physiological effects of temperature on salmonids. U.S. Environmental Protection Agency, Region 10, Seattle, WA.; EPA-910-D1-005.

US EPA (Environmental Protection Agency). (2003) EPA region 10 guidance for Pacific Northwest state and tribal temperature water quality standards. U.S. Environmental Protection Agency, Region 10, Seattle, WA.; EPA 910-B-03-002.

Vannote, RL; Sweeney, BW. (1980) Geographic analysis of thermal equilibria: a conceptual-model for evaluating the effect of natural and modified thermal regimes on aquatic insect communities. Am Naturalist 115:667-695.

Ward, JV; Stanford, JA. (1982) Thermal responses in the evolutionary ecology of aquatic insects. Annu Rev Entom 27:97-117.

Webb, BW; Walling, DE. (1993) Temporal variability in the impact of river regulation on the thermal regime and some biological implications. Freshwater Bio 29:167-182.

Webb, BW; Walling, DE. (1996) Long-term variability in the thermal impact of river impoundment and regulation. App Geog 16(3):211-223.