Increases In Stream Temperature Research Articles

Modeling instream temperature from solar insolation under varying timber harvesting intensities using RPAS laser scanning

Stream temperatures are influenced by the amount of solar insolation they receive. Increasing stream temperatures associated with climate warming pose detrimental health risks to freshwater ecosystems. In British Columbia (BC), Canada, timber harvesting along forested streams is managed using riparian buffer zones of varying widths and designations. Within buffer zones, depending on distance from the stream, selective thinning may be permitted or harvest may be forbidden. In this study, we used airborne laser scanning (ALS) point cloud data acquired via a remotely piloted aircraft system (RPAS) to derive forest canopy characteristics that were then used to estimate daily incoming summer and fall solar insolation for five stream reaches in coastal conifer-dominated temperate forests in Vancouver Island, BC, Canada. We then examined empirical relationships between estimated insolation and actual instream temperature measurements. Based on these empirical relationships, the potential effects of timber harvest on instream temperatures were simulated by comparing scenarios of different riparian forest harvest intensities. Our results indicated that modeled solar insolation explained 43–90 % of the variation in observed stream reach temperatures, and furthermore, when a single cold-water stream reach was excluded explained an overall 81 % of variation. Simulated harvesting scenarios generally projected increases in maximum stream reach temperatures 1–2 °C in summer and early fall months. However, in a full clearcut scenario (i.e. where all trees were removed), maximum stream reach temperatures increased as much as 5.8 °C. Our results emphasize the importance of retaining riparian vegetation for the maintenance of habitable temperatures for freshwater-reliant fish with thermal restrictions. In addition, we demonstrate the feasibility of RPAS-based monitoring of stream reach shading and canopy cover, enabling detailed assessment of environmental stressors faced by fish populations under climate warming.

The heat is on: Predicting urban stream temperature responses to summer storms

AbstractShort‐term increases in stream temperature in response to storm events have frequently been observed in urban areas, highlighting the need for improved understanding of the factors influencing urban stream temperature. Urban land cover complexity and infrastructure designed for rapid water routing to the sewer system create a direct link between storm events and water release processes, influencing urban stream temperature responses. This study aims to identify predictors of diverse stream temperature response patterns to summer storms. We analysed 403 storm events from six urban and semi‐urban catchments along the US East Coast using dynamic time warping to identify archetype patterns of stream temperature responses. We further disentangled observed stream temperature increase patterns to reveal the drivers associated with ‘heat pulses’, which are characterized by a rapid but high‐magnitude temperature increase followed by a sharp temperature drop at the start of the hydrograph increase. Our results show that stream temperature patterns were event‐specific and linked to pre‐event conditions and rainfall–runoff characteristics, with the shape of the hydrograph and rainfall–runoff response identified as the most important determinators of the observed temperature response patterns. Ponded surface waters and storm drains, as well as cooler water from the shallow subsurface, were identified as potential sources contributing to temperature patterns. These findings have important implications for understanding urban hydrology and the contributions of different source zones in urban catchments. Specifically, our results suggest that stream temperature may serve as a cost‐effective tracer providing information about urban water sources and pathways, thus aiding in the understanding of complex urban hydrology.

Empirical evidence of climate change and urbanization impacts on warming stream temperatures

Climate change and urbanization threaten streams and the biodiversity that rely upon them worldwide. Emissions of greenhouse gases are causing air and sea surface temperatures to increase, and even small areas of urbanization are degrading stream biodiversity, water quality and hydrology. However, empirical evidence of how increasing air temperatures and urbanization together affect stream temperatures over time and their relative influence on stream temperatures is limited. This study quantifies changes in stream temperatures in a region in South-East Australia with an urban-agricultural-forest landcover gradient and where increasing air temperatures have been observed. Using Random Forest models we identify air temperature and urbanization drive increasing stream temperatures and that their combined effects are larger than their individual effects occurring alone. Furthermore, we identify potential mitigation measures useful for waterway managers and policy makers. The results show that both local and global solutions are needed to reduce future increases to stream temperature.

Influences of Urban Discharges and Urban Heat Effects on Stream Temperature

Urban areas with dark and impermeable surfaces are known to have a heating effect on air and still water compared to surrounding areas, called the urban heat island effect (UHI). UHI and stormwater discharges’ collective impact on stream temperature, especially regarding seasonal changes, is a less-studied field. In this study, the temperature effect of the urban village Aarslev on Stream Vindinge in Southern Denmark was examined. Loggers (ID A–L) were placed in Stream Vindinge in 2020–2021, measuring temperature (°C) and pressure (kPa). Outlets were analyzed with respect to origin: Direct stormwater outlets (rain ÷ basin), stormwater delayed by ponds (rain + basin), common overflow, and common sewage from WWTP. Data showed the stream temperature rise through Aarslev village in all months (except March) with 0.3–1.9 °C, most notably in the summer months. A one-way ANOVA confirmed that the upstream station A and downstream station K were significantly different (p-values < 0.001). No significant difference in temperatures between the different outlet types was found. An increase in stream temperature was observed in response to rain events, followed by a temperature decrease. This was assumed to be a “first heat flush”. This was speculated to mean less optimal conditions for trout and sensitive macroinvertebrates not because of heat shock, but rather to lower O2 concentrations and higher mineralization. River and lake temperatures are projected to increase, and this effect might become more pronounced. A decrease in stream temperature was observed after the village (station L). Therefore, it was concluded that the rise in temperature through the village was due to UHI.

Plasticity of upper thermal limits of Australian Paratya spp. (Decapoda, Atyidae) and considerations of climate-change adaptation

Context The ability of ectothermic stream invertebrates to adapt to the predicted increases in mean and extreme stream temperatures is crucial to ensuring they continue to exist. Aims To examine the plasticity of thermal limits of Australian Paratya spp. (Decapoda, Atyidae) from streams in eastern New South Wales (NSW). We hypothesised that the upper lethal temperature (ULT, as indicated by the median lethal temperature, LT50) would be higher for warm water-acclimated shrimp individuals than for winter-acclimatised shrimp individuals because of the importance of acclimatisation temperature. Methods Controlled experiments were undertaken to determine the ULT by using ramping assays for winter field-acclimatised and warm water laboratory-acclimated Paratya spp. Key results Warm water-acclimated shrimp individuals demonstrated a significantly higher LT50 of 36.1°C than did winter-acclimatised shrimp individuals at 34.6°C. Paratya spp. exhibited a limited plasticity for acclimation to warmer temperatures. Conclusions Results demonstrated the potential vulnerability of ectothermic stream invertebrates to climate change if stream temperatures increase as predicted and thermal thresholds are exceeded. Implications Understanding the ULT of stream invertebrates helps predict their ability to respond to temperature variability and response to climate change. Increasing resilience through target management of resorting riparian vegetation for shade and securing environmental flows may reduce the impacts of stream warming.

Heat and smoke from wildfires influence water temperature and dissolved oxygen levels in headwater streams

Wildfire has become increasingly common and severe in forested landscapes across the western United States. Stream and air temperatures within these landscapes are influenced not only by direct heating during the fire but by shading from heavy smoke. In addition, dissolved oxygen (DO) can be affected by increased ash inputs from fire or reduced primary production under lower light conditions. Because collecting data as the event occurs is challenging, most research exploring fire effects on streams has focused on responses months to years after the event as opposed to during and immediately after the fire. We sought to understand how physical stream parameters change as wildfire burns near streams, how stream temperature responses vary through a stream reach, how parameters respond to shading from pervasive smoke during and shortly after the fire, and how fire severity correlates with observed changes. In this study, we report measurements of light, air temperature, stream temperature, and DO across eleven 2nd- to 4th-order streams in the western Cascade Mountains of Oregon, USA, 1 wk before, during, and 1 wk after an extensive wildfire in 2020. Burning near streams resulted in a brief increase in daily maximum stream temperature of 4.5°C at the most severely burned site but small increases or decreases at less affected sites. Longitudinal replication revealed that temperature responses can be patchy at relatively small scales (∼50 m). DO daily minima decreased by 1.3 to 16.9% saturation on the day of the fire, with the magnitude of effect corresponding to burn severity. Across all 11 sites, riparian and watershed estimates of Rapid Assessment of Vegetation Condition after Wildfire and soil burn severity were better correlated with stream temperature responses to fire than percentage of watershed burned. Smoke effects were pervasive, both spatially and temporally, resulting in decreased light, stream temperature maxima, and diurnal variation in DO. Our results suggest that acute changes to physical stream parameters caused minimal harm to aquatic biota at our sites, and the effects of smoke on physical stream parameters will likely impose larger immediate effects on headwater streams than heating from the fire itself.

Effects of land use on thermal enrichment of urban stormwater and potential mitigation of runoff temperature by watershed-scale stormwater control measures

Stream temperature is critical to the survival and reproduction of fauna in cold-water ecosystems. Urbanization causes abrupt increases in ambient stream temperature via stormwater inputs while concurrently decreasing groundwater recharge and stream baseflow. Relatively little is known about how various urban land uses affect thermal load discharged to streams or whether a network of green infrastructure retrofitted at the watershed scale can ameliorate these impacts. Runoff temperature and hydrology were monitored at 17 urban and two forested watershed outlets to assess 1) how urban land use affects stormwater thermal pollution and 2) if thermal pollution can be mitigated through infiltrating stormwater control measures (SCMs). Significantly and substantially larger runoff temperatures (i.e., event mean temperature, event maximum temperature) and thermal loads were observed from every urban land use (without SCMs) in comparison to forested (i.e., surrogate for pre-development) conditions. Event mean temperatures generally followed the land use trend of parking lot > light industrial > commercial > heavy industrial > multi-family residential, single family residential > low density residential > forest. Retrofitted bioretention cells and permeable pavements were not successful in mitigating runoff temperature in residential watersheds compared to (1) watersheds without SCMs or (2) restoring temperature to surrogate pre-development conditions. SCMs were seldomly able to mitigate enough runoff volume to have a significant thermal load reduction compared to watersheds without SCMs. These results suggest stormwater management and cold-water species management have not yet been successfully integrated. More work is needed to optimize green infrastructure SCMs design for sufficient treatment of runoff to protect cold-water ecosystems from urban development. Beyond SCM implementation, wholistic watershed management practices (e.g., riparian buffers, clustered imperviousness, underground storage/conveyance) are needed to stem thermal enrichment of streams and lakes by stormwater.

Loss of riparian forests from wildfire led to increased stream temperatures in summer, yet salmonid fish persisted

AbstractNative salmonid fishes—cutthroat trout (Oncorhynchus clarkii) and steelhead/rainbow trout (Oncorhynchus mykiss)—are ecologically, culturally, and economically important species distributed across western North America. These fish are generally considered “cold‐water” adapted species. As such, recent studies have speculated about the potential effects of climate change on these native salmonids if stream temperature thresholds exceed 16–20°C during the summer. However, the magnitude of stream thermal responses to the slow but steady increases in regional temperatures associated with climate change remains uncertain and hard to predict. Comparatively, abrupt disturbances, such as wildfire, may produce almost instantaneous and substantial increases in stream temperatures that may persist for multiple years until near‐stream vegetation becomes re‐established. In the first summer following a severe wildfire in western Oregon, we observed the initial persistence of populations of O. clarkii clarkii (coastal cutthroat trout) and O. mykiss (rainbow/steelhead trout). The fire burned the entire catchment, including the riparian area (~76% of the watershed area burned at moderate or high severity), resulting in stream temperature that regularly exceeded 20°C and represented increases of 6–7°C relative to prefire conditions. However, the mechanisms enabling the persistence of cold‐water fishes despite the dramatic increases in stream temperature remain unclear and require further investigation. Nevertheless, wildfires represent acute natural disturbances that can substantially alter stream thermal regimes and provide unique insights that allow us to better understand how native fishes in natural systems cope with projected increases in stream temperatures.

Relocated beaver can increase water storage and decrease stream temperature in headwater streams

AbstractMany areas are experiencing increasing stream temperatures due to climate change, and some are experiencing reduced summer stream flows and water availability. Because dam building and pond formation by beaver can increase water storage, stream cooling, and riparian ecosystem resilience, beaver have been proposed as a potential climate adaption tool. Despite the large number of studies that have evaluated how beaver activity may affect hydrology and water temperature, few experimental studies have quantified these outcomes following beaver relocation. We evaluated changes in temperature and water storage following the relocation of 69 beaver into 13 headwater stream reaches of the Skykomish River watershed within the Snohomish River basin, Washington, USA. We evaluated how beaver dams affected surface and groundwater storage and stream temperature. Successful relocations created 243 m3of surface water storage per 100 m of stream in the first year following relocation. Dams raised water table elevations by up to 0.33 m and stored approximately 2.4 times as much groundwater as surface water per relocation reach. Stream reaches downstream of dams exhibited an average decrease of 2.3°C during summer base‐flow conditions. We also assessed how dam age, condition, maintenance frequency, and pond morphology influenced stream temperature at naturally colonized wetland complexes. Our findings demonstrate that dam building can increase water storage and reduce stream temperatures in the first year following successful beaver relocation. Fluvial and floodplain morphology of candidate reaches for relocation is an important consideration because it determines the type and magnitude of response. Relocation to reaches with existing small, abandoned ponds may address thermal criteria by conversion from warming to cooling reaches, whereas relocation within large, abandoned complexes or vacant habitat may result in greater water storage. Although beaver relocation can be an effective climate adaptation strategy to retain more stable hydrologic regimes and water quality in our study area, there appear to be regionally specific environmental and geomorphic factors that influence how beaver affect water storage and temperature. More research is needed to investigate how and why these regional differences affect water storage and stream temperature response in beaver‐influenced systems.

How riparian and floodplain restoration modify the effects of increasing temperature on adult salmon spawner abundance in the Chehalis River, WA.

Stream temperatures in the Pacific Northwest are projected to increase with climate change, placing additional stress on cold-water salmonids. We modeled the potential impact of increased stream temperatures on four anadromous salmonid populations in the Chehalis River Basin (spring-run and fall-run Chinook salmon Oncorhynchus tshawytscha, coho salmon O. kisutch, and steelhead O. mykiss), as well as the potential for floodplain reconnection and stream shade restoration to offset the effects of future temperature increases. In the Chehalis River Basin, peak summer stream temperatures are predicted to increase by as much as 3°C by late-century, but restoration actions can locally decrease temperatures by as much as 6°C. On average, however, basin-wide average stream temperatures are expected to increase because most reaches have low temperature reduction potential for either restoration action relative to climate change. Results from the life cycle models indicated that, without restoration actions, increased summer temperatures are likely to produce significant declines in spawner abundance by late-century for coho (-29%), steelhead (-34%), and spring-run Chinook salmon (-95%), and smaller decreases for fall-run Chinook salmon (-17%). Restoration actions reduced these declines in all cases, although model results suggest that temperature restoration alone may not fully mitigate effects of future temperature increases. Notably, floodplain reconnection provided a greater benefit than riparian restoration for steelhead and both Chinook salmon populations, but riparian restoration provided a greater benefit for coho. This pattern emerged because coho salmon tend to spawn and rear in smaller streams where shade restoration has a larger effect on stream temperature, whereas Chinook and steelhead tend to occupy larger rivers where temperatures are more influenced by floodplain connectivity. Spring-run Chinook salmon are the only population for which peak temperatures affect adult prespawn survival in addition to rearing survival, making them the most sensitive species to increasing stream temperatures.

Regional, multi-decadal analysis on the Loire River basin reveals that stream temperature increases faster than air temperature

Abstract. Stream temperature appears to be increasing globally, but its rate remains poorly constrained due to a paucity of long-term data and difficulty in parsing effects of hydroclimate and landscape variability. Here, we address these issues using the physically based thermal model T-NET (Temperature-NETwork) coupled with the EROS semi-distributed hydrological model to reconstruct past daily stream temperature and streamflow at the scale of the entire Loire River basin in France (105 km2 with 52 278 reaches). Stream temperature increased for almost all reaches in all seasons (mean =+0.38 ∘C decade−1) over the 1963–2019 period. Increases were greatest in spring and summer, with a median increase of + 0.38 ∘C (range =+0.11 to +0.76 ∘C) and +0.44 ∘C (+0.08 to +1.02 ∘C) per decade, respectively. Rates of stream temperature increases were greater than for air temperature across seasons for the majority of reaches. Spring and summer increases were typically greatest in the southern part of the Loire basin (up to +1 ∘C decade−1) and in the largest rivers (Strahler order ≥5). Importantly, air temperature and streamflow could exert a joint influence on stream temperature trends, where the greatest stream temperature increases were accompanied by similar trends in air temperature (up to +0.71 ∘C decade−1) and the greatest decreases in streamflow (up to −16 % decade−1). Indeed, for the majority of reaches, positive stream temperature anomalies exhibited synchrony with positive anomalies in air temperature and negative anomalies in streamflow, highlighting the dual control exerted by these hydroclimatic drivers. Moreover, spring and summer stream temperature, air temperature, and streamflow time series exhibited common change points occurring in the late 1980s, suggesting a temporal coherence between changes in the hydroclimatic drivers and a rapid stream temperature response. Critically, riparian vegetation shading mitigated stream temperature increases by up to 0.16 ∘C decade−1 in smaller streams (i.e. < 30 km from the source). Our results provide strong support for basin-wide increases in stream temperature due to joint effects of rising air temperature and reduced streamflow. We suggest that some of these climate change-induced effects can be mitigated through the restoration and maintenance of riparian forests.

Effects of Stream Thermal Variability on Macroinvertebrate Community: Emphasis on Native Versus Non-Native Gammarid Species

An increase in river water temperatures is being detected worldwide, with some predictions of an up to 4°C rise by 2050. Such stream temperature increase is likely to affect aquatic communities, with predicted declines and range shifts of cold stenothermic species and a facilitated dispersal and population development in temperature-tolerant species, including invasive ones. This study analyzed how thermal changes affect macroinvertebrate community compositions in three differentiated thermal regions within a single stream system. In each thermal region, we used standard surber sampling in combination with in-stream cross-exposure experiments, comparing the thermal response of native vs. non-native gammarids (Gammarus roeselii and Dikerogammarus villosus). Macroinvertebrate species composition was highly dependent on temperature, with indigenous gammarids preferring colder sites and invasive ones such as D. villosus being dominant at higher temperatures. Species composition was also strongly affected by season, substratum and the presence of macrophytes. In light of climate change, consideration of shifts in community composition highly driven by temperature needs to become integrated with biological response patterns related to morphological and flow degradation, chemical pollution and fine sediment. Such an approach is crucial for the effective conservation and restoration of native biodiversity and for a realistic prediction of the ability to reach policy targets for aquatic ecosystems.

Contribution of Glacier Runoff during Heat Waves in the Nooksack River Basin USA

The thirty-eight-year record (1984–2021) of glacier mass balance measurement indicates a significant glacier response to climate change in the North Cascades, Washington that has led to declining glacier runoff in the Nooksack Basin. Glacier runoff in the Nooksack Basin is a major source of streamflow during the summer low-flow season and mitigates both low flow and warm water temperatures; this is particularly true during summer heat waves. Synchronous observations of glacier ablation and stream discharge immediately below Sholes Glacier from 2013–2017, independently identify daily discharge during the ablation season. The identified ablation rate is applied to glaciers across the North Fork Nooksack watershed, providing daily glacier runoff discharge to the North Fork Nooksack River. This is compared to observed daily discharge and temperature data of the North Fork Nooksack River and the unglaciated South Fork Nooksack River from the USGS. The ameliorating role of glacier runoff on discharge and water temperature is examined during 24 late summer heat wave events from 2010–2021. The primary response to these events is increased discharge in the heavily glaciated North Fork, and increased stream temperature in the unglaciated South Fork. During the 24 heat events, the discharge increased an average of +24% (±17%) in the North Fork and decreased an average of 20% (±8%) in the South Fork. For water temperature the mean increase was 0.7 °C (±0.4 °C) in the North Fork and 2.1 °C (±1.2 °C) in the South Fork. For the North Fork glacier runoff production was equivalent to 34% of the total discharge during the 24 events. Ongoing climate change will likely cause further decreases in summer baseflow and summer baseflow, along with an increase in water temperature potentially exceeding tolerance levels of several Pacific salmonid species that would further stress this population.

Investigating the impacts of glacier melt on stream temperature in a cold-region watershed: Coupling a glacier melt model with a hydrological model

Stream temperature is a significant indicator for biodiversity and ecosystem health of stream ecosystem. The impacts of glacier melt processes on stream temperature, such as the cooling effect, have not been explicitly considered in process-based models, which limit model capability in simulating stream temperature in glacierized cold regions. We coupled a temperature index-based glacier melt model with the SWAT equilibrium temperature model to overcome this limitation. The coupled model was tested and applied in the upper Athabasca River basin in western Canada to simulate daily streamflow and stream temperature. It showed a good performance for stream temperature simulation with percent bias (PBIAS) less than ±5% and Nash-Sutcliffe efficiency (NSE) and coefficient of determination (R2) values greater than 0.85. The impact of glacier melt runoff on streamflow and stream temperature simulation was investigated by turning the melt model on and off. The results illustrate that the coupled model accurately reproduces the impacts of glacier melt by simulating higher streamflow and lower stream temperature during the melt season. In particular, the progressive reduction in the cooling effect of glacier melt runoff on stream temperature from headwater to downstream was well represented. Finally, the role of glacier melt runoff affecting stream temperature under climate change was assessed by the scenario simulations. This analysis revealed that an increase in glacier melt runoff in the near future would alleviate the average and peak stream temperature increase during the melt season, while a decrease in glacier melt runoff in the distant future would ultimately aggravate the stream temperature increase. Therefore, the impact of glacier melt runoff should be explicitly considered in process-based models to accurately model stream temperature and predict future climate change impact on the thermal regime in the glacierized watersheds. The model coupling framework developed in this study could be a useful tool for simulating stream temperature in glacierized watersheds.

The Hydrological Urban Heat Island: Determinants of Acute and Chronic Heat Stress in Urban Streams

AbstractDuring and after rainfall events, the interaction of precipitation with hot urban pavements leads to hot runoff, and its merger with urban streams can result in an abrupt change in water temperature that can harm aquatic ecosystems. To understand this phenomenon and its relation to land cover and hydrometeorological parameters, we analyzed data spanning two years from 100 sites in the eastern United States. To identify surges, we first isolated temperature jumps of at least 0.5°C over 15 min occurring simultaneously with water flow increase. Surge magnitude was defined as the difference between peak stream temperature and baseflow temperature right before the jump. At least 10 surges were observed in 53 of the studied streams, with some surges exceeding 10°C. Our results demonstrate that the watershed developed area and vegetation fraction are the best descriptors of surge frequency (Spearman correlation of 0.76 and 0.77, respectively). On the other hand, for surge magnitude and peak temperature, the primary drivers are stream discharge and stream temperature immediately before the surge. In general, the more urbanized streams were found to be already warmer than their more “vegetated” counterparts during baseflow conditions, and were also the most affected by temperature surges. Together, these findings suggest the existence of a hydrological urban heat island, here defined as the increase in stream temperature (chronic and/or acute), caused by increased urbanization.

High‐resolution temperature modeling of stream reconstruction alternatives

AbstractStreams are complex where biology, hydrology, and atmospheric processes are all important. Because quantifying and modeling of these systems can be challenging, many teams go directly to prescribed restoration treatments and principles. Restoration on the Middle Fork of the John Day River in Oregon, USA, shows how a project that was designed according to widely accepted restoration principles may lead to outcomes contrary to one of the project's stated goals: reducing peak temperatures for endangered salmonids on the site. This study employed the most sophisticated equipment available for stream temperature monitoring, including approximately 1 million independent hourly measurements in the 2‐week period considered. These data were collected along the river channel with fiber optic–distributed temperature sensing and were used to quantify thermal dynamics. These observations were paired with a physically based stream temperature model which was then employed to predict temperature change from design alternatives. Restored‐reach impact on peak temperature was directly correlated with the air–water interfacial area and the percentage of effective shade (R2 > 0.99). The increase in air–water area of the proposed design was predicted to increase daytime stream temperature by as much as 0.5°C upon completion of the work. Shade from riparian vegetation was found to potentially mitigate stream temperature increases, though only after decades of growth. A moderately dense canopy of 5 m tall trees blocking 17% of daily shortwave solar radiation is predicted to mitigate predicted temperature increases over the 1,800 m reach but also increases nighttime temperatures due to blocking of long‐wave radiation. These outcomes may not be intuitive to restoration practitioners and show how quantitative analysis can benefit the design of a project. This is significant in an area where riparian vegetation has been difficult to reestablish. Without quantitative analysis, restoration efforts can lead to outcomes opposite to stated goals and may be costly and disruptive interventions to fragile stream systems.

Thermal Characteristics of a Beaver Dam Analogues Equipped Spring-Fed Creek in the Canadian Rockies

Beaver dam analogues (BDAs) are becoming an increasingly popular stream restoration technique. One ecological function BDAs might help restore is suitable habitat conditions for fish in streams where loss of beaver dams and channel incision has led to their decline. A critical physical characteristic for fish is stream temperature. We examined the thermal regime of a spring-fed Canadian Rocky Mountain stream in relation to different numbers of BDAs installed in series over three study periods (April–October; 2017–2019). While all BDA configurations significantly influenced stream and pond temperatures, single- and double-configuration BDAs incrementally increased stream temperatures. Single and double configuration BDAs warmed the downstream waters of mean maxima of 9.9, 9.3 °C by respective mean maxima of 0.9 and 1.0 °C. Higher pond and stream temperatures occurred when ponding and discharge decreased, and vice versa. In 2019, variation in stream temperature below double-configuration BDAs was lower than the single-configuration BDA. The triple-configuration BDA, in contrast, cooled the stream, although the mean maximum stream temperature was the highest below these structures. Ponding upstream of BDAs increased discharge and resulted in cooling of the stream. Rainfall events sharply and transiently reduced stream temperatures, leading to a three-way interaction between BDA configuration, rainfall and stream discharge as factors co-influencing the stream temperature regime. Our results have implications for optimal growth of regionally important and threatened bull and cutthroat trout fish species.

Genome-wide diversity and habitat underlie fine-scale phenotypic differentiation in the rainbow darter (Etheostoma caeruleum).

Adaptation to environmental change requires that populations harbor the necessary genetic variation to respond to selection. However, dispersal‐limited species with fragmented populations and reduced genetic diversity may lack this variation and are at an increased risk of local extinction. In freshwater fish species, environmental change in the form of increased stream temperatures places many cold‐water species at‐risk. We present a study of rainbow darters (Etheostoma caeruleum) in which we evaluated the importance of genetic variation on adaptive potential and determined responses to extreme thermal stress. We compared fine‐scale patterns of morphological and thermal tolerance differentiation across eight sites, including a unique lake habitat. We also inferred contemporary population structure using genomic data and characterized the relationship between individual genetic diversity and stress tolerance. We found site‐specific variation in thermal tolerance that generally matched local conditions and morphological differences associated with lake‐stream divergence. We detected patterns of population structure on a highly local spatial scale that could not be explained by isolation by distance or stream connectivity. Finally, we showed that individual thermal tolerance was positively correlated with genetic variation, suggesting that sites with increased genetic diversity may be better at tolerating novel stress. Our results highlight the importance of considering intraspecific variation in understanding population vulnerability and stress response.

Impacts of small dams on stream temperature

Small, surface-release dams are ubiquitous features of the landscape that typically slow water flow and decrease canopy cover through impounded reaches, potentially increasing stream temperatures. However, reported effects of small dams on water temperature are variable, likely due to differences in landscape and dam characteristics. To quantify the range of thermal effects of small dams, we deployed continuous temperature loggers for one to four years at 30 dam sites across a range of environmental settings throughout Massachusetts (USA). Most dams (67%) warmed downstream waters, with August mean temperatures 0.20–5.25 °C higher than upstream. Downstream temperatures cooled with increased distance from the dam at 68% of sites, such that the warmest temperatures were observed closest to the dam. Where there was both a significant downstream warming effect and cooling pattern (seven sites), elevated temperatures persisted for an average of 1.31 km downstream of the dam. Dams with impoundments that caused the greatest relative widening of the stream channel and those on coldwater streams had the most warming, while streams with short dams in forested watersheds cooled most quickly downstream of the dam. Flow had a homogenizing effect on water temperatures at over half of the sites, whereby summer thermal impacts were more pronounced (e.g., more warming, faster cooling rates) under periods of lower flows. Downstream warming may reduce habitat for coldwater fishes and invertebrates, particularly where dams shift coldwater/coolwater habitat to warmwater. These results suggest that dam removal may mitigate elevated stream temperatures and increase ecosystem resilience in the face of a changing climate via restoration of critical coldwater habitats.

Stream temperature responses to experimental riparian canopy gaps along forested headwaters in western Oregon

Streamside (riparian) areas in the western United States and across much of North America are dominated by young, regenerating forests with closed canopies, which shade the understory and reduce light to streams. The addition of canopy gaps has been suggested as a management tool to accelerate development of riparian forest complexity, create stream light conditions that mimic those in late successional forests, and enhance in-stream productivity. Although gaps form naturally in late-successional forests, explicitly adding gaps is a concern because the increases in light that accompany a canopy gap may have the potential to increases stream temperature, which is an important ecological driver and regulatory metric in streams. The goal of this study was to determine whether and to what degree riparian forest canopy gaps that reflect localized disturbance events (mortality of one to a few dominant canopy trees) affect stream temperatures. We created experimental gaps in young regenerating riparian forests along six replicate headwater streams in western Oregon. Gaps increased light along 90 m study reaches by 3.91 (±1.63) moles of photons m−2 day−1, similar to that of a naturally occurring gap in a late-successional forest. Using a Before-After-Control-Impact study design, we assessed stream temperature by tracking multiple responses in each reach including: maximum seven day moving average of daily maximums (T7DayMax), maximum seven day moving average of daily means (T7DayMean), daily maximum, and mean summer temperatures, as well as within-reach (every 30 m) responses, and downstream recovery. Over a 40-day period in summer (July 22nd - August 30th), the mean response in T7DayMax across the six replicate streams was 0.21 °C ± 0.12, and the mean response in T7DayMean was 0.15 °C ± 0.14. Although the mean response in T7DayMax (a key regulatory metric) was small, changes varied across individual study streams, and the magnitude of the relative increase in stream T7DayMax as a result of the canopy gap was strongly negatively correlated with stream size. T7DayMax was not correlated with the size of the canopy opening or change in reach-scale light availability (within the range of gap sizes from 514 m2 to 1,374 m2 (0.05 to 0.14 ha)). Overall, riparian forest canopy gaps have the potential to increase stream temperatures, but in the western Cascade Mountain headwaters studied here, gap effects were small (all < 0.5 °C), and temperature responses declined as stream size increased.