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During much of the 21st century, natural runoff in the Colorado River basin has declined, while consumption has remained relatively constant, leading to historically low reservoir storage. Between January 2000 and April 2023, the amount of water stored in Lake Mead and Lake Powell, the two largest reservoirs in the United States, declined by 33.5 million acre feet (41.3 billion cubic meters). As of April 2023, total basin‐wide storage was sufficient to support the 21st century average rate of basin‐wide consumption for only 15 months. Runoff in spring 2023 is predicted to be large, providing a short‐term reprieve. However, it will take four to five additional unusually wet years in succession to refill Lake Powell and Lake Mead if basin‐wide water use remains unchanged. Increasing evapotranspiration and dry soils associated with global climate change makes such a scenario unlikely. To stabilize reservoir storage, basin‐wide use needs to equal modern runoff. To recover reservoir storage, basin‐wide use needs to decline even more. Based on 21st century average runoff, a 13%–20% decline in basin‐wide use would allow for stabilization and some reservoir storage recovery. Future policy debate about reservoir operations will inevitably concern whether most, or all, reservoir storage should be in Lake Mead or in Lake Powell. The choice of one or the other will result in significantly different environmental and recreational outcomes for Glen Canyon and the Grand Canyon. This article is categorized under:Water and Life > Stresses and Pressures on EcosystemsHuman Water > Water GovernanceScience of Water > Water and Environmental Change [ABSTRACT FROM AUTHOR]

The cold and clear water conditions present below many large dams create ideal conditions for the development of economically important salmonid fisheries. Many of these tailwater fisheries have experienced declines in the abundance and condition of large trout species, yet the causes of these declines remain uncertain. Here, we develop, assess, and apply a drift-foraging bioenergetics model to identify the factors limiting rainbow trout (Oncorhynchus mykiss) growth in a large tailwater. We explored the relative importance of temperature, prey quantity, and prey size by constructing scenarios where these variables, both singly and in combination, were altered. Predicted growth matched empirical mass-at-age estimates, particularly for younger ages, demonstrating that the model accurately describes how current temperature and prey conditions interact to determine rainbow trout growth. Modeling scenarios that artificially inflated prey size and abundance demonstrate that rainbow trout growth is limited by the scarcity of large prey items and overall prey availability. For example, shifting 10% of the prey biomass to the 13 mm (large) length class, without increasing overall prey biomass, increased lifetime maximum mass of rainbow trout by 88%. Additionally, warmer temperatures resulted in lower predicted growth at current and lower levels of prey availability; however, growth was similar across all temperatures at higher levels of prey availability. Climate change will likely alter flow and temperature regimes in large rivers with corresponding changes to invertebrate prey resources used by fish. Broader application of drift-foraging bioenergetics models to build a mechanistic understanding of how changes to habitat conditions and prey resources affect growth of salmonids will benefit management of tailwater fisheries. Les conditions d'eau froide et claire presentes en aval de nombreux grands barrages creent les conditions ideales pour le developpement de peches aux salmonides d'importance economique. Si bon nombre de ces peches en aval de barrages ont connu des baisses d'abondance et d'embonpoint de grandes especes de truites, les causes de ces baisses demeurent incertaines. Nous elaborons, evaluons et appliquons un modele bioenergetique d'approvisionnement a la derive afin de cerner les facteurs qui limitent la croissance des truites arc-en-ciel (Oncorhynchus mykiss) dans un grand cours d'eau en aval d'un barrage. Nous examinons l'importance relative de la temperature, de la quantite de proies et de la taille des proies en elaborant des scenarios dans lesquels ces variables, individuellement ou combinees, sont modifiees. La croissance predite concorde avec les estimations empiriques du poids selon l'age, particulierement pour les groupes d'ages plus jeunes, ce qui demontre que le modele decrit avec exactitude l'effet de l'interaction de la temperature et des conditions relatives aux proies sur la croissance des truites arc-en-ciel. Des scenarios de modelisation qui gonflent artificiellement la taille et l'abondance des proies demontrent que la croissance des truites arc-en-ciel est limitee par la rarete de grosses proies et la disponibilite globale de proies. Par exemple, le fait de transferer 10 % de la biomasse de proies dans la classe de longueur de 13 mm (grande) sans accroitre la biomasse globale de proies se traduit par une augmentation de 88 % du poids maximum durant la vie des truites arc-en-ciel. En outre, des temperatures plus chaudes se traduisent par une croissance predite plus faible pour un degre de disponibilite de proies egal ou plus faible que le degre actuel; cependant, la croissance est semblable pour toutes les temperatures pour des degres de disponibilite de proies plus eleves. Les changements climatiques modifieront vraisemblablement les regimes d'ecoulement et de temperature dans les grandes rivieres, entrainant des changements correspondants aux ressources de proies invertebrees utilisees par les poissons. L'application elargie de modeles bioenergetiques d'approvisionnement a la derive pour etablir une comprehension mecaniste de l'influence des modifications des conditions de l'habitat et des ressources en proies sur la croissance des salmonides sera utile pour la gestion des peches en aval de barrages. [Traduit par la Redaction], Introduction Tailwaters below large dams are often characterized by cold and clear hypolimnetic water releases, creating habitat conditions ideal for salmonids. These conditions support salmonid populations that serve as economically [...]

We estimated the abundance, survival, movement, and recruitment of non-native rainbow trout (Oncorhynchus mykiss) in the Colorado River in Grand Canyon to determine what controls their abundance near the Little Colorado River (LCR) confluence where endangered humpback chub (Gila cypha) rear. Over a 3-year period, we tagged more than 70 000 trout and recovered over 8200 tagged fish. Trout density was highest (10 000-25 000 fish x [km.sup.-1]) in the reach closest to Glen Canyon Dam, where the majority of trout recruitment occurs, and was 30- to 50-fold lower (200-800 fish x [km.sup.- 1]) in reaches near the LCR confluence ~100 km downstream. The extent of rainbow trout movement was limited with less than 1% of recaptures making movements greater than 20 km. However, because of high trout densities in upstream source areas, this small dispersal rate was sufficient to explain the threefold increase in the relatively small population near the LCR. Reducing dispersal rates of trout from upstream sources is the most feasible solution to maintain low densities near the LCR to minimize negative effects of competition and predation on humpback chub. Nous avons estime l'abondance, la survie, les deplacements et le recrutement de truites arc-en-ciel (Oncorhynchus mykiss) non indigenes dans le fleuve Colorado, dans le Grand Canyon, afin de determiner ce qui controle leur abondance pres de sa confluence avec la Petite riviere Colorado (LCR), un lieu de grossissement pour les chevaines a bosse (Gila cypha). Sur une periode de 3 ans, nous avons marque plus de 70 000 truites et recupere plus de 8200 poissons marques. La densite des truites etait la plus forte (10 000-25 000 poissons x [km.sup.-1]) dans le troncon le plus proche du barrage de Glen Canyon, oU a lieu la majorite du recrutement de truites, et etait de 30 a 50 fois plus faible (200-800 poissons x [km.sup.-1]) dans les troncons pres de la confluence avec la LCR, environ 100 km en aval. L'ampleur des deplacements des truites arc-en-ciel etait limitee, moins de 1% des individus repris s'etant deplaces sur plus de 20 km. Cependant, en raison des fortes densites de truites dans les regions sources en amont, ce faible taux de dispersion suffit pour expliquer une augmentation par trois de la population relativement petite pres de la LCR. La reduction de taux de dispersion des truites provenant de sources en amont est la solution la plus faisable pour maintenir de faibles densites pres de la LCR, afin de minimiser les effets negatifs de la concurrence et de la predation sur les chevaines a bosse. [Traduit par la Redaction], Introduction Native fish are commonly threatened by increased competition and predation from non-native fish (Strayer 2010). This issue is of particular concern in regulated rivers, where changes in physical characteristics [...]

Aim: Grasslands cover a third of Earth's landmass and provide critical ecosystem services. Anticipating how perennial C3 (cool‐season) and C4 (warm‐season) grasses respond to climate change will be key to predicting future composition and functioning of grasslands. Here, we evaluate environmental drivers of C3 and C4 perennial distributions and assess how C3 and C4 grass distributions shift in response to future climate change. Location: Western United States. Methods: We developed integrated species distribution models to identify climate and soil drivers of relative abundance of C3 and C4 perennial grasses. We then created projections of species abundances under future climate and evaluated when and where projected shifts in relative abundance were robust across climate models. Results: Historically, C3 grasses occupied areas with lower temperature and more variable precipitation regimes, while C4 grasses occupied areas of higher temperature, greater temperature variability and greater warm‐season precipitation. C4 species also occupied narrower soil texture niches. In response to future climate change, C3 grass abundance declined across 74% of areas, while C4 abundance increased across 66% of areas. C3 grasses expanded in mid‐ to higher‐latitude areas with increasing temperature and decreasing seasonality of precipitation. In contrast, C4 grasses increased in higher‐latitude regions, but declined in lower‐latitude, dryer regions. Results were surprisingly robust across climate scenarios, suggesting high confidence in the direction of these future changes. Main Conclusions: Findings imply C3 and C4 perennial grasses will have highly divergent responses to climate change that may result in grassland functional compositional changes. Increasing temperatures and precipitation variability may favour some C4 grasses, but C4 habitat expansion may be constrained by soil conditions in western USA. Results provide actionable insights for anticipating the impacts of climate change on grass‐dominated and co‐dominated ecosystems and improving large‐scale conservation and restoration efforts. [ABSTRACT FROM AUTHOR]

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Habitat suitability is a consequence of interacting environmental factors. In riparian ecosystems, suitable plant habitat is influenced by interactions between stream hydrology and climate, hereafter referred to as "hydroclimate". We tested the hypothesis that hydroclimate variables would improve the fit of ecological niche models for a suite of riparian species using occurrence data from the western United States. We focus on the climate conditions (temperature, precipitation and vapor pressure deficit) during the months of lowest and highest streamflow as integrative hydroclimate metrics of resource and stress levels. We found that the inclusion of hydroclimate variables improved model fit for all species in the western USA dataset. We then tested the utility of the improved habitat suitability models by projecting them onto a regulated segment of the Colorado River to assess potential impacts of streamflow seasonality on vegetation metrics of management concern. Species frequency derived from independent survey data in the Colorado River segment was significantly higher for species with predicted suitable habitat than for species without predicted suitable habitat. Under different simulated hydrographs for the Colorado River, overall species richness was predicted to be greatest with peak streamflows during summer, and native‐to‐non‐native species ratios were predicted to be greatest with lowest streamflows in winter. Summer high flows were particularly associated with higher predicted habitat suitability for species that have increased in cover over recent decades (e.g., Pluchea sericea, Baccharis species). We conclude that hydroclimate covariates can be useful tools for predicting how riparian vegetation communities respond to changes in the seasonal timing of low and high streamflows. [ABSTRACT FROM AUTHOR]

We documented a significant mortality event affecting a southwestern pond turtle (Actinemys pallida) population living in a lake in southern California, USA. The area around the lake was impacted by a large wildland fire in 2013 that occurred during a protracted drought. As the mortality event was still unfolding, we collected data in 2014 on water quality, demographic structure, and short-term survivorship of the population. Water quality was poor with low levels of dissolved oxygen and high salinity of up to 45.90 ppt. Many turtles were severely emaciated and coated with a pale mineralized layer on their shells and skin. Estimated survival rate was low leading to a projected 90% decline in 134 days and a high probability of extirpation within a year. The lake was dry in September 2015 with no evidence of live turtles. Necropsies and low volumetric body condition indices suggested death by starvation. Although this semi-aquatic species has the ability to aestivate in upland habitats during periods of low water or move to other nearby water bodies, it is unlikely that many were able to do so because of their extremely poor condition and the severity of the drought conditions throughout the area.