ABSTRACT Rivers supplied by glacial ice provide critical habitat for aquatic organisms that survive only in cold water, including federally threatened Bull Trout ( Salvelinus confluentus ). However, as glaciers retreat due to climate change, stream temperatures in key habitats could warm. This issue is particularly relevant to heavily glaciated Mount Rainier (Washington State, USA), where rivers sourced from cold glacial meltwater provide crucial spawning habitats for Bull Trout. To explore current and future conditions in these habitats, we used stream temperature data collected in the glacially fed White and Carbon Rivers to build spatial stream network models, representing contemporary thermal conditions as a function of glacial extent and air temperature. We focused predictions on Bull Trout spawning season, using September 7‐day average daily maximum (7DADM) stream temperatures. To simulate future climate change impacts, we adjusted models to predict stream temperature in mid‐ and late‐century scenarios of air temperature rise and decreased glacial extent. The average 7DADM temperature predicted for contemporary conditions was 6.3°C and 8.1°C in the White and Carbon River watersheds, respectively. With higher air temperatures and smaller glaciers, stream temperatures warmed by up to 9.4°C in the White River and 4.6°C in the Carbon by late century. The future proportion of thermally viable habitat for Bull Trout spawning significantly declined in both watersheds. Predictions for specific spawning streams, however, found that a few locations may continue to provide cold conditions. These results and methods can be utilized in the conservation of sensitive species and their enduring habitats in glacial rivers.
ABSTRACT Flow regime alteration is among the most pervasive threats to freshwater ecosystems globally, and migratory species are particularly vulnerable to these impacts. Despite this, specific aspects of flow regimes that shape spawning migrations remain poorly understood for most threatened species. Functional flow frameworks offer a promising tool for identifying ecologically meaningful flow targets, especially for threatened species, like the Sacramento River population of green sturgeon ( Acipenser medirostris ). Aspects of this species' life‐history are highly flow‐dependent, and its migratory corridor overlaps with heavily regulated waterways, making it an interesting case study for understanding how flow dynamics shape migratory behavior. We used an acoustic telemetry dataset (2007–2022) to explore how river flows impact spawning migration behavior of green sturgeon ( n = 220) through the California Sacramento‐San Joaquin Delta. A cluster analysis identified three migration patterns: mainstem Sacramento River routing, Delta routing, and incomplete migrants. Increased discharge during migration was positively associated with migration success and fish selecting the Sacramento River, with a threshold near 10,000 cfs, below which fish frequently routed through the Delta where water diversions and altered hydrology may pose additional risks. Higher pre‐migration temperatures were positively correlated with greater Delta use and strongly negatively correlated with migration success. Functional flow metrics related to wet season magnitude, wet season timing, and spring recession characteristics were the strongest predictors of route selection, migration success, and duration. Overall, this study demonstrates how functional flow frameworks combined with telemetry can uncover critical linkages between flow dynamics and spawning migrations, with direct implications for management in regulated rivers.
Rivers, particularly large river systems, constitute one of the most fundamental life-support systems that have sustained civilisations and are projected to be a critical determinant for the future sustenance of human civilisations. A river can be termed a healthy ecosystem only when all the flora and fauna are in and along the river, the cultivation in floodplains is healthy, and the river's form is in equilibrium. The Ganga River system in India is a large river system with high geodiversity and biodiversity, where about one-third of India's population depends on the river directly or indirectly. This study focuses on the development of a Geomorphic River Health Index (gRHI) based on eight geomorphic attributes derived from geospatial data, including sinuosity, braid-channel ratio, width/depth ratio, occupancy, floodplain development, bar density, bar complexity and stream power. The selected river health parameters were analysed using a multi-criteria decision-making technique, the Analytic Hierarchy Process (AHP), in a GIS framework. We adopted a five-fold river health classification framework (i.e., near-pristine, good, fair, poor and degraded) for the entire Ganga River system from Gomukh to Farakka. Our results, based on geospatial data and supported by field observations, indicate that most reaches of the Ganga River in the higher mountainous stretch are in 'near pristine' condition, while some in the lower reaches fall into the 'good' and 'fair' classes. In contrast, several reaches in the alluvial part show 'poor' river health, primarily associated with flow reduction and sediment accumulation leading to morphological degradation, which is well manifested in geomorphic indicators used for river health assessment in this study. The geomorphic framework presented in this study can serve as a good template to evaluate river health and link this to scenarios of habitat suitability, environmental flows and flood risk associated with large river systems.
In France, it is estimated that shads (Alosa spp.) have lost 69% of their historical range, mainly due to disrupted river connectivity. After an absence of one century, signs of their return in the Seine River have been recorded over the past two decades: adult and juvenile specimens have been caught by anglers or recorded at video-counting stations up to 400 km from the sea. To investigate the migratory behaviour and routes of allis shad in the lower navigable reaches of the Seine, an acoustic telemetry study was conducted. Nineteen individuals were captured, tagged and released at the first weir from the sea and tracked along a 188-km channelized reach, fragmented by five navigation weirs and locks, up to Paris. Seventeen individuals promptly resumed their upstream migration. Our results underscore the importance of quantifying the overall passage rate, the time fish spend waiting downstream of weirs before passing, and the cumulative effects of navigation weirs on migration delays. Low rates of passage through fishways were found, with most shad (93%) passing through locks without any specific management measures for this species. Throughout the day, trajectories were direct in open-river reaches, with median cruising speeds of approximately 0.8 m s-1. Time delays varied downstream of navigation weirs (median time of 27-87 h) and passage was observed during daytime. Ultimately, 16% of individuals reached Paris, passing five weirs with a cumulative delay of 6-12 days.
The middle reaches of the Yangtze River, especially the Jiujiang region with unconsolidated soil, suffer frequent erosion-induced collapses, yet the mechanism by which immersion duration regulates soil properties and drives differential erosion remains unclear. To address this gap, field investigations and sampling were focused on Jiujiang's riverbanks, laser diffraction analysis, potassium dichromate oxidation-spectrophotometry, and dry-wet sieving methods were utilized to examine soil property evolution under four immersion durations (severe, moderate, mild, slight). Results indicate: (1) As immersion duration shortens, soil particles transition from fine-dominated (clay, silt) to coarse-dominated (sand), with the lowest sand content in the severe immersion zone (43.1%-76.3%) and highest in the moderate immersion zone (69.08%-98.98%); (2) Organic carbon content first decreased, then increased with shorter immersion duration, reaching a trough in the moderate immersion zone (minimum < 0.2%) and a peak in the mild immersion zone (maximum > 1.0%); (3) Aggregate stability was poorest in the moderate immersion zone ( R-0.25= 50.12%-67.35%, highest PAD) and optimal in the slight immersion zone ( R-0.25= 78.39%-82.20%, lowest PAD); (4)Erosion phenomena progressively transition from Category IV (collapse, scouring) to Category I (minor topsoil loss) as immersion duration shortens. This phenomenon originates from immersion duration modifying soil particle size and aggregate stability, thereby changing anti-erosion capacity. These findings provide a scientific basis for projecting erosion-prone zones and offering targeted bank protection strategies, thereby serving as a valuable reference for similar studies on large alluvial rivers with regulated flow regimes and comparable dual-layer bank structures.
