Dryland River Systems Research Articles

Landscape-Scale Modeling to Forecast Fluvial-Aeolian Sediment Connectivity in River Valleys.

Sedimentary landforms on Earth and other planetary bodies are built through scour, transport, and deposition of sediment. Sediment connectivity refers to the hypothesis that pathways of sediment transport do not occur in isolation, but rather are mechanistically linked. In dryland river systems, one such example of sediment connectivity is the transport of fluvially deposited sediment by wind. However, predictive tools that can forecast fluvial-aeolian sediment connectivity at meaningful scales are rare. Here we develop a suite of models for quantifying the availability of river-sourced sediment for aeolian transport as a function of river flow, wind regime, and land cover across 168 km of the Colorado River in Grand Canyon, USA. We compare and validate these models using topographic changes observed over 10 years in a coupled river sandbar-aeolian dunefield setting. The models provide a path forward for directly linking fluvial hydrology with the management and understanding of aeolian landscapes.

Inter‐decadal variation in diadromous and potamodromous fish assemblages in a near pristine tropical dryland river

AbstractFreshwater ecosystems are both incredibly biodiverse and highly threatened globally. Variation in environmental parameters including habitat and flow can substantially affect many ecological processes within riverine aquatic communities, but the ties between such parameters and ecology are neither well studied nor understood. In highly variable tropical dryland river systems, assessing such relationships requires data collection over inter‐decadal time scales, which is not typically permitted on development schedules driven over short periods (including election and funding cycles). Here, we used seine net sampling data collected over an 18‐year period in the tropical dryland Fitzroy River, Western Australia, to assess how environmental and temporal factors including habitat, seasonality, and inter‐annual variation in wet season magnitude affect the community assemblage structure, recruitment, and growth of aquatic species in dryland rivers. Results demonstrated that macrohabitat (main channel vs floodplain creek) and the magnitude of wet season rains and resultant flooding both had a substantial influence on biotic communities, alongside seasonal and diel variation. The magnitude of wet season flooding (measured as river discharge volume) had the greatest impact on assemblage composition within floodplain creek habitats and was a significant driver of recruitment rates and growth of recruits and adults of several species examined. This study highlights key considerations for conserving dryland river systems and constituent biota. Specifically, these are maintaining (a) rhythmicity of flow within each year, (b) diversity of flow volume between years, and (c) a variety of habitat types including ephemeral, semi‐permanent, and permanent shallow floodplain and deeper main channel pools, in order to support a diverse array of generalist and specialist diadromous and potamodromous fishes.

Valley-scale controls and hydrogeomorphic processes driving channel breakdown in a dynamic floodplain wetland system: The Mara River, Tanzania

The character and behaviour of dryland rivers are driven by complex interactions between extrinsic and intrinsic controls. A long-term perspective is necessary to assess the sensitivity of dryland rivers to extrinsic forcing, such as hydroclimatic changes, and to contextualise the threshold channel responses. The semi-arid Mara River in northern Tanzania is the only perennial river in the UNESCO World Heritage Serengeti-Mara ecosystem and provides water resources for human and wildlife populations. Satellite imagery and discharge modelling were used to assess the valley-scale controls and hydrogeomorphic processes driving the morphological adjustments of the lower Mara River and its floodplain wetland. Both the modern and palaeochannels undergo downstream morphological and hydrological changes as they traverse the broad, alluvial floodplain. In response to marked downstream declines in discharge (from ~300 m3·s−1 to ~45 m3·s−1) and stream power (from ~40 W·m−2 to ~0.5 W·m−2), the modern channel exhibits a nonequilibrium river response resulting in downstream declines in channel size (from ~60 m to ~10 m wide) and eventual channel breakdown in the floodplain wetland. Palaeochannels in the unconfined reaches are typically larger with higher palaeodischarges than the modern channel. While some palaeochannels exhibit similar downstream trends to the modern channel, others may have maintained a continuous channel farther into the wetland. Comparison of the Mara River to other dryland river systems highlights the importance of valley setting and hydroclimate as long-term controls that influence geomorphic river response over time. The lower Mara River represents a nonequilibrium river response to declining discharges related to transmission losses that leads to channel bed aggradation and avulsion, constituting an inherent condition in many dryland alluvial rivers. An improved understanding of the interplay between intrinsic and extrinsic controls is important for the assessment of river character, behaviour, and floodplain wetland development, particularly under future projections of more variable or extreme climate conditions.

Body size dictates physiological and behavioural responses to hypoxia and elevated water temperatures in Murray cod (Maccullochella peelii).

Increasing drought frequency and duration pose a significant threat to fish species in dryland river systems. As ectotherms, fish thermal and hypoxia tolerances directly determine the capacity of species to persist in these environments during low flow periods when water temperatures are high and waterbodies become highly stratified. Chronic thermal stress can compound the impacts of acute hypoxic events on fish resulting in significant fish mortality; however, it is not known if all size classes are equally susceptible, or if the allometric scaling of physiological processes means some size classes are disproportionately affected. We investigated the physiological responses of Murray cod (Maccullochella peelii) over a four-fold body size range (0.2-3000g) to acute changes in water temperature and oxygen concentration following 4 weeks of acclimation to representative spring (20°C) and summer (28°C) water temperatures. We recorded maximum thermal tolerance (CT max), oxygen limited thermal tolerance (PCTmax ), lowest tolerable oxygen level (as the oxygen level at which lose equilibrium; O2,LOE), gill ventilation rates and aerial surface respiration threshold, blood oxygen transport capacity and lactate accumulation. Acclimation to elevated water temperatures improved thermal and hypoxia tolerance metrics across all size classes. However, body size significantly affected thermal and hypoxia responses. Small M. peelii were significantly less hypoxia tolerant than larger individuals, while larger fish were significantly less thermal tolerant than smaller fish. Hypoxia constrained thermal tolerance in M. peelii, with both small and large fish disproportionally compromised relative to mid-sized fish. Our findings indicate that both very small/young (larvae, fry, fingerlings) and very large/older M. peelii in dryland rivers are at significant risk from the combined impacts of a warming and drying climate and water extraction. These data will inform policy decisions that serve to balance competing demands on precious freshwater resources.

Reservoir characteristics and diagenesis of the Buntsandstein sandstones in the Campine Basin (NE Belgium)

The red beds of the Buntsandstein (Early Triassic) in the Campine Basin (NE Belgium) display porosities between 5.3–20.2% (average 13.7%) and permeabilities varying between 0.02–296.4 mD (average 38.7 mD). Knowledge of their reservoir controlling properties, which today are missing, is important in view of potential geological storage of CO2 or natural gas and geothermal reservoir potential within these sandstones. Therefore the effects of diagenesis were assessed based on petrography, stable isotope analyses, fluid inclusion microthermometry, X-ray diffraction, electron microprobe and porosity-permeability core analyses. These sandstones were deposited by a dryland river system, in a warm, mostly arid climate with episodic rainfall and high evaporation rates. During wetter periods especially feldspars were dissolved. Strong evaporation during dry periods led to reprecipitation of the dissolved species as K-feldspar and quartz overgrowths, smectite and calcite/dolomite. Sediment reworking resulted in framework grains becoming clay coated. The clay coats are better developed in finer than in coarser grained sediments. The original smectite composing the rims converted to illite during burial. The tangential orientation of the clay platelets in the rims led to illite-mica-induced dissolution of quartz during burial/compaction, which is manifested as bedding parallel dissolution seams that are filled with clays and micas, especially in the fine-grained sandstone/siltstone/claystone. These constitute important barriers to the vertical flow within the reservoir. The released silica did not really affect the red sandstones but was exported (often on mm to cm scale) to nearby bleached horizons, where nucleation inhibiting clay rims are less well developed. The red colour of the sandstones arises from the presence of small amounts of Fe-oxides in the inherited clay rims. Migration of fluids enriched in organic acids, expelled from underlying Carboniferous coal-bearing strata, resulted in local bleaching of coarser grained horizons. In the finer grained sediments, the red colour was mostly preserved, which suggests that the reductive capacity of the fluid was limited.

Reproductive Strategies and Population Genetic Structure in Two Dryland River Floodplain Plants, Marsilea drummondii and Eleocharis acuta.

Aquatic plants share a range of convergent reproductive strategies, such as the ability to reproduce both sexually and asexually through vegetative growth. In dryland river systems, floodplain inundation is infrequent and irregular, and wetlands consist of discrete and unstable habitat patches. In these systems, life history strategies such as long-distance dispersal, seed longevity, self-fertilisation, and reproduction from vegetative propagules are important strategies that allow plants to persist. Using two aquatic plants, Marsilea drummondii and Eleocharis acuta, we investigated the proportions of sexual and asexual reproduction and self-fertilisation by employing next-generation sequencing approaches, and we used this information to understand the population genetic structure of a large inland floodplain in western New South Wales (NSW), Australia. Asexual vegetative reproduction and self-fertilisation were more common in M. drummondii, but both species used sexual reproduction as the main mode of reproduction. This resulted in a highly differentiated genetic structure between wetlands and a similar genetic structure within wetlands. The similarity in genetic structure was influenced by the wetland in the two species, highlighting the influence of the floodplain landscape and hydrology on structuring population genetic structure. The high levels of genetic variation among wetlands and the low variation within wetlands suggests that dispersal and pollination occur within close proximity and that gene flow is restricted. This suggests a reliance on locally sourced (persistent) seed, rather than asexual (clonal) reproduction or recolonisation via dispersal, for the population maintenance of plants in dryland rivers. This highlights the importance of floodplain inundation to promote seed germination, establishment, and reproduction in dryland regions.

Monitoring vital signs: Wetland vegetation responses to hydrological resources in the Macquarie Marshes NSW, Australia

AbstractHuman intervention through damming, diversion and extraction of water resources has created regulated dryland river systems. As a result, connectivity between river channels and floodplain wetlands has diminished. Wetland function is fundamentally linked to water availability: flooding exchanges water, nutrients, sediments and biota with the floodplain. Water allocation for environmental flows may mitigate wetland degradation following river regulation by improving hydrological connectivity between rivers and floodplain wetlands. We report the results (2014–15 to 2018–19) of a vegetation monitoring program in the Macquarie Marshes in north western New South Wales. As part of the long‐term program, we monitored semi‐permanent wetland (Water Couch (Paspalum distichum) grassland or sedgeland dominated) and River Red Gum (Eucalyptus camaldulensis) forest/woodland communities along with environmental and water resource availability predictors. During the five‐year period, we found linear and nonlinear responses of functional group abundance and species richness to the water year and to time since last inundation. We also found differences in species assemblages in response to the flow and flooding regime, particularly water year and inundation duration. These results indicated that wetlands continued to express dry and wet phase responses. We also observed a depressed response to the large 2016–17 flood event by a subset of semi‐permanent wetland sites that had not been inundated in 3 years, compared with sites that were more recently inundated. This result indicated that environmental water management, along with protection of natural inundation events, may improve the resilience of floodplain wetlands: increased hydrological connectivity may promote a stronger wetland response when floods occur. Our results indicate that inundation, in part comprised of managed environmental flows, is a primary driver of species assemblage and functional group representation in semi‐arid floodplain wetlands.

Dryland avulsion sequences: Insights from data-model comparison of a terminal dryland river system

An advection–diffusion model of fluvial processes was used to analyze the stratigraphic expression of avulsions in terminal river systems and understand their control on basin-fill architecture. The initial and boundary conditions of the model runs (i.e., catchment area, smoothed initial topographic surface, grain-size distribution and sediment supply rates) were extracted from the modern Rio Colorado dryland terminal river system in the Altiplano Basin (Bolivia). Water-discharge and sediment-load values were derived from global regression curves and the BQART equation, respectively. To evaluate the robustness of the simulations, the model was tested under increasing sediment-load scenarios ranging from 0.003 m3/s to 0.095 m3/s. Data-model comparison provided insights into the role of avulsions in the geomorphological evolution of terminal river systems. The observed stacking of sediments, as captured by geospatial and geochronological data from the Rio Colorado, is consistent with the high sediment-load scenarios, which start with a single-thread fluvial channel that in time radially expands over the floodplain by successive river avulsions on account of alluvial-ridge aggradation and channel-floor elevation above the surrounding floodplain. The model output shows a laterally extensive, convex-upwards lobate topography which is in agreement with the lateral and longitudinal geomorphology in the upper and lower coastal plain of the Rio Colorado. The simulated inter-avulsion period, which is the time period between two successive full (or stabilized) avulsions in the model, varies from 0.18 to 1.2 kyr and is consistent with the OSL-age determination in the Rio Colorado with inter-avulsion periods up to 1.28 ± 0.34 kyr.

Spatio‐temporal reconstruction of avulsion history at the terminus of a modern dryland river system

AbstractFluvial depositional architecture in an unconfined environment is governed by sediment dispersal across the alluvial plain through river‐path switching by avulsion. Documented inter‐avulsion periodicity from modern rivers ranges from tens to over a thousand years. In this study, a quantitative spatio‐temporal reconstruction of avulsion history is presented of the non‐vegetated and pristine modern Río Colorado dryland river system in the semi‐arid Altiplano Basin (Bolivia), based on the integrated analysis of satellite imagery and absolute age dating using optically stimulated luminescence, complemented with sedimentological and geomorphological ground‐truth data. This approach enables us to reconstruct the chronological order of channel belts of the Río Colorado, to determine avulsion recurrence time and inter‐avulsion periodicity, to identify mechanisms for flow path changes, and to present a morphodynamic model for the spatio‐temporal evolution of fluvial deposits in a semi‐arid environment. In a maximum timespan of 12.71 ± 1.5 ka, successive avulsions of the Río Colorado created a sheet of interconnected fluvial deposits, consisting of diverging and juxtaposed alluvial ridges that formed by sediment aggradation in point bars, crevasse splays, levees, and on the channel floor. The ridges show lateral onlap and amalgamation as the result of repeated avulsion and compensational stacking, whereby the river avoided the positive alluvial‐ridge relief of its precursors. The resultant morphology is fan‐shaped, convex‐up with a surface area of approximately 500 km2 and a maximum observed thickness of 3 m. The results show inter‐avulsion periods of the river of up to 1.28 ± 0.34 ka. A paucity in fluvial activity around 2 ka BP, and at present, is interpreted as the result of low river discharge related to long‐term dry periodicity in the El Niño Southern Oscillation circulation system. Each river path started as a low sinuous, single‐thread channel in a narrow belt, and in time increased its width and sinuosity by point‐bar expansion and rotation.

Determining the seasonal relationships between upstream flows and channel transmission losses along a dryland river reach

River transmission loss is the reduced amount of flow at downstream locations along a channel due to infiltration into the riverbed and riverbank, evaporation from water surface, transpiration by riparian vegetation and, temporary storage in the river and flood plains. Transmission losses reduce flood stage, recharge local and regional aquifers and, are main sources of groundwater recharge in dryland areas. This study determines the seasonal relationships between upstream flows and channel transmission losses along a river reach in Runde River catchment in Zimbabwe. Using regression equations, channel transmission loss volumes were modelled as the response variables while upstream flow volumes were the predictor variables. The study shows significant (p = 0.000) positive linear relationships between reach upstream flow and transmission loss during the cool, hot and rainy seasons. Also, during the post rainy season, a significant (p = 0.000) power relationship exist between reach upstream flow and transmission loss. This simple approach helps to understand water balances and transmission losses in data scarce dryland river systems.

Dynamics and scales of transmission losses in dryland river systems: a meta-analysis

ABSTRACT In this paper, 245 studies were reviewed to understand approaches used for estimating river channel transmission losses. Findings indicate that regression equations, differential equations, flow routing, experimental approaches and water balances are most widely used. Geographic Information Systems are becoming a convenient framework to display model results showing spatial variability of losses. In the United States, regression equations and experimental approaches involving controlled releases are widely used to assess transmission losses whereas in the dryland regions of Australia, water balance and flow routing approaches are popular. In Africa and Asia, regression equations and water balances are common approaches to estimate transmission losses. By using regression equations on data pooled from studies done in different dryland regions of the world, statistically significant (p<0.05) relationships were observed between transmission loss volume and, reach length, inflow, flow contributing area and runoff coefficient. Overall, the review underscores the importance of channel and catchment characteristics in shaping the dynamics of transmission losses. Two main limitations of the current approaches are that they are site-specific and require high amounts of data not always available in dryland regions due to sparse network of monitoring stations. The review also highlights existing knowledge gaps and future research needs.

Diatom-based inference model for conductivity reconstructions in dryland river systems from north Patagonia, Argentina

Diatom-based inference model for conductivity reconstructions in dryland river systems from north Patagonia, Argentina

Restoring the ecological integrity of a dryland river: Why low flows in the Barwon–Darling River must flow

SummaryFor dryland rivers globally, understanding hydro‐ecological function is fundamental to informing trade‐offs between consumptive water use and aquatic ecosystem integrity. The Barwon–Darling is an Australian dryland river system recognised for its hydrological variability, which is considered a primary driver of the riverine ecosystem. Emphasis has been placed on extremes of zero flow and flood but examining low flow hydrology and hydraulics – through historical and modern droughts – demonstrates that under natural conditions, the river system also exhibits persistent and predictable flow characteristics. From 1885 to 1950, prior to flow regulation, the Barwon–Darling flowed 92% of the time, and throughout severe droughts (1895–1903 and 1939–1945) the river system was characterised by: near‐perennial flows (85% of the time), with lotic (flowing water) habitats; and near‐annual, in channel, flow pulses. Furthermore, evidence of lotic biota is found consistently in Aboriginal middens dating over the past 15,000 years, thus indicating the long‐term persistence of lotic conditions. We propose these consistent hydrological and hydrodynamic features have shaped the ecology of aquatic biota in the Barwon–Darling River but are now experiencing unprecedented change. Flow storage and diversion have increased the frequency and duration of zero flows in some reaches, but arguably the most substantial impacts, along the entire river, are on: (i) low flows, which are now frequently below lotic thresholds, and (ii) the magnitude of near‐annual flow pulses, which are reduced by over 90%. Consequently, in modern droughts, the river becomes predominantly lentic (still‐water), an impact that is exacerbated by weirpools which create artificial lentic conditions for approximately 1000 km (40%) of river. The ecological impacts of these changes are increasingly apparent, with the loss of lotic biota and a reduction in biodiversity. An ecohydraulic perspective explains present impacts, provides new directions and some immediate solutions for river management, and clarifies choices for stakeholders.

Phytoplankton in dryland riverine waterholes: environmental drivers, variability and ecosystem-monitoring potential using different levels of taxonomic resolution and dataset reduction

Waterholes that remain in the dry season in intermittent dryland rivers are important biotic refugia, but detailed ecological descriptions of these habitats and their plankton are scarce. We aimed to determine spatial and temporal variation in phytoplankton assemblages in a tropical Australian dryland river system, their main environmental determinants and the potential of the phytoplankton for ecosystem monitoring. We sampled nine sites in three rivers over 2 years using standard methods. Water quality and phytoplankton assemblages varied considerably among sites, rivers and seasons, reflecting lithology, hydrology, bathymetry and local catchment influences. Major environmental drivers included conductivity, pH, temperature and species of N and P. We analysed several derived versions of the original dataset by using density and presence–absence data, eliminating rarer species and grouping species into higher taxa. We found substantial consistency among analyses in environmental drivers, identified using distance-based linear modelling, and in variability among systems, identified using nested permutational multivariate analysis of variance (PERMANOVA). Responsiveness of the algal assemblages to environmental drivers and consistency among analyses, even using subsamples at low taxonomic resolution, suggests potential for ecosystem monitoring and optimising of sample throughput, although variability among systems requires substantial effort to determine the range of reference conditions.

Genetic analysis suggests extensive gene flow within and between catchments in a common and ecologically significant dryland river shrub species; Duma florulenta (Polygonaceae).

AimThe conservation of plant species biodiversity has been identified as a crucial factor for the resilience of dryland ecosystems in the face of climate change and desertification. Duma florulenta (lignum) is a keystone species that facilitates biodiversity in the floodplains and wetlands of Australia's dryland river systems. This paper explores spatial genetic structure of lignum and investigates factors influencing dispersal and gene flow within and among river catchments of the northern Murray–Darling Basin.LocationNorthern Murray–Darling Basin, eastern Australia.MethodsA total of 122 individual plants from subpopulations located on rivers in four adjacent catchments were genotyped using 10 microsatellite markers. Microsatellite data were then analyzed using population genetic techniques to evaluate levels of gene flow and genetic structure and identify factors influencing dispersal.ResultsResults suggest high levels of gene flow between lignum subpopulations of the northern Murray–Darling Basin. AMOVA revealed small but significant differences between subpopulations, and STRUCTURE analysis did not detect meaningful structure when sampling information was not provided. However, when sampling information was supplied using the LOCPRIOR model, three genetic clusters were identified. All Lower Balonne subpopulations were assigned to cluster 1 while a number of the other subpopulations showed mixed ancestry. Weak relationships were identified between pairwise genetic distance and geographic as well as river distance, although the R 2 value of the former was only half that of the latter.Main conclusionsPatterns of genetic variation suggest frequent long‐distance overland gene flow largely as a result of the movement of seeds via floodwater. Therefore, maintenance of natural variability in flow regime is key both to maintain conditions favorable to recruitment and to promote dispersal and gene flow across the landscape. However, given future climate change projections persistence may be more reliant on the species ability to endure long periods of drought between flood events.

Cascades of sub‐decadal, channel‐floodplain changes in low‐gradient, non‐vegetated reaches near a dryland river terminus: Salar de Uyuni, Bolivia

AbstractThe terminus of the ephemeral Río Colorado is located at the margins of Salar de Uyuni, Bolivia, the world's largest salt lake. The low‐gradient (&lt;0.0006 m m‐1), non‐vegetated reaches approaching the terminus provide an excellent natural laboratory for investigating cascades of channel‐floodplain changes that occur in response to quasi‐regular flows (at least once annually) and fine‐grained sediment supply (dominantly silt and clay). High‐resolution satellite imagery (&lt;0.65 m, various dates from 2004 onwards) and field data reveal widespread, pronounced and rapid morphodynamics on sub‐decadal timescales, including channel erosion and chute cutoff formation, and development of crevasse channels and splays, floodouts (unchannelled surfaces at channel termini), and erosion cells (floodplain scour‐transport‐fill features). In particular, following high annual precipitation (&gt;400 mm) in 2004–2005 and two subsequent high magnitude daily precipitation events (~40 mm), all of which led to widespread flooding, numerous crevasse splays formed between 2004 and 2016, avulsions occurred at nearby floodouts, and erosion cells downstream of the splays and floodouts underwent striking morphological changes. High‐precision GPS data reveal two preferential localities for erosion cell development: partially or fully abandoned channels with crevasse splay remnants, and topographic lows between channels. In this overall low‐gradient setting, comparatively high gradients (up to ~0.0006 m m‐1) at the edge of splay deposits and topography created by crevasses and abandoned channels may initiate knickpoint retreat and thereafter erosion cell development. Abandoned channels with splays tend to give rise to narrow, deep erosion cells, while topographic lows promote relatively shallow, wide erosion cells. In both situations, erosion cells may extend upslope and downslope, and eventually connect to form straight channels. The channel‐floodplain morphodynamics near the Río Colorado terminus extend previous analyses of low‐gradient, dryland river systems, particularly because the lack of vegetation and quasi‐regular floods drive cascades of rapid changes on sub‐decadal timescales. © 2018 John Wiley &amp; Sons, Ltd.

Does the reintroduction of large wood in a large dryland river system benefit fish assemblages at the reach scale?

Benefits of reintroduced large wood in river channels are largely based on studies at site scales in high-energy systems. By comparison, relatively little is known of the benefit of reintroduced large wood in low-energy systems at larger, reach scales. The present study assessed the effects of reintroducing large wood on fish assemblages along the Barwon–Darling River, Australia. Fish were sampled in replicated reaches subject to three treatments: six reference (wooded), six control (unwooded) and six managed (wood reintroduced) reaches. Sampling was conducted before and several months after wood addition, and then during a period following several large floods. Results demonstrate that reintroducing large wood had limited effects on fish. There were significant differences between treatments in fish length, but not in total abundance or species composition between treatments. Significant differences were detected in total abundance, species composition and fish length over time. There was an interaction recorded between treatments and time for fish length, but not total abundance or species composition. It is suggested that the lack of response by fish was because the physical character and position of the reintroduced wood pieces did not replicate ‘natural’ reference conditions. However, high variability in fish assemblages through time, likely in response to hydrological variation, reduced the power of the study to detect differences between fish over the shorter time period of the study (&lt;5 years).

Direct and indirect effects of predatory young‐of‐year fishes in a dryland river food web

Summary Young‐of‐year (YOY) fishes are sometimes numerically dominant vertebrate consumers in many large river systems, but their effects as predators are not as well understood as those of adult fishes. We predicted that YOY fishes influence community composition and abundance of invertebrate prey. Predation effects could be especially important in recruitment‐driven dryland river systems, where YOY fishes seasonally comprise a large portion of overall fish biomass. We conducted a mesocosm experiment to quantify effects of YOY fishes on trophic dynamics and interactions with environmental factors in a dryland river food web. We manipulated presence of YOY fishes (an assemblage of cypriniform species) and supplemental allochthonous carbon (LEAF treatments) in 24 mesocosms, and measured invertebrate abundance and diversity over 6 weeks. Experimental conditions mimicked a seasonal river drying regime that occurs during YOY fish growth. Seasonal drying in the Rio Grande frequently results in isolated pools with altered habitat complexity and riparian connectivity. Fishes exerted direct top‐down control of invertebrate assemblage composition through differential prey selection, and indirect control mediated by non‐lethal responses of invertebrates to predatory fishes. We observed enhanced water clarity in LEAF treatments associated with greater prey selectivity in fishes, presumably through enhanced visual feeding. Stable isotope analysis (C and N) was used to measure indirect impacts of YOY fishes on invertebrate prey items. YOY fishes caused a significant decrease in “isotopic niche breadth” among invertebrate taxa, suggesting that fish predators limit foraging opportunities for these consumers. However, allochthonous carbon sources were predominant in invertebrate diets in all LEAF treatments, suggesting consumer access to more biologically available autochthonous production is limited by factors other than fish predation. Although the roles of abiotic drivers and adult fishes in determining food web structure have received more attention, our results indicate that YOY fishes exert biologically relevant top‐down control of invertebrate assemblage composition and trophic complexity in dryland river ecosystems against a backdrop of substantial changes in carbon availability and abiotic conditions (e.g. water quality and habitat volume) over the growing season.

Multiscale influence of woody riparian vegetation on fluvial topography quantified with ground‐based and airborne lidar

Coupling between riparian vegetation and river processes can result in the coevolution of plant communities and channel morphology. Quantifying biotic-abiotic interactions remains difficult because of the challenges in making and analyzing appropriately scaled observations. We measure the influence of woody vegetation on channel topography at the patch and reach scales in a sand-bedded, dryland river system (Santa Maria River, Arizona) with native Populus and invasive Tamarix. At the patch scale, we use ground-based LiDAR to relate plant morphology to “tail bars” formed in the lee of vegetation. We find vegetation roughness density (λf) to most influence tail bar shape and size, suggesting coherent flow structures associated with roughness density are responsible for sediment deposition at this scale. Using airborne LiDAR, we test whether relationships between topography and vegetation morphology observed at the patch scale are persistent at the reach scale. We find elevation of the channel (relative to the local mean) covaries with a metric of vegetation density, indicating analogous influences of vegetation density on topography across spatial scales. While these results are expected, our approach provides insight regarding interactions between woody riparian vegetation and channel topography at multiple scales, and a means to quantify such interactions for use in other field settings.

Landsat and GRACE observations of arid wetland dynamics in a dryland river system under multi-decadal hydroclimatic extremes

Arid wetlands are important for biodiversity conservation, but sensitive and vulnerable to climate variability and hydroclimatic events. Amplification of the water cycle, including the increasing frequency and severity of droughts and wet extremes, is expected to alter spatial and temporal hydrological patterns in arid wetlands globally, with potential threats to ecosystem services and their functioning. Despite these pressing challenges, the ecohydrological interactions and resilience of arid wetlands to highly variable water regimes over long time periods remain largely unknown. Recent broad-scale drought and floods over Australia provide unique opportunities to improve our understanding of arid wetland ecosystem responses to hydroclimatic extremes. Here we analysed the ecohydrological dynamics of the Coongie Lakes arid wetland in central Australia, one of the world’s largest Ramsar-designated wetlands, using more than two decades (1988–2011) of vegetation and floodwater extent retrievals derived from Landsat satellite observations. To explore the impacts of large-scale hydrological fluctuations on the arid wetland, we further coupled Landsat measurements with Total Water Storage Anomaly (TWSA) data obtained from the Gravity Recovery and Climate Experiment (GRACE) satellites.Pronounced seasonal and inter-annual variabilities of flood and vegetation activities were observed over the wetland, with variations in vegetation growth extent highly correlated with flood extent (r=0.64, p<0.05) that ranged from nearly zero to 3456km2. We reported the hydrological dynamics and associated ecosystem responses to be largely driven by the two phases (El Niño and La Niña) of the El Nino-Southern Oscillation (ENSO) ocean-atmosphere system. Changes in flood and vegetation extent were better explained by GRACE–TWSA (r=0.8, lag=0month) than rainfall (r=0.34, lag=3months) over the water source area, demonstrating that TWS is a valuable hydrological indicator for complex dryland river systems. The protracted Millennium Drought from 2001 to 2009 resulted in long-term absence of major flood events, which substantially suppressed wetland vegetation growth. However, the 2010–11 La Niña induced flooding events led to an exceptionally large resurgence of vegetation, with a mean vegetation growth extent anomaly exceeding the historical average (1988–2011) by more than 1.5 standard deviations, suggesting a significant resilience of arid wetland ecosystems to climate variability. This study showed the ecological functioning of arid wetlands is particularly sensitive to large-scale hydrological fluctuations and extreme drought conditions, and vulnerable to future altered water regimes due to climate change. The methods developed herein can be applied to arid wetlands located in other dryland river systems across the globe.