Dryland Streams Research Articles

Alluvial ground water influences dissolved organic matter biogeochemistry of pools within intermittent dryland streams

Summary The dissolved organic matter (DOM) biogeochemistry of intermittent rivers is affected by periods of disconnection and drying after cessation of surface flow, with a pronounced concentration of solutes as pools evaporate and contract. However, inputs of alluvial ground water to stream pools may offset the effects of evaporation and influence the quantity and composition of DOM. We used δ18O and δ2H values of surface water and alluvial ground water (AW) together with DOM fluorescence excitation–emission spectroscopy to investigate changes in DOM biogeochemistry of pools from four intermittent streams in semi‐arid north‐west Australia. Our objective was to determine whether the fluorescence characteristics of DOM in pools are driven by their connectivity to alluvial ground water. Pool water δ18O values ranged from −8 to 13.1 ‰ and showed progressive enrichment as pools became disconnected from AW and subsequently evaporated. Parallel factor analysis of DOM fluorescence revealed that DOM in pools was generally dominated by humic‐like components, with lesser contributions from tryptophan‐like and tyrosine‐like components. DOC concentrations were higher in more evaporated pools. Humic‐like and tryptophan‐like DOM were also highest in pools isolated from AW, suggesting that inputs of AW had a dilution effect while also moderating evaporative concentration of solutes. In contrast, tyrosine‐like DOM was only weakly correlated with the hydrological status of pools. The proportion that each component contributed to total DOM fluorescence remained mostly consistent across both AW‐connected and evaporating pools. However, some pools with AW throughflow had higher contributions from tyrosine‐like fluorescence and lower contributions from humic‐like fluorescence. Disconnection from AW and subsequent evaporative concentration of solutes is an important control on DOM fluorescence characteristics in these intermittent streams. However, production and removal of DOM were only rarely influenced by AW connectivity, likely reflecting the high hydrological variability of intermittent rivers, even following cessation of surface flow.

Dispersal strength determines meta‐community structure in a dendritic riverine network

AbstractAimMeta‐community structure is a function of both local (site‐specific) and regional (landscape‐level) ecological factors, and the relative importance of each may be mediated by the dispersal ability of organisms. Here, we used aquatic invertebrate communities to investigate the relationship between local and regional factors in explaining distance decay relationships (DDRs) in fragmented dendritic stream networks.LocationDryland streams distributed within a 400‐km2 section of the San Pedro River basin, south‐eastern Arizona, USA.MethodsWe combined fine‐scale local information (flow and habitat characteristics) with regional‐scale information to explain DDR patterns in community composition of aquatic invertebrate species with a wide range of dispersal abilities. We used a novel application of a landscape resistance modelling approach (originally developed for landscape genetic studies) that simultaneously assessed the importance of local and regional ecological factors as well as dispersal ability of organisms.ResultsWe found evidence that both local and regional factors influenced aquatic invertebrate DDRs in dryland stream networks, and the importance of each factor depended on the dispersal capacities of the organisms. Local and weak dispersers were more affected by site‐specific factors, intermediate dispersers by landscape‐level factors, and strong dispersers showed no discernable pattern. This resulted in a strongly hump‐shaped relationship between dispersal ability and landscape‐level factors, where only moderate dispersers showed evidence of DDRs. Unlike most other studies of dendritic networks, our results suggest that overland pathways, using perennial refugia as stepping‐stones, might be the main dispersal route in fragmented stream networks.Main conclusionsWe suggest that using a combination of landscape and local distance measures can help to unravel meta‐community patterns in dendritic systems. Our findings have important conservation implications, such as the need to manage river systems for organisms that span a wide variety of dispersal abilities and local ecological requirements. Our results also highlight the need to preserve perennial refugia in fragmented networks, as they may ensure the viability of aquatic meta‐communities by facilitating dispersal.

Evaluating recharge to an ephemeral dryland stream using a hydraulic model and water, chloride and isotope mass balance

Dewatering associated with mining below water table to achieve dry mining conditions may exert significant pressure on water balance in terms of lowering the water table and change in the dynamics of interactions between surface water and groundwater. The discharge of surplus mine water into ephemeral streams may also affect the water balance, by elevating groundwater levels and altering the exchange rate between streams and underlying aquifers. However, it is unclear whether volumes and recharge processes are within the range of natural variability. Here, we present a case study of an ephemeral creek in the semi-arid Hamersley Basin of northwest Australia that has received continuous mine discharge for more than six years. We used a numerical model coupled with repeated measurements of water levels, chloride concentrations and the hydrogen and oxygen stable isotope composition (δ2H and δ18O) to estimate longitudinal evapotranspiration and recharge rates along a 27 km length of Weeli Wolli Creek. We found that chloride increased from 73 to 120 mg/L across this length, while δ18O increased from −8.2‰ to −7.00‰. Groundwater is directly connected to the creek for the first 13 km and recharge rates are negligible. Below this point, the creek flows over a highly permeable aquifer and water loss by recharge increases to a maximum rate of 4.4 mm/d, which accounts for ∼65% of the total water discharged to the creek. Evapotranspiration losses account for the remaining ∼35%. The calculated recharge from continuous flow due to surplus water discharge is similar to that measured for rainfall-driven flood events along the creek. Groundwater under the disconnected section of the creek is characterised by a much lower Cl concentration and more depleted δ18O value than mining discharge water but is similar to flood water generated by large episodic rainfall events. Our results suggest that the impact of recharge from continuous flow on the water balance of the creek has not extended beyond 27 km from the discharge point. Our approach using a combination of hydrochemical and isotope methods coupled with classical surface flow hydraulic modelling allowed evaluation of components of the water budget otherwise not possible in a highly dynamic system that is mainly driven by infrequent but large episodic floods.

Climate change poised to threaten hydrologic connectivity and endemic fishes in dryland streams.

Protecting hydrologic connectivity of freshwater ecosystems is fundamental to ensuring species persistence, ecosystem integrity, and human well-being. More frequent and severe droughts associated with climate change are poised to significantly alter flow intermittence patterns and hydrologic connectivity in dryland streams of the American Southwest, with deleterious effects on highly endangered fishes. By integrating local-scale hydrologic modeling with emerging approaches in landscape ecology, we quantify fine-resolution, watershed-scale changes in habitat size, spacing, and connectance under forecasted climate change in the Verde River Basin, United States. Model simulations project annual zero-flow day frequency to increase by 27% by midcentury, with differential seasonal consequences on continuity (temporal continuity at discrete locations) and connectivity (spatial continuity within the network). A 17% increase in the frequency of stream drying events is expected throughout the network with associated increases in the duration of these events. Flowing portions of the river network will diminish between 8% and 20% in spring and early summer and become increasingly isolated by more frequent and longer stretches of dry channel fragments, thus limiting the opportunity for native fishes to access spawning habitats and seasonally available refuges. Model predictions suggest that midcentury and late century climate will reduce network-wide hydrologic connectivity for native fishes by 6-9% over the course of a year and up to 12-18% during spring spawning months. Our work quantifies climate-induced shifts in stream drying and connectivity across a large river network and demonstrates their implications for the persistence of a globally endemic fish fauna.

Ecology, management, and conservation implications of North American beaver(Castor canadensis)in dryland streams

ABSTRACTAfter near‐extirpation in the early 20th century, beaver populations are increasing throughout many parts of North America. Simultaneously, there is an emerging interest in employing beaver activity for stream restoration in arid and semi‐arid environments (collectively, ‘drylands’), where streams and adjacent riparian ecosystems are expected to face heightened challenges from climate change and human population growth.Despite growing interest in reintroduction programmes, surprisingly little is known about the ecology of beaver in dryland streams, and science to guide management decisions is often fragmented and incomplete.This paper reviews the literature addressing the ecological effects and management of beaver activity in drylands of North America, highlighting conservation implications, distinctions between temperate and dryland streams, and knowledge gaps.Well‐documented effects of beaver activity in drylands include changes to channel morphology and groundwater processes, creation of perennial wetland habitat, and substantial impacts to riparian vegetation. However, many hypothesized effects derived from temperate streams lack empirical evidence from dryland streams.Topics urgently in need of further study include the distribution and local density of beaver dams; consequences of beaver dams for hydrology and water budgets; and effects of beaver activity on the spread of aquatic and riparian non‐native species.In summary, this review suggests that beaver activity can create substantial benefits and costs for conservation. Where active beaver introductions or removals are proposed, managers and conservation organizations are urged to implement monitoring programmes and consider the full range of possible ecological effects and trade‐offs.Copyright © 2014 John Wiley & Sons, Ltd.

Comparison of Unsteady and Quasi‐Unsteady Flow Models in Simulating Sediment Transport in an Ephemeral Arizona Stream1

Hummel, Ryan, Jennifer G. Duan, and Shiyan Zhang, 2012. Comparison of Unsteady and Quasi‐Unsteady Flow Models in Simulating Sediment Transport in an Ephemeral Arizona Stream. Journal of the American Water Resources Association (JAWRA) 48(5): 987‐998. DOI: 10.1111/j.1752‐1688.2012.00663.xAbstract: Hydrodynamic and sediment transport models are useful engineering tools for predicting unsteady flood flow and sediment transport. Many models such as HEC‐RAS, HEC‐6, and IALLUVIAL apply quasi‐unsteady flow model, whereas others apply the unsteady flow model. It remains unknown if a quasi‐unsteady flow model is sufficiently accurate for simulating sediment transport in rapidly varied unsteady flood events, especially in ephemeral rivers in arid and semiarid regions. This study compared the quasi‐unsteady HEC‐RAS 4.1 model with one‐dimensional (1D) Finite Volume Method (FVM) based model in simulating flood flow and sediment transport in the Pantano Wash, a dryland river in the state of Arizona. The objective is to determine which sediment transport method is appropriate in predicting bed elevation changes in an ephemeral stream, Pantano Wash, and if an unsteady model is more accurate than a quasi‐unsteady flow model in predicting sediment transport. Results showed that the quasi‐unsteady HEC‐RAS model and the 1D FVM yielded similar results of bed degradation and aggradation for this dryland stream, although the FVM model predicted better flood hydrographs. Among the seven sediment transport formulas embedded in HEC‐RAS, Yang’s and Engelund‐Hansen’s equations gave the best matches with the field measurements for this particular case study.

The role of vegetation in mitigating the effects of landscape clearing upon dryland stream response trajectory and restoration potential

ABSTRACTDryland rivers are recognized for limited research and high uncertainties with respect to understanding biogeomorphic processes. This study uses aerial photography, sediment analysis, palynology indicators and hydraulic modelling to investigate the role of riparian vegetation in influencing the response of systems to disturbance, the trajectory of channel evolution and the potential for management. The study focuses on cleared and uncleared sites in the Yerritup catchment, along the south coast of Western Australia, that occur along a transect with a consistent stream gradient and landscape topographic setting.Downstream reaches show no gross botanical change, but gradual sediment deposition across the floodplain of up to 40 cm based on palynological and sedimentary indicators. Channel response in the cleared section by incision, widening and floodplain degradation began rapidly after land clearing, but is driven by large flood events. Degradation of riparian vegetation has significantly increased the sensitivity of the system. The cleared reaches have transformed from a low‐capacity channel, under‐adjusted to the prevailing flow regime, to the large present channel that is now over‐adjusted to the predominantly low to moderate seasonal (occasional flood) flow regime. Modelling of pre‐settlement erosive potential reveals that the entire system was naturally sensitive to change, and was primed to erode once riparian vegetation was removed.The trajectory of channel evolution and the role of riparian vegetation is examined in relation to undisturbed reaches in the system and an appreciation of the historical range of variability in geomorphic response. Analysis of the patterns of contemporary vegetation growth identify the potential to re‐establish vegetation where it is elevated from saline baseflow. However, the system is assessed as being close to a threshold where restoration is no longer possible and remediation options become more limited as eco‐hydraulic and hydrochemical changes continue. Copyright © 2011 John Wiley & Sons, Ltd.

Environment–human relationships in historical times: The balance between urban development and natural forces at Leptis Magna (Libya)

The relationships between human modification of the environment and natural events in the Roman city of Leptis Magna (UNESCO world heritage), western Libya, are analyzed. For the first time, the history of Leptis Magna is tested against a geomorphological and stratigraphical reconstruction and radiocarbon dating. Historical and archaeological interpretations or analyses indicate the occurrence of different extreme natural events as the cause of the town’s decline: earthquakes, flooding, and tsunami. Geological and geomorphological surveys investigated the dynamics of the nearby Wadi Lebda, a major dryland stream that forms the depositional and erosional systems of the settlement area. Alluvial phases were studied by applying traditional stratigraphic analyses of outcrops and hand-cores. Additionally, the mapped flights of inset terrace surfaces provided insights into the human modifications of the natural depositional/erosional environment during historical times and the following alluvial phases affecting the Leptis Magna harbor. The results integrate the archaeological knowledge by providing some independent chronological constraints, and indicate that Leptis Magna history was tightly linked to the Wadi Lebda. Aware of the hazards related to devastating flooding, the Romans were able to cope with the threat posed by the wadi by performing engineering defensive hydraulic works around the town (dam and artificial channels). Once the economic decay began and the society could no longer guarantee the ongoing maintenance of these structures, the decline of the settlement started and the occurrence of destructive floods reclaimed the populated areas. Conversely, the occurrence of a large earthquake (365 CE), or of a tsunami that caused the disruption of the hydraulic systems and the infill of the harbor, has been discarded as primary cause of the decline of Leptis Magna.

Reconciling arroyo cycle and paleoflood approaches to late Holocene alluvial records in dryland streams

A century of research into the environmental records of alluvial-valley fills in drylands has led to new theories about landscape response to climate change and cultural evolution over the Holocene. In a largely separate line of inquiry, paleoflood hydrologists have scoured bedrock canyons for slackwater deposits, extending the flood record and exploring their climatic significance. Both approaches rely on the analysis and dating of late Holocene alluvium, sometimes along the same drainages, yet they differ in the geomorphic setting in which they should be applied. Studies of arroyo cut-and-fill cycles are focused on broad alluvial valleys, whereas paleoflood hydrology has been focused mainly in narrow bedrock canyons. With a focus on the southwestern U.S., we review and compare the fundamentals of these two approaches and their paradigmatic disconnect, and then discuss potential linkages that could lead to a more complete understanding of how dryland streams adjust to external stimuli. Recent regional compilations provide insight into the broader relation between the two record types over entire physiographic provinces. Meanwhile, new tools such as OSL dating, short-lived isotopes, and improved hydraulic modeling are paving the way for refinement and reconciliation of the two approaches within individual drainages. The relation between past arroyo cut-and-fill cycles and paleofloods must be thoroughly explored if we are to fully understand how drainages will respond to a changing climate over the coming decades to centuries.

Nitrogen cycling in a dryland stream ecosystem as determined by a 15N tracer experiment (LINX)

Nitrogen cycling in a dryland stream ecosystem as determined by a ¹⁵N tracer experiment (LINX)

Geomorphology of heavy metals in the sediments of Queen Creek, Arizona, USA

Data from Queen Creek, a dryland stream in southcentral Arizona, show that the distributions of heavy metals (especially Cu, Pb, Zn, and V) in sediments are strongly related to the spatial aspects of fluvial processes. Cu/Zn and Cu/V ratios permit discrimination between materials polluted by eroded mine tailings and those containing only erosion products from the surrounding metalliferous terrain. In stratigraphic sections, metal ratios discriminate between pre-industrial alluvium and post-industrial flood deposits. On valley floors, channels and active alluvium have the highest metal concentrations, while other process environments have decreasing amounts associated with decreasing frequencies of inundation. Materials from the mine tailings become progressively diluted with increasing distance from the source. Metals appear to be chemically stable once they are precipitated and adsorbed in the alkaline dryland conditions. Spatial variability of metal concentrations are more important than variability associated with particle size in explaining the fate of contaminants in the fluvial sediment system.