River Network Structure Research Articles

Morphological controls on surface runoff: an interpretation of steady-state energy patterns, maximum power states and dissipation regimes within a thermodynamic framework

Abstract. Recent research explored an alternative energy-centred perspective on hydrological processes, extending beyond the classical analysis of the catchment's water balance. Particularly, streamflow and the structure of river networks have been analysed in an energy-centred framework, which allows for the incorporation of two additional physical laws: (1) energy is conserved and (2) entropy of an isolated system cannot decrease (first and second law of thermodynamics). This is helpful for understanding the self-organized geometry of river networks and open-catchment systems in general. Here we expand this perspective, by exploring how hillslope topography and the presence of rill networks control the free-energy balance of surface runoff at the hillslope scale. Special emphasis is on the transitions between laminar-, mixed- and turbulent-flow conditions of surface runoff, as they are associated with kinetic energy dissipation as well as with energy transfer to eroded sediments. Starting with a general thermodynamic framework, in a first step we analyse how typical topographic shapes of hillslopes, representing different morphological stages, control the spatial patterns of potential and kinetic energy of surface runoff and energy dissipation along the flow path during steady states. Interestingly, we find that a distinct maximum in potential energy of surface runoff emerges along the flow path, which separates upslope areas of downslope potential energy growth from downslope areas where potential energy declines. A comparison with associated erosion processes indicates that the location of this maximum depends on the relative influence of diffusive and advective flow and erosion processes. In a next step, we use this framework to analyse the energy balance of surface runoff observed during hillslope-scale rainfall simulation experiments, which provide separate measurements of flow velocities for rill and for sheet flow. To this end, we calibrate the physically based hydrological model Catflow, which distributes total surface runoff between a rill and a sheet flow domain, to these experiments and analyse the spatial patterns of potential energy, kinetic energy and dissipation. This reveals again the existence of a maximum of potential energy in surface runoff as well as a connection to the relative contribution of advective and diffusive processes. In the case of a strong rill flow component, the potential energy maximum is located close to the transition zone, where turbulence or at least mixed flow may emerge. Furthermore, the simulations indicate an almost equal partitioning of kinetic energy into the sheet and the rill flow component. When drawing the analogy to an electric circuit, this distribution of power and erosive forces to erode and transport sediment corresponds to a maximum power configuration.

Predicting invasive signal crayfish (Pacifastacus leniusculus) spread using a traditional survey and river network simulation

AbstractRivers are among the most vulnerable ecosystems to biological invasions. After introduction, predicting the timing and magnitude of population growth and range expansion is critical to decision making on where management tactics will be most efficient. Yet, management is often hindered by the lack of knowledge on species‐specific attributes and the spatially complex structure of river networks that influences connectivity between sites. We analysed the population structure and predicted the range expansion of signal crayfish (Pacifastacus leniusculus), an invasive species in North‐western Italy which was first recorded in the Valla Stream in 2009 and then spread to adjacent rivers including Erro Stream in 2020. A total of 1,284 individual P. leniusculus crayfish were collected in the Valla Stream. No significant difference was found in density or body condition along a downstream gradient for different sex and maturity classes. These empirical data were combined with the available scientific literature to obtain key life‐history information for P. leniusculus, including carrying capacity, downstream versus upstream dispersion and proliferation rate. We used the OCNet R package to simulate the Erro Stream network and applied a meta‐population model to predict the range expansion of this species over time. Results indicate P. leniusculus may completely invade the Erro Stream network in 30–40 years, depending on the proliferation rate and without additional introductions. These findings represent one of the first attempts to use optimal channel networks simulation in R to predict the meta‐population dynamics of aquatic invasive species, a potential key tool to prevent invasive species spread.

Population connectivity of aquatic insects in a dam‐regulated, desert river

AbstractHumans have exaggerated natural habitat fragmentation, negatively impacting species dispersal and reducing population connectivity. Habitat fragmentation can be especially detrimental in freshwater populations, whose dispersal is already constrained by the river network structure. Aquatic insects, for instance, are generally limited to two primary modes of dispersal: downstream drift in the aquatic juvenile life stages and flight during the terrestrial winged adult stage. Yet the impacts of large hydropower dams can make rivers uninhabitable for incoming (drifting) juvenile insects, with remaining refugia found only in tributaries. The ability of adult aquatic insects to traverse such river stretches in search of suitable tributary habitat likely depends on factors such as species‐specific dispersal ability and distance between tributaries. To explore the intersection of natural and human‐induced habitat fragmentation on aquatic insect dispersal ability, we quantified population genetics of three taxa with varying dispersal abilities, a caddisfly (Hydropsychidae, Hydropsyche oslari), a mayfly (Baetidae: Fallceon quilleri), and a water strider (Veliidae: Rhagovelia distincta), throughout tributaries of the Colorado River in the Grand Canyon, Arizona, USA. Using 2bRAD reduced genome sequencing and landscape genetics analyses, we revealed a strong pattern of isolation by distance among mayfly populations. This contrasts with caddisfly and water strider populations, which were largely panmictic. Analysis of thousands of informative single nucleotide polymorphisms showed that realized dispersal ability may not be accurately predicted by species traits for these widespread species. Principal components analysis revealed a strong division between caddisfly populations upstream and downstream of Havasu Creek (279 km through the 390 km study reach), suggesting that the geography of the Grand Canyon imposes a dispersal barrier for this species. Our use of genetic tools in the Grand Canyon to understand population structure has enabled us to elucidate dispersal barriers for aquatic insects. Ultimately, these data may be useful in informing effective conservation management plans for understudied organisms of conservation interest.

Effect of Urbanization on the River Network Structure in Zhengzhou City, China.

Hydrological problems, such as flood disasters, can be caused by the influence of urbanization on river network structures in plain areas. Taking the main urban region of Zhengzhou city as the research area, based on six remote sensing images from 1992 to 2015, the modified normalized difference water index method and a land-use transfer matrix were used to reconstruct river network data to study the temporal and spatial changes in the river system. In addition, the analytic hierarchy process and the entropy weight method were used to construct pattern indexes of the river system to quantitatively evaluate the inner relationship between the urbanization process and the river network structure in the plain area. The results showed that the percentages of arable land, forest and grassland, water, and unused land in Zhengzhou that was transferred to construction land from 1992 to 2015 were 59.10%, 51.05%, 29.83%, and 58.76%, respectively. In the past 34 years, the morphological indices, structural indices, and connectivity indices of the river system experienced a trend of high to low, and then increased, with the structural indices being significantly correlated with construction land use (p < 0.05). The regression equation R2 between urbanization level and river length, water area, river network density, water surface rate, connection rate, and connectivity ranged from 0.677 to 0.966, which could well reflect the response relationship between urbanization and the river network. In addition, the outflow was greater than the inflow, which has destroyed the natural structure of the channel.

Application of HEC-HMS for flood forecasting in hazara catchment Pakistan, south Asia

Floods have become more severe and frequent as a result of climate change around the world, posing a hazard to public safety and economic development. This study investigates the use of distributed hydrological models in flash flood risk management in a small watershed in Hazara, Pakistan, with the goal of improving Pakistan's early warning lead time. First, the HEC-HMS model was built using geographic data and the river network's structure, then calibrated and verified using eight high rainfall events from 2013. demonstrating that the HEC-HMS model could simulate floods in the research area Second, given that rainfall and flood events have happened, this paper proposes an analysis approach for a flood forecasting and warning system, as well as criteria for sending urban-stream flash flood alerts based on rainfall, in order to provide sufficient lead time. The DEMs (digital elevation models) of the research regions were processed using HEC-Geo HMS, an ArcView GIS tool for catchment delineation, terrain pre-processing, and basin processing. The model was calibrated and verified using previously observed data. The proposed flood prediction and risk reduction methodology is nonstructural. The Hydrologic Modeling System (HEC-HMS), which provides a sufficient lead time forecast and computes the runoff/stage threshold conditions, is at the heart of the flood warning application. For flood risk assessment, data from the Pakistan Meteorological Department (PMD) is entered into a hydro-meteorological database and then into the HEC-HMS. A server-client application was utilised to visualise the real-time flood scenario and send out an early warning message. The outcomes of this study will be used to develop flood validation measures in the Hazara stream watershed to deal with potential flash floods.

Quantification of river network types based on hierarchical structures

The development of river networks is closely related to landscape evolution processes and terrain analysis. Several methods for classifying river networks based on qualitative evaluation or quantitative indices have been developed. However, these methods pay little attention to the hierarchical structures of river networks and omit some generalized and detailed information about basins. In this work, a classification method based on the hierarchical structure of river networks is established. Drainage texture, flow direction and aspect ratios are employed to describe river network characteristics. The specific attributes calculated include the drainage density, the stream frequency of river networks and the maximum frequency, and the distribution uniformity of the flow direction of river reaches on each order. Classification trees are established using these metrics. The method is applied to classify 83 river networks in the Yellow River source zone ranging in size from 17 to 241 km2. A previous classification method using basin invariance attributes was contrasted with our network structure approach, and cross-validation (CV) accuracies were obtained of 66.8% and 82.0% respectively. To verify the applicability of the proposed method, another 45 river networks classified in previous studies are considered. A CV accuracy of 76.7% is achieved with the proposed method. These results demonstrate that the proposed method based on the hierarchical structures of river networks is more accurate than methods that ignore the hierarchical structure.

On the efficacy of restoration in stream networks: comments, critiques, and prospective recommendations

Swan and Brown (2017) recently addressed the effects of restoration on stream communities under the meta-community framework. Using a combination of headwater and mainstem streams, Swan and Brown (2017) evaluated how position within a stream network affected the outcome of restoration on invertebrate communities. Ostensibly, their hypotheses were partially supported as restoration had stronger effects in headwater streams: invertebrate taxonomic richness was increased and temporal variability decreased in restored reaches; however, these results were not consistent upon closer scrutiny for both the original paper (Swan and Brown 2017) and the later erratum (Swan and Brown 2018). Here, I provide a secondary analysis of the data, with hypotheses and interpretations in the context of stream, meta-community, and restoration ecology. Swan and Brown (2017, 2018) evaluated the effect of restoration on sites receiving various combinations of in-channel manipulation and riparian reforestation treatments. Given the difference in the relative importance of environmental filtering and dispersal between headwaters and mainstems and the structure of river networks, I contend that different restoration treatments have differential effects between headwaters and mainstems. I hypothesized in-channel manipulations would have more consistent effects between headwaters and mainstems compared to riparian reforestation, and I used this hypothesis to guide site selection in the re-analysis. I then compared results from the re-analysis to those presented by Swan and Brown (2017, 2018). I did not find any effects of restoration on local diversity, spatial dissimilarity, or temporal variability, let alone differential effects of restoration between headwaters and mainstems; these results are contrary Swan and Brown (2017, 2018), who reported that restoration increased taxonomic richness, increased spatial dissimilarity, and decreased temporal variability in restored headwater streams. I demonstrate further that the statistical tests conducted by Swan and Brown (2017, 2018) were invalid and, therefore, recommend the use of the results presented here. More broadly, I suggest, in agreement with Swan and Brown (2017, 2018) and a growing body of research, that river and stream restoration will likely have greater success if a regional approach is taken to designing and implementing restoration projects.

A Note on the Role of Seasonal Expansions and Contractions of the Flowing Fluvial Network on Metapopulation Persistence

AbstractDoes a dynamic drainage density have a role on species persistence in the river basin? The general viability of a focus species under time‐varying hydrologic connectivity and habitat quality is a topic gaining traction in view of recent advances in our understanding of flowing fluvial network dynamics and of ecological interactions occurring on directed trees. Here, we combine metapopulation dynamics and scaling theory to investigate how the structure of river networks and time‐changing hydrological and geomorphological attributes control local metapopulation survival. This is done by introducing seasonal fluctuations of the drainage density subsuming overall time‐changing connectivity and distributed changes in habitat quality of the fluvial domain. Suitable replicas of channel networks within an assigned domain are used to compute the statistics of evolving metapopulation capacities, properties of a landscape matrix measuring the viability of the focus species. To obtain consistent replicas of the substrate for ecological interactions, we employ constructs whose suitability for the task has long been established. We find that the river network structure blends the fluctuations into a nontrivial scaling of the metapopulation capacity with the sum of total active contributing sites at any point of the flowing network. The latter is proportional to the mean distance to the outlet of the flowing dendrite and to the tree diameter—a measure of the overall connectivity of the active stream links. Scaling emerges as a robust ensemble property that enables the linkage of ecological patterns across a river network to clearly identified hydrological and geomorphological factors.

Multifractal Analysis of River Networks under the Background of Urbanization in the Yellow River Basin, China

Multifractal theory provides an effective method for the scientific quantification of the river network features. This method has been applied to estimate river network structure in previous research, but there are few temporal and spatial analysis studied for large river basins based on multifractals. In this study, the Yellow River Basin and its nine provinces were selected as the study area, and the generalized fractal dimension and multifractal spectrum of the study area during the period 2000–2020 were calculated and analyzed. We analyzed the response relationship between the river network changes and the urbanization process. Results indicated that the river network of the study area exhibited obvious multifractal properties, which were mainly affected by dense river networks. The complexity of the studied river network has shown a tendency to decrease over time. The rate of change in the river network structure has a high positive correlation with urbanization, that is, the higher the rate of urbanization, the greater the impact on the river network structure. Additionally, the changes of the river network structure are more affected by urbanization during the rapid urbanization stage. We applied multifractal analysis to study the river network structure changes, which is of great significance for scientifically quantifying fluvial characteristics and studying the development and evolution of river networks.

The linkage between box-counting and geomorphic fractal dimensions in the fractal structure of river networks: the junction angle

AbstractIn the past, a great deal of research has been conducted to determine the fractal properties of river networks, and there are many kinds of methods calculating their fractal dimensions. In this paper, we compare two most common methods: one is geomorphic fractal dimension obtained from the bifurcation ratio and the stream length ratio, and the other is box-counting method. Firstly, synthetic fractal trees are used to explain the role of the junction angle on the relation between two kinds of fractal dimensions. The obtained relationship curves indicate that box-counting dimension is decreasing with the increase of the junction angle when geomorphic fractal dimension keeps constant. This relationship presents continuous and smooth convex curves with junction angle from 60° to 120° and concave curves from 30° to 45°. Then 70 river networks in China are investigated in terms of their two kinds of fractal dimensions. The results confirm the fractal structure of river networks. Geomorphic fractal dimensions of river networks are larger than box-counting dimensions and there is no obvious relationship between these two kinds of fractal dimensions. Relatively good non-linear relationships between geomorphic fractal dimensions and box-counting dimensions are obtained by considering the role of the junction angle.

Multiphase distribution and spatial patterns of perfluoroalkyl acids (PFAAs) associated with catchment characteristics in a plain river network

Perfluoroalkyl acids (PFAAs) have emerged as global concerning contaminants because of their persistence, bioaccumulation, and toxicological effects. The transport and fate of PFAAs on dimension of plain river networks (PRNs) are difficult to model due to the unique regional characteristics (i.e., undirectional flows, low slope, complicated structure and connectivity) and the lack of data on PFAAs concentrations and compositions. A typical PRN (Taihu Basin, China) was selected to elucidate the spatial patterns of PFAAs in multi-matrices, including colloidal phase, soluble phase, suspended particles, and sediment. PFAAs were ubiquitously detected in plain rivers with total concentrations of 18.48–1220 ng/L in colloids, 139.07–721.37 ng/L in soluble phase, 97.69–2247 ng/g dw in suspended particles, and <72.04–178.12 ng/g dw in sediment. PFAAs were more likely to transport via dissolved phase and accumulate into sediment. Colloids carried 45.46–62.59% of ∑PFAAs in overlying water, while suspended particles contained <36.63% of ∑PFAAs, suggesting the important role of colloids in preloading PFAAs. Moreover, PFAAs variability was correlated with indicators of the structure and connectivity of river network by gray relational analysis. The mean gray relational degrees can be sorted as edge-node ratio (0.7609) > network connectivity (0.7191) > river density (0.7012) > water surface ratio (0.6887) > river development coefficient (0.6504) > functional connectivity (0.4780). These results suggested that the effects of catchment characteristics should be taken into account in understanding PFAAs fate in the PRNs.

Is the River a Chemostat?: Scale Versus Land Use Controls on Nitrate Concentration‐Discharge Dynamics in the Upper Mississippi River Basin

AbstractThe Upper Mississippi River Basin is the largest source of reactive nitrogen (N) to the Gulf of Mexico. Concentration‐discharge (C‐Q) relationships offer a means to understand both the terrestrial sources that generate this reactive N and the in‐stream processes that transform it. Progress has been made on identifying land use controls on C‐Q dynamics. However, the impact of basin size and river network structure on C‐Q relationships is not well characterized. Here, we show, using high‐resolution nitrate concentration data, that tile drainage is a dominant control on C‐Q dynamics, with increasing drainage density contributing to more chemostatic C‐Q behavior. We further find that concentration variability increases, relative to discharge variability, with increasing basin size across six orders of magnitude, and this pattern is attributed to different spatial correlation structures for C and Q. Our results show how land use and river network structure jointly control riverine N export.

Topological structures of river networks and their regional‐scale controls: A multivariate classification approach

AbstractLandscape evolution is governed by the interplay of uplift, climate, erosion, and the discontinuous pattern of sediment transfer from the proximal source of erosion to distal sedimentary sinks. The transfer of sediment through the catchment system is often referred to as a cascade, the pattern of which is modulated by the interaction of key network characteristics such as the distribution of transport capacity and resultant zones of sediment storage. An understanding of how sediment production is modulated through river networks with different topological structures at the associated timescales has remained elusive but presents significant implications for the knowledge of river response to disturbance events, and floodplain asset management.A multivariate method of identifying representative topological structures from a range of river networks is presented. Stream networks from 59 catchments in the South Island of New Zealand were extracted from a digital elevation model and their key topological parameters quantified. A principal component analysis was implemented to reduce these to two‐dimensional axes that represent the magnitude of network branching and the topographic structure of each catchment, respectively. An agglomerative hierarchical clustering analysis revealed five network ‘types’, which are examined in terms of their internal structural characteristics and relationships to potential regional‐scale controls. Implications for sediment transfer in these network ‘types’, and their use as representative networks for further analysis, are discussed. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd

A stronger statistical test of stream restoration experiments

Swan and Brown (2017) recently addressed the effects of restoration on stream communities under the meta-community framework. Using a combination of headwater and mainstem streams, Swan and Brown (2017) evaluated how position within a stream network affected the outcome of restoration on invertebrate communities. Ostensibly, their hypotheses were partially supported as restoration had stronger effects in headwater streams: invertebrate taxonomic richness was increased and temporal variability decreased in restored reaches; however, these results were not consistent upon closer scrutiny for both the original paper (Swan and Brown 2017) and the later erratum (Swan and Brown 2018). Here, I provide a secondary analysis of the data, with hypotheses and interpretations in the context of stream, meta-community, and restoration ecology. Swan and Brown (2017, 2018) evaluated the effect of restoration on sites receiving various combinations of in-channel manipulation and riparian reforestation treatments. Given the difference in the relative importance of environmental filtering and dispersal between headwaters and mainstems and the structure of river networks, I contend that different restoration treatments have differential effects between headwaters and mainstems. I hypothesized in-channel manipulations would have more consistent effects between headwaters and mainstems compared to riparian reforestation, and I used this hypothesis to guide site selection in the re-analysis. I then compared results from the re-analysis to those presented by Swan and Brown (2017, 2018). I did not find any effects of restoration on local diversity, spatial dissimilarity, or temporal variability, let alone differential effects of restoration between headwaters and mainstems; these results are contrary Swan and Brown (2017, 2018), who reported that restoration increased taxonomic richness, increased spatial dissimilarity, and decreased temporal variability in restored headwater streams. I demonstrate further that the statistical tests conducted by Swan and Brown (2017, 2018) were invalid and, therefore, recommend the use of the results presented here. More broadly, I suggest, in agreement with Swan and Brown (2017, 2018) and a growing body of research, that river and stream restoration will likely have greater success if a regional approach is taken to designing and implementing restoration projects.

Impact of urbanization on stream structure and connectivity of plain river network in the Taihu Basin

Impact of urbanization on stream structure and connectivity of plain river network in the Taihu Basin

Modelling the effect of directional spatial ecological processes for a river network in Northern Italy

• Distribution of macroinvertebrate communities in mountain waters was studied. • The competing RCC and RES river theories allow for different interpretations. • The statistical analysis allows to discuss some of the features of the two theories. • Longitudinal gradient prevailed on local and lateral dimensions according to RCC. • Land use effects predominate only in human impacted landscapes according to RES. The River Continuum Concept (RCC) and the Riverine Ecosystem Synthesis (RES) are two different theories proposed by river ecologists to describe the response of biotic communities to environmental variability. River network directional patterns are conveniently described by asymmetric eigenvector maps, an eigenfunction-based spatial filtering method specifically proposed for situations where a hypothesized directional spatial process influences the species distribution. In this work asymmetric eigenvector maps are used in conjunction with canonical redundancy analysis and variation partitioning analysis to describe the distribution of macroinvertebrate communities of a river system in Northern Italy and to test the link between the river theories and the available data. Benthic macrofauna data were collected during the summer of 2009–2013 in 16 rivers, for a total of 283 replicates. We investigate the effects of some measured environmental factors on the benthic macrofauna community, accounting for directional effects intrinsic to the river network structure. The proposed protocol allows to highlight and discuss some of the features relevant to the two river theories. According to the RCC theory, altitude and temperature were relevant factors affecting the macrozoobenthic community, together with the distance from the spring and water depth. Environmental factors representing local and lateral dimensions were less relevant for explaining the variability of the community composition. Nonetheless a role of the surrounding land use was also found, suggesting the presence of lateral effects due to human activities. Overall, the results demonstrated that RCC is a reliable model to describe the distribution of macrobenthic communities in river networks. In socio-ecological systems, the local and lateral dimensions postulated by the RES theory could be mainly related to surrounding land use and naturalness degree.

Examining Observed Rainfall, Soil Moisture, and River Network Variabilities on Peak Flow Scaling of Rainfall‐Runoff Events with Implications on Regionalization of Peak Flow Quantiles

AbstractThe scaling of peak flows associated with a probability of exceedance (Qp) or a specific rainfall‐runoff event (QR) with respect to drainage area (A) is known as flood scaling and it has been widely used in peak flow regionalization. The attenuation and aggregation processes within the hillslopes and river network in a rainfall‐runoff event, provide a framework to test the scaling of QR. Although scaling of Qp has been reported in empirical studies, its physical interpretation is compromised, since Qp at each site could come from different rainfall‐runoff events. To address this problem, the authors explored the effect of actual variabilities of rainfall and soil moisture fields, and the effect of the river network structure, in the scaling of peak flows of 85 rainfall‐runoff events and peak flow quantiles that were observed in the Iowa River Basin at 43 streamflow gauges. The authors established empirical evidence that addresses two questions: (1) What does control the performance of the scaling of observed QR? (2) What is the interplay between sampling errors and the selection of explanatory variables in the construction of regional regression models for QR and Qp? For the first question, the authors found that the slope magnitude in the scaling of the rainfall intensity fields with respect to A controls the scaling' performance of QR. Regarding the second question, the authors demonstrate that the inclusion of river network descriptors should improve the regional equations to estimate peak flow quantiles unless stream gauging sampling errors affect the analysis.

Using Physically Based Synthetic Peak Flows to Assess Local and Regional Flood Frequency Analysis Methods

AbstractAlthough prior studies have evaluated the role of sampling errors associated with local and regional methods to estimate peak flow quantiles, the investigation of epistemic errors is more difficult because the underlying properties of the random variable have been prescribed using ad‐hoc characterizations of the regional distributions of peak flows. This study addresses this challenge using representations of regional peak flow distributions derived from a combined framework of stochastic storm transposition, radar rainfall observations, and distributed hydrologic modeling. The authors evaluated four commonly used peak flow quantile estimation methods using synthetic peak flows at 5,000 sites in the Turkey River watershed in Iowa, USA. They first used at‐site flood frequency analysis using the Pearson Type III distribution with L‐moments. The authors then pooled regional information using (1) the index flood method, (2) the quantile regression technique, and (3) the parameter regression. This approach allowed quantification of error components stemming from epistemic assumptions, parameter estimation method, sample size, and, in the regional approaches, the number of pooled sites. The results demonstrate that the inability to capture the spatial variability of the skewness of the peak flows dominates epistemic error for regional methods. We concluded that, in the study basin, this variability could be partially explained by river network structure and the predominant orientation of the watershed. The general approach used in this study is promising in that it brings new tools and sources of data to the study of the old hydrologic problem of flood frequency analysis.

Multifractal Analysis of River Networks in an Urban Catchment on the Taihu Plain, China

Multifractal analysis was successfully used to investigate the structure of river networks. In this paper, we performed a multifractal analysis of river networks in an urban catchment that is located on the Taihu Plain in the lower part of the Yangtze River Delta, China. Spatial and temporal variations in the river networks during the period 1960–2010 were investigated. The generalized multifractal dimensions (Dq) and the multifractal spectrum (f(α)) were calculated using a box-counting method. The results indicate that: (i) the river networks in Wuchengxiyu (WXCY), Yangchengdianmao (YCDM), and Hangjiahu (HJH) had obvious multifractal features with capacity dimensions between 1.90 and 1.91 during the period 1960–2010. The multifractal spectrums are asymmetrical inverted-hook-shaped curves with a dominant left arm. The variation in the singularity component (∆α) changed the most in WCXY (an increase of ~ 7.9%), and the height variation in the multifractal spectrum (∆f) increased by ~ 17.5% in HJH; (ii) the changes in ∆α and ∆f of the tributaries in the three areas during the period 1960–2010 were consistent with those of the overall river network, demonstrating the decisive role that the tributaries play in the complexity of the river networks; (iii) compared to the natural factors, the influences of urbanization on the river networks significantly changed with a higher urbanization level; and (iv) there were no border effects. Further applications of multifractal theory in analyses of the relationship between a flood-forming regime and the multifractal structures of river networks will attract more attention. Generally, this approach, when successfully applied to studies of changes in river networks, is of theoretical significance for better describing and quantifying the evolution of river networks’ structures.

Structural characteristics of river networks and their relations to basin factors in the Yangtze and Yellow River basins

The integration of rivers and basins highly implies the possible existence of certain relationships between hierarchical characteristics of river networks and primary basin factors. Here we investigated river networks in two large basins, the Yangtze River and the Yellow River, characterized with basic factors such as annual precipitation, slope, soil erodibility and vegetation. Hierarchical analysis demonstrated a fair self-similarity of river networks at the stream-order 1–5 in both rivers, described by the structural parameters including bifurcation ratio, side-branching ratio, drainage density, and length of headwater-river. Besides precipitation, basin slope was essential in shaping river networks in both basins, showing a significant positive correlation ( R 2=0.39–0.85) to bifurcation ratio, side-branching ratio, and drainage density. Given the same basin slope (5°–15°), thehigher soil erodibility and sparse vegetation would promote greater side-branching ratio and drainage density in the Yellow River, which were estimated 11.97 % and 63.70 % larger, respectively than those in the Yangtze River. This study highlights the importance to formulate basin-specific strategies for water and soil conservation in terms of different structures of river networks.