Flood Disturbance and the Distribution of Riparian Species Diversity

. Biodiversity varies considerably in Southern Californian riparian vegetation. The intermediate disturbance hypothesis posits greatest diversity in settings that are subject to moderate-intensity disturbance. Flood intensity tends to vary systematically in watersheds, potentially imposing patterns of biodiversity. In two study watersheds, species richness increases with flood severity. Diversity, or heterogeneity, is less predictable: Biodiversity patterns in these watersheds are complicated by atypical patterns of flood severity. Although riparian diversity may be intimately dependent on flood disturbance, the relationship is predictable only with due attention to the physiographic details of individual stream networks.

We investigated the influence of channel migration and expansion on riparian plant species diversity along the lower Colorado River near the United States–Mexico border. Using repeat aerial photography in a GIS we identified and classed areas of low, moderate, and high disturbance frequency caused by channel expansion and migration. Replicate vegetation plots (12 m × 12 m) were sampled in each of the three disturbance classes. One-way ANOVA was used to test for differences in species richness, species diversity (using the Shannon–Weiner Index) and overall percent ground cover of plants between the three disturbance classes. Regardless of disturbance class, plots were dominated by trees or shrubs, especially the non-native Tamarix ramosissima, as well as Pluchea sericea, Baccharis salicifolia and Salix goodingii. Clearly woody species constitute the great bulk of overall species richness, percent ground cover, and species diversity (H′) in each disturbance group. No overall statistically significant differences were revealed among the disturbance groups for values of species richness, percent ground cover, or the Shannon–Wiener Index, though paired contrasts of means revealed that total percent ground cover on low disturbance plots was significantly higher than on moderately disturbed plots. Spatial and temporal variability in riparian diversity in the study area appears to hinge on factors other than disturbance frequency such as salt or drought stress. Alternately, our results could be interpreted as suggesting that in the presence of intensive flow regulation, disturbance plays a secondary role to ecological stresses, similar to that demonstrated by others. Intentional flood pulses are advocated as a restorative management strategy for improving plant productivity, management of exotic species (particularly T. ramosissima), and restoration of overall biodiversity.

Riparian plant species richness along lateral and longitudinal gradients of water stress and flood disturbance, San Pedro River, Arizona, USA

A global assessment of change in flood volume with surface air temperature

Floodplain soils are subject to quasi-periodic flood disturbances. This flooding serves to enrich floodplain soils, increasing their fertility and often making them ideal locations for agriculture. However, what is less well understood is how the frequency of flooding impacts on soil fertility and the diversity of soil character. This study investigates how flood frequency influences the heterogeneity (assessed using 26 physical and geochemical soil properties) of floodplain soils in a semi-arid floodplain wetland system in New South Wales, Australia. The study includes an investigation of soil properties across four flood frequency (or disturbance frequency) categories ranging from frequent through to infrequent flood disturbance. Thirty samples were collected from each zone and the physical and geochemical soil data were analyzed using a suite of univariate and multivariate statistical tests. The results show that sites subject to an intermediate level of flood disturbance have a greater level of diversity in soil properties than those sites subject to frequent flood disturbances. These results reflect those of the Intermediate Disturbance Hypothesis, an ecological theory that posits the highest biological diversity will also be found in intermediately disturbed environments and suggests that there might be physical habitat drivers of biological diversity in intermediately disturbed floodplains.

Accurately understanding flood evolution and its attribution is crucial for watershed water resource management as well as disaster prevention and mitigation. The source region of the Yellow River (SRYR) has experienced several severe floods over the past few decades, but the driving factor influencing flood volume variation in the SRYR remains unclear. In this study, the Budyko framework was used to quantify the effects of climate change, vegetation growth, and permafrost degradation on flood volume variation in six basins of the SRYR. The results showed that the flood volume decreased before 2000 and increased after 2000, but the average value after 2000 remained lower than that before 2000. Flood volume is most sensitive to changes in precipitation, followed by changes in landscape in all basins. The decrease in flood volume was primarily influenced by changes in active layer thickness in permafrost-dominated basins, while it was mainly controlled by other landscape changes in non-permafrost-dominated basins. Meanwhile, the contributions of changes in potential evapotranspiration and water storage changes to the reduced flood volume were negative in all basins. Furthermore, the impact of vegetation growth on flood volume variation cannot be neglected due to its regulating role in the hydrological cycle. These findings can provide new insights into the evolution mechanism of floods in cryospheric basins and contribute to the development of strategies for flood control, disaster mitigation, and water resource management under a changing climate.

Persistence, a measure of presence or absence of species, and stability, an estimate of assemblage equilibrium measured as constancy in species ranks or densities, should both be considered when assessing temporal change in natural assemblages. We measured persistence and stability in two distinct animal assemblages frequently disturbed by natural and severe flooding events, the fishes and benthic invertebrates in a Sonoran Desert stream. A persistence index (derived from colonization/extinction analyses) indicates high persistence of fishes for several decades, while benthic invertebrates were persistent except in periods of severe flooding. Stability of taxon rankings (measured by Kendall's W) was high for both assemblages, even though absolute population sizes fluctuated. Fish populations resisted even the most severe flood disturbances, whereas benthic invertebrates were decimated by particularly frequent and intensive flooding. The latter were resilient, however, and quickly recovered due to life history characters favoring rapid postflood recolonization. Although absolute numbers of organisms varied through orders of magnitude, more general aspects of assemblage structure (species' presence or absence, and relative rankings) remained relatively constant despite repeated and potentially devastating natural perturbations. INTRODUCTION In their synthesis of ecological stability and persistence, Connell and Sousa (1983) emphasized the utility of a multifaceted approach in temporal analyses of community structure. Drawing upon the works of Holling (1973), Boesch (1974), Orians (1974) and Whittaker (1974), among others, they presented qualitative and quantitative criteria to be considered when assessing changes in community structure over time. Qualitatively, persistence of species focuses upon their continued presence, particularly with respect to potentially devastating forces. Quantitatively, stability is the relative constancy of species abundances over time despite disturbances. Stability may result from resistance, when relative species abundances are unchanged despite potentially disruptive forces, or resilience, rapid return to the former state following disturbance. Both persistence and stability should be considered when attempting to understand community pattern and process (Holling, 1973). The possibility of destructive disturbance is a necessary feature of communities that are used to investigate temporal change. . . for any situation to be included under the concept of stability . . .there must exist disturbing forces; without them, simple lack of change in numbers is of little interest (Connell and Sousa, 1983). Consequently, responses of natural communities to disturbance are central to current ecological research (Dayton, 1971; Sousa, 1979, 1984; Matthews, 1986), and disturbance provides a framework upon which mechanistic understandings of community structure and function may be developed. We examined persistence and stability of fish and benthic invertebrate assemblages in a Sonoran Desert stream that repeatedly experiences large, quantifiable natural dis-

Resilience of benthic macroinvertebrates to extreme floods in a Catskill Mountain river, New York, USA: Implications for water quality monitoring and assessment

Multivariable flood risk and its dynamics considering project reasonable service life in a changing environment

Urbanization growth and climate change have increased the frequency and severity of floods in urban areas. One of the effective methods for reducing stormwater volume and managing urban floods is the low-impact development best management practice (LID-BMP). This study aims to mitigate flood volume and peak discharge caused by land use changes in the Darabad basin located in Tehran, Iran, using LID-BMPs. For this purpose, land use maps were extracted for a period of 23 years from 2000 to 2022 using Landsat satellite images. Then, by using a combination of geographic information system-based multi-criteria decision analysis (GIS-MCDA) method and spatial criteria, four types of LID-BMPs, including bioretention basin, green roof, grass swale, and porous pavement, were located in the study area. Next, rainfall–runoff modeling was applied to calculate the changes in the mentioned criteria due to land use changes and the application of LID-BMPs in the area using soil conservation service curve number (SCS-CN) method. The simulation results showed that the rise in built-up land use from 43.49 to 56.51 percent between the period has increased the flood volume and peak discharge of 25-year return period by approximately 60 percent. The simulation results also indicated that the combined use of the four selected types of LID-BMPs will lead to a greater decrease in stormwater volume and peak discharge. According to the results, LID-BMPs perform better in shorter return periods in a way that the average percentage of flood volume and peak discharge reduction in a 2-year return period were 36.75 and 34.96 percent, while they were 31.37 and 26.5 percent in a 100-year return period.

Spatiotemporal evolution and driving factors of urban form resilience under flood hazard disturbance: A case study of 137 cities in the southeast monsoon influence zone of China.

<p>We present a model for detecting historical floods in ungauged remote hyperarid regions using satellite vegetation indices (SatVITS-Flood). Our model is based on observations that floods in hyperarid regions are the primary cause of change in the growth and expansion of local vegetation. We used 40 years of hydrological data from reliable gauge stations at four sites from three regions over three continents to develop and evaluate the model. MODIS, Landsat, and AVHRR time series of the normalized difference vegetation index (NDVI), the modified soil-adjusted vegetation index (MSAVI), and the normalized difference water index (NDWI) were used to detect vegetation changes. The model uses two different time series analysis metrics, (1) a trend change detection from the Breaks For Additive Season and Trend (BFAST-trend) and (2) a newly developed seasonal change metric based on Temporal Fourier Analysis (TFA) and the growing-season integral anomaly (TFA-GSI<sub>anom</sub>). The idea was to capture major changes in perennial species following extreme, rare floods with BFAST-trend and small changes in the ephemeral vegetation following more frequent, less severe floods with TFA-GSI<sub>anom</sub>. We compared these metrics with flood events and the magnitude of change with the flood volume (Mm<sup>3</sup>) and duration (days). Our model was able to predict flood occurrence with an accuracy of 78% and precision of 67% (Recall = 0.69 and F1 = 0.68; <em>p</em><0.01), and the flood volume with NSE of 0.79 (RMSE = 15.4 Mm<sup>3</sup> event<sup>–1</sup>) and flood duration with R<sup>2</sup> of 0.69 (RMSE = 5.7 days). SatVITS-Flood proved useful for detecting historical floods and may provide valuable hydrological information in poorly-documented areas, which can help understand the impacts of climate change on the hydrology of hyperarid regions.</p>

Soil sealing increases surface runoff in a watershed and decreases infiltration into the soil. Consequently, urbanization poses a significant challenge for watershed management to mitigate faster runoff accumulation downstream and associated floods. Hydrological models are often employed to assess the impact of land-use dynamics on flood events. Alternatively, data-driven approaches combining time series of land use geodatasets and georeferenced flooded zones also allow to assess the relationship between soil sealing and flood severity. This study presents such data-driven analysis using a spatially explicit archive of flooded areas dating back to 1988 in the Flanders region of Belgium, which is characterized by urban sprawl. This archived data, along with time series of rainfall and land use, were analyzed for three middle-sized river subbasins using two machine learning methods: boosted regression trees and support vector regression. The machine learning methods were found suitable for this type of analysis, since their flexibility allows for spatially explicit models with larger sample sizes. However, the relationship between soil sealing and flood volume and extent could not be conclusively confirmed by our models. This may be due to data limitations, such as the limited number of recorded historical floods, inaccuracies in recorded historical flood polygons and inconsistencies in the land use classifications. It is therefore stressed that continued consistent monitoring of floods and land use changes is required.

River corridors along non‐perennial stream networks provide diverse physical and ecological functions that are thought to be related to the spatial and temporal variability of geomorphic units, also known as geomorphic heterogeneity. While studies on the characteristics and drivers of geomorphic heterogeneity have been developed in perennial streams, similar studies in ephemeral streams are lacking. Given the ubiquity of non‐perennial streams globally, we aim to answer questions regarding the magnitude and drivers of geomorphic heterogeneity in ephemeral river corridors as well as how geomorphic unit assemblages reflect processes related to flood disturbance. Geomorphic units were mapped in 30 unconfined river corridors within six non‐perennial watersheds in Utah and Arizona, USA. Landscape heterogeneity metrics—Shannon's Diversity Index, Shannon's Evenness Index, and patch density—were used to quantify geomorphic heterogeneity within each reach. Additionally, variables that potentially constrain or drive heterogeneity were quantified, including floodplain shape, grain size, large wood abundance, and proxies for flood disturbance. While heterogeneity positively correlated with metrics for morphology and disturbance, statistical models suggest that morphologic context, particularly river corridor width, was a more important predictor for estimating geomorphic heterogeneity. Still, geomorphic units reflected aggradation processes indicative of a range of flood energies, suggesting a strong tie between heterogeneity and disturbance. Results suggest that geomorphic heterogeneity may be resilient to changes in flood disturbance frequency or magnitude, but future studies investigating long‐term temporal heterogeneity are needed.

Floodplains are characterized by high spatial and temporal heterogeneity. Despite low active mobility, Central European floodplain gastropod communities show a high species diversity. They are supposed to have developed a large range of resistance and/or resilience strategies to survive in the highly variable and frequently disturbed floodplain habitats. Relating gastropod diversity and species traits to hydrological conditions, we tested how different groundwater and flood regimes affect gastropod diversity and identified the main species traits favouring their survival in highly dynamic floodplain grasslands. Species richness, species diversity and functional diversity peaked at intermediate flood disturbance and moisture levels. Harsher environmental conditions in either dryer or frequently flooded habitats restricted the gastropod communities to a few specialized species. Morphological and life-history traits showed significant variations along the hydrological gradient. Shell character and mode of reproduction proved to be important functional determinants for gastropod community composition. Species with strongly calcified shells, which limit the risk of injuries in case of dislodgment, were more often found in flood prone sites. Uniparental reproduction dominated in the driest as well as in highly flood-disturbed habitats, providing reproductive assurance where harsh environmental conditions may reduce the number of potential mates. Intermediate disturbance and moisture levels favoured local gastropod diversity whereas dryer or highly flood-disturbed habitats sheltered specialized species. Therefore, the maintenance of areas with different disturbance and moisture levels is of major importance in favouring taxonomical and functional mollusc diversity across the whole floodplain. Copyright © 2011 John Wiley & Sons, Ltd.