An official website of the United States government
Abstract The strength of interactions between plants and river processes is mediated by plant traits and fluvial conditions, including above‐ground biomass, stem density and flexibility, channel and bed‐material properties, and flow and sediment regimes. In many rivers, concurrent changes in (1) the composition of riparian vegetation communities as a result of exotic species invasion and (2) shifts in hydrology have altered physical and ecological conditions in a manner that has been mediated by feedbacks between vegetation and morphodynamic processes. We review howTamarix, which has invaded many southwestern US waterways, andPopulusspecies, woody pioneer trees that are native to the region, differentially affect hydraulics, sediment transport, and river morphology. We draw on flume, field, and modelling approaches spanning the individual seedling to river‐corridor scales. In a flume study, we found that differences in the crown morphology, stem density, and flexibility ofTamarixcompared toPopulusinfluenced near‐bed flow velocities in a manner that favoured aggradation associated withTamarix. Similarly, at the patch and corridor scales, observations confirmed increased aggradation with increased vegetation density. Furthermore, long‐term channel adjustments were different forTamarix‐ versusPopulus‐dominated reaches, with faster and greater geomorphic adjustments forTamarix. Collectively, our studies show how plant‐trait differences betweenTamarixandPopulus, from individual seedlings to larger spatial and temporal scales, influence the co‐adjustment of rivers and riparian plant communities. These findings provide a basis for predicting changes in alluvial riverine systems which we conceptualize as a Green New Balance model that considers how channels may adjust to changes in plant traits and community structure, in addition to alterations in flow and sediment supply. We offer suggestions regarding how the Green New Balance can be used in management and invasive species management.
more »
« less
An Ecogeomorphic Framework Coupling Sediment Modeling With Invasive Riparian Vegetation Dynamics
Abstract Feedbacks between geomorphic processes and riparian vegetation in river systems are an important control on fluvial morphodynamics and on vegetation composition and distribution. Invasion by nonnative riparian species alters these feedbacks and drives management and restoration along many rivers, highlighting a need for ecogeomorphic models to assist with understanding feedbacks between plants and fluvial processes, and with restoration planning. In this study, we coupled a network‐scale sediment model (Sediment Routing and Floodplain Exchange; SeRFE) that simulates bank erosion and sediment transport in a spatially explicit manner with a recruitment potential analysis for a species of riparian vegetation (Arundo donax) that has invaded river systems and wetlands in Mediterranean climates worldwide. We used the resulting ecogeomorphic framework to understand both network‐scale sediment balances and the spread and recruitment ofA. donaxin the Santa Clara River watershed of Southern California. In the coupled model, we simulated a 1‐year time period during which a 5‐year recurrence interval flood occurred in the mainstem Santa Clara River. Outputs identify key areas acting as sources ofA. donaxrhizomes, which are subsequently transported by flood flows and deposited in reaches downstream. These results were validated in three study reaches, where we assessed postflood geomorphic and vegetation changes. The analysis demonstrates how a coupled model approach is able to highlight basin‐scale ecogeomorphic dynamics in a manner that is useful for restoration planning and prioritization and can be adapted to analogous ecogeomorphic questions in other watersheds.
more »
« less
- PAR ID:
- 10447348
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Earth Surface
- Volume:
- 126
- Issue:
- 6
- ISSN:
- 2169-9003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)The goal of this research was to characterize the impact of invasive riparian vegetation on burn severity patterns and fluvial topographic change in an urban Mediterranean riverine system (Med-sys) after fire in San Diego, California. We assessed standard post-fire metrics under urban conditions with non-native vegetation and utilized field observations to quantify vegetation and fluvial geomorphic processes. Field observations noted both high vegetation loss in the riparian area and rapidly resprouting invasive grass species such as Arundo donax (Giant Reed) after fire. Satellite-based metrics that represent vegetation biomass underestimated the initial green canopy loss, as did volumetric data derived from three-dimensional terrestrial laser scanning data. Field measurements were limited to a small sample size but demonstrated that the absolute maximum topographic changes were highest in stands of Arundo donax (0.18 to 0.67 m). This work is the first quantification of geomorphic alterations promoted by non-native vegetation after fire and highlights potential grass–fire feedbacks that can contribute to geomorphic disruption. Our results support the need for ground-truthing or higher resolution when using standard satellite-based indices to assess post-fire conditions in urban open spaces, especially when productive invasive vegetation are present, and they also emphasize restoring urban waterways to native vegetation conditions.more » « less
-
Given the widespread presence of non-native vegetation in urban and Mediterranean watersheds, it is important to evaluate how these sensitive ecosystems will respond to activities to manage and restore native vegetation conditions. This research focuses on Del Cerro, a tributary of the San Diego River in California, where non-native vegetation dominates the riparian zone, creating flooding and fire hazards. Field data were collected in 2018 to 2021 and consisted of water depth, streamflow, and stream temperature. Our data set also captured baseline conditions in the floodplain before and after the removal of burned non-native vegetation in November 2020. Observed changes in hydrologic and geomorphic conditions were used to parameterize and calibrate a two-dimensional hydraulic model to simulate urban floodplain hydraulics after vegetation removal. We utilized the U.S. Army Corps of Engineers’ Hydrologic Engineering Center River Assessment System (HEC-RAS) model to simulate the influence of canopy loss and vegetation disturbance and to assess the impacts of vegetation removal on stream restoration. We simulated streamflow, water depth, and flood extent for two scenarios: (1) 2019; pre-restoration where non-native vegetation dominated the riparian area, and (2) 2021; post-restoration following the removal of non-native vegetation and canopy. Flooding after restoration in 2021 was more frequent compared to 2019. We also observed similar flood extents and peak streamflow for storm events that accumulated half the amount of precipitation as pre-restoration conditions. Our results provide insight into the responses of small urban stream reaches to the removal of invasive vegetation and canopy cover.more » « less
-
Abstract Sediment regimes, i.e., the processes that recruit, transport, and store sediment, create the physical habitats that underpin river‐floodplain ecosystems. Natural and human‐induced disturbances that alter sediment regimes can have cascading effects on river and floodplain morphology, ecosystems, and a river's ability to provide ecosystem services, yet prediction of the response of sediment dynamics to disturbance is challenging. We developed the Sediment Routing and Floodplain Exchange (SeRFE) model, which is a network‐based, spatially explicit framework for modeling sediment recruitment to and subsequent transport through drainage networks. SeRFE additionally tracks the spatially and temporally variable balance between sediment supply and transport capacity. Simulations using SeRFE can account for various types of watershed disturbance and for channel‐floodplain sediment exchange. SeRFE is simple, adaptable, and can be run with widely available geospatial data and limited field data. The model is driven by real or user‐generated hydrographs, allowing the user to assess the combined effects of disturbance, channel‐floodplain interactions and particular flow scenarios on the propagation of disturbances throughout a drainage network, and the resulting impacts to reaches of interest. We tested the model in the Santa Clara River basin, Southern California, in subbasins affected by large dams and wildfire. Model results highlight the importance of hydrologic conditions on postwildfire sediment yield and illustrate the spatial extent of dam‐induced sediment deficit during a flood. SeRFE can provide contextual information on reach‐scale sediment balance conditions, sensitivity to altered sediment regimes, and potential for morphologic change for managers and practitioners working in disturbed watersheds.more » « less
-
Abstract River channels, riparian and floodplain forms and dynamics are all influenced strongly by biological processes. However, the influence of macroinvertebrates on entrainment and transport of river sediments remains poorly understood. We use an energy‐based approach to explore the capacity of benthic animals to move surficial, gravel‐bed particles in field and laboratory settings and use the results to assess the relative significance of biological and physical benthic processes. Our results showed that in 11 British gravel‐bed rivers, the maximum energy content (i.e., calorific content) of macroinvertebrate communities generally matched the flow energy associated with median discharges and, at multiple sites, exceeded that of the 10‐year return interval flood. A series of laboratory experiments used to estimate the minimum energy expended by signal crayfish (Pacifastacus leniusculus) when performing geomorphic work established that crayfish move gravel particles at energy levels below that expected of the flow, complicating direct comparisons of the capacity for macroinvertebrates and fluvial flows to influence bed mobility. Our findings suggest that the influence of macroinvertebrate communities in either promoting or suppressing, the mobilisation of the bed may be large compared to equivalent values of fluvial energy. Based on these findings, we conclude that in the gravel‐bed rivers studied, the macroinvertebrate community's potential to perform geomorphic work matches or exceeds the stream power during most of the year. Although our study examined biological and fluvial energy systems separately, it is important to recognise that in nature, these systems are highly interactive. It follows that utilising the energy framework presented in this paper could lead to rapid advances in both fluvial biogeomorphology and river management and restoration.more » « less
