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Managing Flow Regimes and Landscapes Together: Hydrospatial Analysis for Evaluating Spatiotemporal Floodplain Inundation Patterns with Restoration and Climate Change Implications

  • Author(s): Whipple, Alison A
  • et al.
Abstract

Riverine landscapes are shaped by dynamic and complex interactions between streamflow and floodplain landforms, and these physical processes drive productive and diverse freshwater ecosystems. However, human activities have fundamentally altered river-floodplain processes and degraded ecosystems. Flow regime variability has been homogenized and floodplains disconnected from rivers due to dams, diversions, levee building, and land use change. Reconciling competing demands to support ecosystems and resilience to future change is a core scientific and management challenge. This dissertation describes spatiotemporal dynamics of floodplain environments, introducing a method for flood regime classification and establishing a methodological approach for hydrospatial analysis to quantify and evaluate the response of floodplain inundation patterns and related physical habitat to restoration and flow regime change under climate change. It is motivated by the need to develop process-based and landscapescale strategies to better manage flow regimes and landscapes together, such as coordinated levee-removal floodplain restoration and environmental flow allocations. River restoration literature is synthesized herein to examine trajectories from form-based to process-based approaches, recognize that highly modified large rivers may require coordinated physical habitat restoration and environmental flows implementation, and suggest opportunities for improved integration of restoration strategies. A river’s flood regime drives a variety of different physical and ecological functions. Characterizing different floods of a flood regime informs understanding of climate and watershed processes and the management of natural floodplain dynamics. Following cluster analysis approaches used in flow regime classification, a flood regime typology was developed for the Cosumnes River, the only major unregulated river of the west slope Sierra Nevada, California, USA. A primary contribution of this study is the establishment of flood regime classification that moves beyond typical flood frequency analysis to address a range of ecologically-relevant flood characteristics, including duration and timing. Rehabilitating freshwater ecosystems of highly modified rivers under a changing future requires improved understanding and quantification of land-water interactions. Despite ecological implications, quantification of spatiotemporal variability is rare, particularly for management applications. An approach for evaluating spatiotemporal floodplain inundation patterns, or the hydrospatial regime, is presented in several studies. Physical inundation characteristics and associated habitat were quantified in space and time, and responses to restoration and climate change induced flow scenarios were evaluated and compared. The multi-metric approach is demonstrated for a recent levee-removal restoration site along the lower Cosumnes River. The novel hydrospatial analytical approach developed and presented herein applies twodimensional hydrodynamic modeling and spatial analysis to quantitatively summarize, in space and time, a range of ecologically-relevant physical metrics relating to inundation extent, depth, velocity, frequency, -iiiduration, timing, rate of change, connectivity, and heterogeneity. Comparison of metrics before and after levee-removal restoration on the Cosumnes River floodplain showed that while inundation extent greatly increased with restoration, responses varied in space and time and were different for different metrics. Changes in metrics were most substantial at intermediate flood flows. Subsequently, habitat criteria for a native floodplain fish species, Sacramento splittail (Pogonichthys macrolepidotus), were applied to the physical metrics. Findings suggest that restoration nearly doubled overall habitat availability, though benefits varied considerably in space and time. Flow-habitat relationships were nonlinear and not oneto- one, indicating habitat availability mediated by the physical complexity of the floodplain. Finally, floodplain responses to climate change induced streamflow scenarios were compared and the relative impacts of levee-removal restoration across the scenarios were evaluated. Results reflected the balance of increasing extreme winter flooding and declining spring flooding under future climate change scenarios. Magnitude and direction of change depended on the climate change scenario and metric. Levee removal had the general effect of dampening climate change impacts, though the relative impacts of climate change scenarios were greater than that of restoration in some cases. This body of work presents a new methodology to analyze flow-landscape interactions, and in turn contributes to understanding of flow-ecology relationships, susceptibility to anthropogenic change, and improvements to water and land management. Several broad implications emerge from this research. It demonstrates the capacity of a riverine landscape to serve different functions at different times and supports improved management toward variable conditions. Another contribution is advancing the use of hydraulic metrics over hydrologic metrics for better connections between physical processes and ecological functions. Further, the approach allows for ecologically-relevant criteria that are spatially and temporally dependent to be evaluated explicitly (e.g., duration, connectivity, temporal sequence of flood events). Findings show that, for habitat evaluation within complex floodplain environments, habitat availability is not likely to be a simple function of flow. Floodplain hydrospatial regime responses to climate change will be mediated by flow-landscape interactions, with the potential for physical restoration activities to mitigate impacts of climate change. Despite highly modified physical processes, climate change, and freshwater diversity and productivity declines globally, there is great capacity to better balance human and ecosystem requirements. This dissertation expands scientific understanding of and informs management toward dynamic and heterogeneous riverine landscapes that support functional and resilient ecosystems.

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