This dissertation addresses the role that dynamic flow characteristics play in shaping the potential for significant ecosystem benefits from floodplain restoration. Mediterranean-climate river systems present challenges for restoring healthy floodplains because of the inter and intra-annual variability in stream flow, which has been dramatically reduced in an effort to control flooding and to provide a more consistent year-round water supply for human use. Habitat restoration efforts require that this reduced stream flow be altered in order to recover more naturally dynamic flow patterns and reconnect floodplains. This thesis defines and takes advantage of an eco-hydrology modeling framework to reveal how the ecological returns of different hydrologic alterations or restoration scenarios--including changes to the physical landscape and flow dynamics--influence habitat connectivity for freshwater biota. A method for quantifying benefits of expanding floodplain connectivity can highlight actions that might simultaneously reduce flood risk and restore ecological functions, such as supporting fish habitat benefits, food web productivity, and riparian vegetation establishment.
Pending climate change increases the uncertainty of restoration treatment outcomes yet must be addressed as part of the restoration planning process. An ecologically-oriented assessment of the current and potential future stream flow characteristics of selected Central Valley rivers makes it clear that climate change will affect future floodplain habitat function. Findings show that the low emissions (warm-wet) climate change scenario allows for higher flows at longer durations compared to the historical post-dam record and the high emissions (hot-dry) scenario. In fact, the low-emissions scenario flows might be more similar to pre-dam flow regimes--peak magnitudes in particular--than to the current regulated flow regime. The high emissions scenario can serve as a measure for the lower bounds of functional floodplain area for ecological benefit. Planning for potential impacts of climate change on flow dynamics will be essential if restoration managers are to minimize negative consequences of climate change and maximize the potential benefits that it may offer for species recovery.
Efforts to plan and evaluate floodplain reconnection projects for ecological benefits have been hindered by a lack of metrics that allow for comparisons among alternative restoration sites with respect to the type and quality of dynamic habitat potential. This dissertation presents a framework for quantifying the benefits of floodplain restoration projects by coupling the spatial and temporal characteristics of floodplains to express the functional habitat they create. First, habitat was quantified using Area-Duration-Frequency (ADF) curves for several durations and across multiple frequencies of flood occurrence. From these data, a value was then generated for expected annual habitat (EAH). The method has advantages in framing the potential restored area in terms of probabilities based on dynamics of flow timing, durations, and frequencies. The EAH metric captures a comprehensive picture of the likelihood of flooded areas appearing in any given year. This method can be used to design projects to meet specific and measurable habitat objectives. These methods and new metrics provide a transparent and replicable means to examine the effects and relative importance of policy decisions and river restoration projects.
To illustrate this modeling method, statistical flow characteristics needed to support floodplain benefit for species were coupled with topographic alteration scenarios for increasing beneficial habitat along the Vernalis to Mossdale corridor on the San Joaquin River, California. Findings for a suite of species that span a range of necessary flow requirements exemplify a wide array of impacts associated with flow scenarios for the San Joaquin River system. Most importantly, the modeled results predict significant declines in the availability of required flow related habitat conditions for splittail spawning and rearing and Chinook salmon rearing in the future under two climate change scenarios. Physical habitat restoration must be paired with additional in-stream flows to meet frequency, duration, and seasonal requirements for these species. Thus, restoration treatment considerations for floodplain habitat should not only include physical alterations for additional channel floodplain connectivity, but also restore a more natural flow regime to increase habitat area and frequency of inundation.
Restoration planning often fails to follow strategies based on assessments of ecological benefit outcomes and cost effectiveness. A hydro-ecological approach was applied to multiple modeled floodplain restoration sites along California's Sacramento River and was integrated with socio-economic considerations into a prioritization scheme. The new EAH and ADF metrics were used to assess probabilities for ecological outcomes for increased salmon rearing habitat and combined with land value cost for parcels in the restoration areas. The model was used to assess individual and cumulative benefits of 26 floodplain rehabilitation options involving levee setbacks and examine the consequences of changing topography and climate for floodplain habitat along a large expanse of the Sacramento River. Cumulative effects of projects implemented concurrently showed only small changes in functional floodplain habitat creation. Climate change flow scenarios for this section of the Sacramento River indicate that the functional EAH habitat under a low emissions (warm-wet) regime overlaps with that created for restoration sites under the current flow regime. However, the high emissions (hot-dry) regime will create less functional habitat and serves as a good lower bound of expectations for any restoration plan. By adding to ecological outcome measures and integrating environmental benefits into a cost effectiveness ratio, some projects' priority rankings shift. Thus, cost effectiveness is relevant for informing decisions about restoration site priorities and could improve the way funds are allocated to restoration options. This study advances mitigation planning at a local and regional scale by providing tools for quantitative estimates of potential habitat that could be restored, for assessing projects individually and cumulatively, and for comparing and prioritizing sites using an analytical cost effectiveness approach.
In sum, this dissertation presents a modeling framework and new quantitative metrics that can be used to plan and evaluate floodplain restoration projects that address connectivity and dynamic flows, whether they are the result of climate change or prescribed reservoir release flows. Restoration options for multiple locations in California's Central Valley were investigated to demonstrate the utility of this approach. The method has advantages in estimating the potential restored area in terms of probabilities based on dynamics of flow timing, durations, and frequencies. Ultimately, using integrative hydro-ecological models offers support for decision makers considering where to rehabilitate floodplain processes upon which biological and social benefits depend.