DRERIP Ecosystem Conceptual Model: Floodplain

Elements common to all models (Figure 1). Gray shapes (“plaques” in Word parlance) are other DRERIP models. Brown polygons are modifying factors. Pink rectangles are primary outputs of direct interest to Delta restoration planners (e.g., splittail). Blue rectangles are hydrological characteristics or variables, primarily pertaining to the primary river that is the source of inundation to the floodplain of interest but some rectangles represent “secondary hydrology” such as water deriving from direction precipitation on the floodplain, and groundwater and tributary inputs. Blue-green rectangles represent inundated habitat characteristics – properties of the floodplain during periods of inundation.

Model 1 (Creating the Template) captures the linkages and processes that create the habitat mosaic—the physical template of a given floodplain, such as topography and vegetative communities (Figure 2). Note that habitat mosaic is more than vegetative communities as it also includes topographic features like side channels, oxbows, and wetlands. This is a very basic model, so we didn’t attempt to use the information-coded arrows to indicate importance, predictability, etc. This model describes how floodplain topography and vegetation, important features treated as more or less static in the other models, are created and maintained. This model encompasses time scales ranging from a single flood event (e.g., bank erosion) to decades or centuries (successional processes in a floodplain forest). Delta restoration planners can use model 1 to understand management options for creating and maintaining habitat mosaics on a given floodplain.

Model 2 (Inundating the template) depicts how a given floodplain, with topography and vegetation created within Model 1, is inundated by river flows and other sources of water to create specific conditions within the inundated floodplain that are important to the species or processes described in model set 3 (Figure 3). The hydrology first encounters river-floodplain topography (e.g., the relative elevations of floodplain to river stage) to determine if the floodplain becomes inundated; the other linkages only occur if flow magnitude is capable of exceeding the inundation threshold. Inundation is a function of flow magnitude in the river, along with contributions from other hydrological sources (e.g., local tributaries, high water table), and the relative elevation and connectivity of the river-floodplain topography. If inundation occurs, the floodwaters interact with the topography and vegetation created in Model 1. As inundation occurs across this mosaic it results in a variety of inundated habitat characteristics—conditions that directly affect biota and processes during the period of inundation. These inundated habitat characteristics are the primary inputs for Model Set 3. This model is primarily focused at the temporal scale of a single flood season. Delta restoration planners can use model 2 to evaluate how management actions can influence the inundation of floodplain habitat mosaics and characteristics of the inundated habitat.

Model Set 3 (Management outputs) illustrates how the inundated habitat characteristics, developed in Model 2, interact with a few other key elements to influence the production of biota of direct interest to delta restoration planners, including algae, zooplankton, splittail, and juvenile Chinook salmon (Figures 4 – 7). Model Set 3 encompasses a temporal scale of a single flood season. Model Set 3 can be used to evaluate how specific characteristics of the inundated floodplain affect specific outputs of management interest.

Figure 4 provides a basic overview of the inputs and outputs and relationships between the models in Model Set 3. Figure 5 focuses on the production of algae (phytoplankton and epibenthic algae) and zooplankton, structured as a food web. Figures 6 and 7 show how the inundated habitat characteristics and base of the food web (algae and zooplankton) affect the production of splittail and Chinook salmon, respectively.

Primary management outputs from floodplain models:

1. Primary productivity, in terms of phytoplankton, the most nutritious organic matter for the downstream delta (Muller-Solger et al. 2002) and secondary productivity (zooplankton and macroinvertebrates). Because several important species in the delta are food-limited, as indicated by low first-year survival, floodplain restoration has been promoted as a means of increasing productivity to these species and ecosystems (Jassby and Cloern 2000).
2. Juvenile Chinook
3. Splittail
4. Habitat mosaic and riparian structure for a variety of species.

The first three management outputs are Model 3 outputs. For these a user can work backwards through the models to understand how management options can increase the productivity of a specific desired output. For example, if the desired output is biologically available Carbon for downstream ecosystems (algae), then Figure 5 (Model 3) indicates that residence time and intra-annual frequency are important characteristics; for these characteristics of inundation to occur, Figure 3 (Model 2) indicates that the frequency of inundation can be influenced either through hydrologic or topographic manipulations, and the narrative for Model 1 provides background information on the processes that create and maintain floodplain topography. The management output ‘riparian structure’ is a Model 1 output.