Executive Summary

The Cosumnes River Experience and Recommendations for Restoration Monitoring

Floodplains are among the most productive and diverse ecosystems on Earth and are now being recognized globally for the ecosystem services they provide; however, they are also one the more impacted ecosystems globally and are at risk of further degradation by a fusillade of anthropogenic stressors and consumptive demands. Natural floodplain ecosystems are a product of, and adapted to, highly variable hydrologic regimes – typified by droughts, catastrophic floods, and frequent periods of inundation - expressed across seasonal, yearly, and decadal dimensions. This hydrologic variability acts to reset various biotic populations within aquatic, riparian, and wetland ecosystems through disturbance, acting as an essential ecological process in maintaining complex ecosystem pathways. Multipath ecological relationships, expressed as trophic food webs or transition states, promote high biodiversity and biological integrity. In floodplain ecosystems, these ecological relationships are underpinned by the fundamental linkage between floodplains and river systems, forming a critical linkage that creates and maintains a mosaic of habitats for groundwater recharge, primary productivity and biogeochemistry, the reproductive cycle of fishes, nesting and foraging of birds, and regeneration of riparian vegetation.

It is estimated that less than 5% of the Central Valley’s original riparian forest remain intact. This fact, coupled with the simultaneous loss of floodplain wetlands to channel modification, agriculture and urbanization, has resulted in the desire for ecosystem scale restoration. For Central Valley lowland river floodplains, the only cost-effective method for large scale restoration is semi-passive; in essence, through structural modification – such as levee breaches or levee setbacks, followed by natural succession of flood dependent ecological communities – where nature does most of the work. However, past semi-passive restoration efforts have not been uniformly successful either in generating high quality, productive native-dominated vegetation or in reestablishing functioning food webs supporting floodplain sentinel species (e.g.,birds, fishes, etc.). The lower Cosumnes River floodplain provides an unprecedented opportunity to examine semi-passive restoration techniques in relation to successional trajectory and opportunities for improved management and also the development of monitoring methodologies, as there is a 20-year history of wetland and riparian habitat restoration – both active and semi-passive in technique – that allows for temporal and comparative analyses.

To help elucidate important patterns and processes in semi-passive restoration, we examine the interaction and feedback pathways between the physical regime and the ecological communities in the Cosumnes River floodplain, which is created and maintained by a natural hydrologic discharge regime that promotes habitat heterogeneity, nutrient transformation and exchange, and spatiotemporal fluxes in productivity. Other important pathways include the interactions between the hydrologic discharge regime and groundwater, subsequent effects of groundwater on establishment of riparian forests, and possible changes in groundwater regime caused by forest evapotranspiration. In turn, forest productivity and structure provide both a trophic and a structural basis for both aquatic and terrestrial productivity, wherein biological indicators such as birds respond to the mosaic of qualitative and quantitative differences created by the differential effects of ecosystem disturbance and restoration.

Freshwater ecosystems, including floodplain and riparian habitats, are among the Earth’s most productive and diverse ecological systems providing innumerable services to humans. In addition to these ecosystem services, such as water supply, freshwater ecosystems have productive floodplain and riparian habitats that are essential for sustaining and recovering at-risk resident and migratory populations of native plants and animals. A challenge for floodplain restoration and management in California’s Bay-Delta, which is heavily populated by humans and used for agricultural production, is to configure floodplains and their food webs so that populations of beneficial species thrive. As we restore and manage riparian zones and floodplains, we need to learn how their spatial and temporal characteristics affect ecological function and processes, so we can sustain their ecological services. From our studies, presented here, we understand that natural floodplain ecosystems are a product of, and adapted to, highly variable hydrologic regimes. It is important that the general public, and resource managers, understand and adapt to a hydrologic regime that is typified by droughts, catastrophic floods, and frequent periods of inundation. Our studies document that hydrologic variability is expressed across seasonal, yearly, and decadal units of time, but is also changing its mode. Therefore, restoration and management that uses hydrologic variability to reset various biotic populations through disturbance, must be cognizant of the alternate trajectories that may occur. Understanding the multiple ecological paths, expressed in our studies asphysical processes governing trophic food webs, will also allow us to consider long-term consequences to biodiversity and biological integrity.

Using the Cosumnes River floodplain ecosystem as a living laboratory, our studies focused on the ecological relationships that form critical linkages between and maintain a mosaic of habitats for groundwater recharge, primary productivity and biogeochemistry in floodwaters, reproductive cycle of fishes and resulting communities, nesting and foraging of birds, the presence and utilization of bats, as well as the continued regeneration of riparian vegetation. The Cosumnes River Preserve and the river itself has provided us – in part due to its unimpaired flood flow regime maintained by its undammed status – with the unique opportunity to study two approaches to restoration. Passive restoration, defined as letting the ecosystem self-organize following early intervention, clearly affects larger are with less monetary investment. Active restoration, the continued and direct manipulation of ecosystem members and process, does provide more certainty in short-term effects. Thus, this study and the preceding ones have benefited from the implementation of both restoration methods to study restoration success metrics. There are clear ecological benefits to passive restoration, accomplished through floodplain reconnectivity, through increases in structural complexity and subsequent functional diversity, which in turns gives rise to increased biodiversity through creating additional pathways for species interaction and energy transfer and transformation. From our studies, we emphasize the importance of large floods which can export large woody debris and coarse particulate organic matter from the floodplain to the channel, creating habitat, and creating important avenues for energy transfer across and within the river-floodplain system.