Rivers of the Southeastern United States traverse a wide coastal plain where they are characterized by broad floodplains and expansive estuaries. Because of ample rainfall and the flat terrain, these rivers were historically subject to large, frequent floods, particularly in the spring or associated with tropical storms or other major rain events. Because floods can be hazardous to human settlements, extensive networks of dams have been built on many, if not most, major Southeastern rivers, and river flows have been closely regulated to minimize flooding. This has made it difficult for managers to meet competing interests for water such as property protection, hydropower, or recreational needs, along with the needs of downstream habitats and species. The services provided by healthy functioning floodplains are rarely considered in management plans. Floodplains provide flood protection, water reclamation, pollution abatement, aquifer recharge, wildlife habitat, and recreation opportunities. Estuaries and their associated tidal marshes are vital natural areas, providing habitat for fish, shellfish, and wildlife; maintenance of water quality via waste assimilation and nutrient processing; climate control via carbon sequestration; prevention of sediment erosion; and protection from storms. Riverine, floodplain, and estuarine habitats may become degraded by regulation, because many biological functions in these crucially important ecosystems rely on natural variation in flows including flooding.
The periodic inundation of floodplains is believed to be the major factor in sustaining the ecological functions of river-floodplain systems (see review in Junk and Wantzen, 2006). The high overall productivity and biodiversity of these systems is due largely to the many-fold increase in aquatic areas occurring upon floodplain inundation. Flood pulses can exert a range of influences such as shaping the morphology of river channels and floodplains, affecting soil oxygen and moisture levels, and impacting biological and chemical transformations among nutrient pools. Flood pulses regulate plant reproduction, with seedling establishment only successful under certain hydrologic conditions (Schneider and Sharitz, 1988). Flood pulses during the plant dormant season (October-March) can increase soil moisture, disperse seeds, and inhibit germination of upland species (those species not considered wetland residents), on lower elevation sites.
In addition, healthy floodplains support a community of invertebrates (insects and crustaceans) and fish specifically adapted to live and breed in wetlands. Most fish activity in Southeastern floodplains occurs during the winter and spring flood pulses. Floodplain habitat availability to riverine fishes depends on the physical extent and duration of floodplain inundation, the rate of rise and fall of the flood-pulse, and pulse seasonality. Rapidly rising and falling pulses, which also tend to be short in duration, are unlikely to provide meaningful benefits to fishes. Pulses that occur when water temperatures are low also are less likely to benefit fishes than pulses that coincide with spawning and juvenile growth periods (typically spring and early summer months). Perhaps the primary reason that fish move onto floodplains is to exploit invertebrates as food, and like fish the resident invertebrate community is strongly influenced by the character of flood pulses.
Nonaquatic animals can also be affected by flood pulses. Late spring floods can destroy nests of ground-nesting birds, but prolonged late flooding provides canopy birds with protection from a variety of nest predators. Longer-term effects on bird communities will likely reflect the frequency (both within the year and annual rate) of flood pulses, as well as changes occurring in the plant community.
As flood pulses make their way downstream they ultimately influence estuaries and their associated intertidal marshes. Because tidal wetlands are regularly inundated to levels that are primarily influenced by sea level (Figure 1), flood pulses have a different effect in these areas as compared to upstream. Rather than increasing the period or extent of inundation, a flood pulse in a tidally-influenced area will be manifest as a slug of low salinity water. The distribution of many estuarine resources, including marsh plants, plants, animals or bacteria that live on tidal creek sediments or living within the water column can shift in response to changes in salinity. Changes in the pattern of inflow will also affect the delivery of nutrients, organic matter, and sediment, which can in turn affect estuarine productivity rates and trophic structure (Alber 2002). The effects of freshwater pulses are likely to vary depending on the length of the pulse as well as the individual life histories of estuarine organisms, but freshwater regulation can affect migration patterns, spawning habitat, and species diversity of both fish and invertebrates (Drinkwater and Frank 1994).
A short-term period of reduced salinity (i.e. days to weeks), that occurs with a flood pulse, is likely to affect the distribution of motile organisms as they move to stay in water of a preferred salinity. Wetland plants and sedentary organisms (such as oysters or benthic infauna) are generally able to tolerate a range of salinities. Over the longer term (years), however, a regular regime of flood pulses may well cause more permanent shifts in the distributions of all organisms, with effects on population dynamics and community composition. To the extent that decreased flow tends to “squeeze” the estuary this may result in decreased areas of appropriate habitat. Upstream habitats in particular, such as tidally influenced forests and marshes, are most likely to show an immediate and measurable response to changes in freshwater inflow than habitats closer to the ocean. It is not clear how a change in the intertidal marshes, particularly a shift from tidal fresh to salt marsh, will affect the ecosystem services that marshes provide. Plant diversity is higher in tidal fresh as compared to salt marshes, but trends in overall biodiversity are less clear. Recent studies have shown that, while primary production is equivalent in fresh and saltwater marshes, rates of decomposition are much slower in fresh than salt marshes, leading to increased accumulation of soil and higher concentrations of organic carbon and nitrogen in fresh marshes (Craft 2007).
In addition to upstream management of dams and reservoirs, other types of perturbations will also affect estuarine areas. Channel dredging and sea level rise will both result in increased upstream penetration of seawater and an associated decrease in the low salinity habitat, whereas an increase in storm activity will increase freshwater flow. Understanding the implications of changing flooding regimes and the delivery of freshwater to estuarine habitats will remain an important area of study.
THE SAVANNAH RIVER PROJECT
The Nature Conservancy (TNC) scientists are working with partners to protect downstream aquatic habitats in the Savannah River basin, Georgia, by advancing ecosystem flow restoration and adaptive management. The U.S. Army Corps of Engineers (Corps) maintains three large dam and reservoir projects within the basin. Under the water management regime of the last 50 years, there has been a removal of natural variability in the hydrograph including a reduction in peak flow volumes and frequency (Figure 2).
The Corps in partnership with TNC has released experimental controlled flood pulses per scientific recommendations developed specifically for the Savannah River. In order for the Corps to adopt ecological water management as standard practice, flow recommendations, or flow restoration hypothesis, must be tested in a scientifically credible manner. TNC and partners such as the U.S. Fish and Wildlife Service, Georgia and South Carolina Departments of Natural Resources, The University of Georgia, Savannah State University, The Southeastern Natural Sciences Academy, and Augusta State University have begun to implement monitoring approaches to determine pre-controlled and post-controlled flood release conditions (Figure 3).
This effort of supplying science to support ecosystem flow restoration has been successful because of the work of professors, students, agency biologists, Corps hydrologists, and volunteers. Initially flood pulses have been measured with dynamic response indicators such as floodplain fish use, quantity and duration of floodplain flooding, anadromous fish movements, shifts in floodplain invertebrate communities, water quality, and a temporary shift in the average location of the saltwater/ freshwater interface in the estuary (Wrona et al., 2007). Bench mark, or long-term indicators such as riverine freshwater mussel populations or floodplain tree recruitment and growth will need to be measured over decades. Although the adaptive management process essential to ecosystem flow restoration in the Savannah depends on rigorous scientific measures, funding sources for this type of science, especially long-term monitoring, are scarce. The cost of both short and long-term monitoring is high and is difficult to “sell” to the public as an important conservation action.
The Savannah River provides an opportunity to give much needed definitions to the concept of adaptive management as well as the opportunity to test large scale
aquatic ecosystem response to flow restoration. By continuing to monitor the effects of flow restoration and feeding these results back into the adaptive management
framework, we will be able to advance the Savannah River as successful example of ecologically sustainable water management.
Alber, M., 2002. A Conceptual Model of Estuarine Inflow Management. Estuaries 25:1246-1261.
Craft, C., 2007. Freshwater Input Structures Soil Properties, Vertical Accretion, and Nutrient Accumulation of Georgia and U.S. Tidal Marshes. Limnol. Oceanogr. 52 (In Press).
Drinkwater, K.F. and K.T. Frank, 1994. Effects of River Regulation and Diversion on Marine Fish and Invertebrates. Aquatic Conservation: Marine and Freshwater 4(2):135-151.
Junk, W.J. and K.M. Wantzen, 2006. Flood Pulsing and the Development and Maintenance of Biodiversity in Floodplains. In: Ecology of Freshwater and Estuarine Wetlands, D.P. Batzer and R.R. Sharitz (Editors). University of California Press, Berkeley, California, pp. 407-435.
Schneider, R.L. and R.R. Sharitz, 1988. Hydrochory and Regeneration in a Bald Cypress-Water Tupelo Swamp Forest. Ecology 69:1055-1063.
Wrona A., D. Wear, J. Ward, R. Sharitz, J. Rosenzweig, J.P. Richardson, D. Peterson, S. Leach, L. Lee, C.R. Jackson, J. Gordon, M. Freeman, O. Flite, G. Eidson, M. Davis, and D. Batzer, 2007, Restoring Ecological Flows to the Lower Savannah River: A Collaborative Scientific Approach to Adaptive Management. In: Proceedings of the 2007 Georgia Water Resources Conference, T.C. Rasmussen, G.D. Carroll, and A.P. Georgakakos (Editors), pp. 538-549.
Amanda Wrona Meadows, Dr. Meadows earned a M.S. in Zoology and a Ph.D. in Marine Science from the University of Georgia where she did her research at the Marine Institute at Sapelo Island. Between degrees she worked with the South Carolina Dept. of Natural Resources Marine Resources Research Inst. in Charleston as a Marine Biologist. She has been the director of the Savannah River Project for The Nature Conservancy for over three years and works as a scientist, educator, fundraiser, and in government relations. She is coordinating the continued ecosystem flow restoration and monitoring effort of the Savannah River with partners such as the U.S. Army Corps of Engineers, South Carolina, and Georgia Departments of Natural Resources, U.S. Fish and Wildlife Service, Augusta State University, and the University of Georgia.