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Connectivity of wetlands to downstream waters: Conceptual framework and review
Citation:
LEIBOWITZ, S. G., L. ALEXANDER, B. C. AUTREY, J. DeMeester, C. R. LANE, S. D. LEDUC, AND C. E. RIDLEY. Connectivity of wetlands to downstream waters: Conceptual framework and review. Presented at Society of Wetland Scientists/Intecol, Orlando, FL, June 03 - 08, 2012.
Impact/Purpose:
A river represents the time-integrated combination of all waters contributing to it.
Description:
A river represents the time-integrated combination of all waters contributing to it. Understanding the factors that influence a river’s health and sustainability, as well as its degradation, requires an integrated systems perspective. This considers all the components of the river system and their spatiotemporal interactions. However, the effects of wetlands on rivers have received little attention. Such information could help address science needs regarding the relationship between wetlands and navigable waters arising from the 2006 U.S. Supreme Court’s Rapanos case. Here we first describe a conceptual framework for understanding wetland effects on rivers. Wetlands affect downstream waters by altering material fluxes through the river network, thereby affecting river structure and function. This requires wetland functions that alter material fluxes and connectivity between system components. Wetlands can affect material fluxes by serving as sources, sinks, refuges, lags, and/or transformers. Movement of water is the primary mechanism for connectivity between wetlands and rivers, but biological movement also is important. In some cases, lack of connectivity (isolation) is important, as in limiting sediment inputs. Connectivity is not a fixed characteristic, but varies over space and time due to expansion and contraction of the river network and transient connection with wetlands and other components. Wetland impacts can increase or decrease connectivity. We also present results of a literature review we conducted to evaluate the physical, chemical, and biological connections between wetlands and downstream waters. The effects of riparian and floodplain wetlands (RFWs) were considered separately from those of non-riparian and channel origin wetlands (NRCWs), since hydrologic connectivity between NRCWs and rivers is unidirectional, while RFW connectivity can also be bidirectional. RFWs are physically, chemically, and biologically connected with rivers via export of channel-forming sediment and woody debris, storage of local groundwater, and transport of stored organic matter. They remove and transform excess nutrients and provide nursery habitat for breeding fish and colonization opportunities for stream invertebrates. RFWs also act as sinks for floodwaters, sediment, nutrients, and contaminants. Even RFWs that rarely flood can have important, long-lasting effects on streams and rivers. NRCWs provide numerous benefits to downstream waters, including storage of floodwater; retention of nutrients, metals, and pesticides; and re-charge of groundwater sources of river baseflow. NRCWs clearly affect downstream waters if the wetland is connected to the river network through a stream. When a direct surface connection is lacking (e.g., geographically isolated wetlands), connectivity varies within a watershed and over time, making generalizations difficult. However, we conclude that: (1) many NRCWs interact with groundwater, which can travel long distances and affect downstream waters; (2) even isolated wetlands can influence downstream rivers by preventing water from entering the river network; and (3) within a watershed or region, wetlands that are closer to rivers and streams have a higher probability of being connected than more distant areas, assuming similar conditions.