This document is a draft for review purposes only and does not constitute Agency policy. 9/10/2013 1-3 DRAFT — DO NOT CITE OR QUOTE: SUMMARY OF MAJOR CONCLUSIONS Based on the review and synthesis of more than 1,000 publications from the peer-reviewed scientific literature, the available evidence supports three major conclusions: 1. The scientific literature demonstrates that streams, individually or cumulatively, exert a strong influence on the character and functioning of downstream waters. All tributary streams, including perennial, intermittent, and ephemeral streams, are physically, chemically, and biologically connected to downstream rivers via channels and associated alluvial deposits where water and other materials are concentrated, mixed, transformed, and transported. Headwater streams (headwaters) are the most abundant stream type in most river networks and supply most of the water in rivers. In addition to water, streams transport sediment, wood, organic matter, nutrients, chemical contaminants, and many of the organisms found in rivers. Streams are biologically connected to downstream waters by the dispersal and migration of aquatic and semiaquatic organisms, including fish, amphibians, plants, microorganisms, and invertebrates, that use both up-and- downstream habitats during one or more stages of their life cycles, or provide food resources to downstream communities. Physical, chemical, and biological connections between streams and downstream waters interact via processes such as nutrient spiraling, in which stream communities assimilate and chemically transform large quantities of nitrogen (N) and other nutrients that would otherwise increase nutrient loading downstream. 2. Wetlands and open-waters in landscape settings that have bidirectional hydrologic exchanges with streams or rivers (e.g., wetlands and open-waters in riparian areas and floodplains) are physically, chemically, and biologically connected with rivers via the export of channel-forming sediment and woody debris, temporary storage of local groundwater that supports baseflow in rivers, and transport of stored organic matter. They remove and transform excess nutrients such as nitrogen and phosphorus(P). They provide nursery habitat for breeding fish, colonization opportunities for stream invertebrates, and maturation habitat for stream insects. Moreover, wetlands in this landscape setting serve an important role in the integrity of downstream waters because they also act as sinks by retaining floodwaters, sediment, nutrients, and contaminants that could otherwise negatively impact the condition or function of downstream waters. 3. Wetlands in landscape settings that lack bidirectional hydrologic exchanges with downstream waters (e.g., many prairie potholes, vernal pools, and playa lakes) provide numerous functions that can benefit downstream water quality and integrity. These functions include storage of floodwater; retention and transformation of nutrients, metals, and pesticides; and recharge of groundwater sources of river baseflow. The functions and effects of this diverse group of wetlands, which we refer to as “unidirectional wetlands,” affect the condition of downstream waters if a surface or shallow subsurface water connection to the river network is present. In unidirectional wetlands that are not connected to the river network through surface or shallow subsurface water, the type and degree of connectivity varies geographically within a watershed and over time. Because such wetlands occur on a gradient of connectivity, it is difficult to generalize about their effects on downstream waters from the currently available literature. This evaluation is further complicated by the fact that, for certain functions (e.g., sediment removal and water storage), downstream effects arise from wetland isolation rather than connectivity. The literature we reviewed does not provide sufficient information to evaluate or generalize about the degree of connectivity (absolute or relative) or the downstream effects of wetlands in unidirectional landscape settings. However, evaluations of individual wetlands or groups of wetlands could be possible through case-by-case analysis. Further, while our review did not specifically address other unidirectional water bodies, our conclusions apply to these water bodies (e.g., ponds and lakes that lack surface water inlets) as well, since the same principles govern hydrologic connectivity between these water bodies and downstream waters.
Posted on: Wed, 26 Mar 2014 20:03:37 +0000
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