Connectivity of Streams & Wetlands to Downstream Waters: A Review & Synthesis of the Scientific Evidence

The physical structure of a river network inherently demonstrates cumulative connectivity (Section 1.2.3) between all streams and their downstream rivers. Substantial evidence supports physical, chemical, and biological connections from headwater streams?including those with ephemeral, intermittent, and perennial flows?to waters immediately downstream through transport of water and associated materials, movement of organisms and their products, and bidirectional geomorphic adjustments. Among the most compelling evidence for the effects of headwater streams on rivers is as sources of water, nitrogen, organic carbon, and sediment (clean and contaminated); as transformers of and sinks for nitrogen, carbon, and contaminants; and as providers of essential habitat for migratory animals such as anadromous salmon. Headwater streams as a class provide substantial quantities of water to larger water bodies. For example, first-order streams cumulatively contribute approximately 60% of the total mean annual flow to all northeastern U.S. streams and rivers. Infrequent, high-magnitude events are especially important for transmitting materials from headwater streams in most river networks. The strongest lines of evidence supporting the effects of headwater streams are from watersheds where headwater streams drain a unique portion of the basin (e.g., hydrology, geology, human alteration). Investigation of connections among river network components continues to be an active area of scientific research, with progress occurring in the development of river network models and connectivity metrics for quantifying connections and their downstream effects. Physical, chemical, and biological connections between headwater streams and downstream waters are fundamental to the structure and function of river networks, and additional empirical data and further breakthroughs that quantify linkages across large spatiotemporal scales will continue to enhance our understanding of river network complexity.