In January 2001, the U.S. Supreme Court ruled that the U.S. Army Corps of Engineers exceeded its statutory authority by asserting Clean Water Act (CWA) jurisdiction over non‐navigable, isolated, intrastate waters based solely on their use by migratory birds. The Supreme Court’s majority opinion addressed broader issues of CWA jurisdiction by implying that the CWA intended some “connection” to navigability and that isolated waters need a “significant nexus” to navigable waters to be jurisdictional. Subsequent to this decision (SWANCC), there have been many lawsuits challenging CWA jurisdiction, many of which are focused on headwater, intermittent, and ephemeral streams. To inform the legal and policy debate surrounding this issue, we present information on the geographic distribution of headwater streams and intermittent and ephemeral streams throughout the U.S., summarize major findings from the scientific literature in considering hydrological connectivity between headwater streams and downstream waters, and relate the scientific information presented to policy issues surrounding the scope of waters protected under the CWA. Headwater streams comprise approximately 53% (2,900,000 km) of the total stream length in the U.S., excluding Alaska, and intermittent and ephemeral streams comprise approximately 59% (3,200,000 km) of the total stream length and approximately 50% (1,460,000 km) of the headwater stream length in the U.S., excluding Alaska. Hillslopes, headwater streams, and downstream waters are best described as individual elements of integrated hydrological systems. Hydrological connectivity allows for the exchange of mass, momentum, energy, and organisms longitudinally, laterally, vertically, and temporally between headwater streams and downstream waters. Via hydrological connectivity, headwater, intermittent and ephemeral streams cumulatively contribute to the functional integrity of downstream waters; hydrologically and ecologically, they are a part of the tributary system. As this debate continues, scientific input from multiple fields will be important for policymaking at the federal, state, and local levels and to inform water resource management regardless of the level at which those decisions are being made. Strengthening the interface between science, policy, and public participation is critical if we are going to achieve effective water resource management.

Supreme Court cases have questioned if jurisdiction under the Clean Water Act extends to water bodies such as streams without year-round flow. Headwater streams are central to this issue because many periodically dry, and because little is known about their influence on navigable waters. An accurate account of the extent and flow permanence of headwater streams is critical to estimating downstream contributions. We compared the extent and permanence of headwater streams from two field surveys with values from databases and maps. The first used data from 29 headwater streams in nine U.S. forests, whereas the second had data from 178 headwater streams in Oregon. Synthetic networks developed from the nine-forest survey indicated that 33 to 93% of the channel lacked year-round flow. Seven of the nine forests were predicted to have >200% more channel length than portrayed in the high-resolution National Hydrography Dataset (NHD). The NHD and topographic map classifications of permanence agreed with~50% of the field determinations across~300 headwater sites. Classification agreement with the field determinations generally increased with increasing resolution. However, the flow classification on soil maps only agreed with~30% of the field determination despite depicting greater channel extent than other maps. Maps that include streams regardless of permanence and size will aid regulatory decisions and are fundamental to improving water quality monitoring and models.

Streamflow duration is used to differentiate reaches into discrete classes (e.g., perennial, intermittent, and ephemeral) for water resource management. Because the depiction of the extent and flow duration of streams via existing maps, remote sensing, and gauging is constrained, field-based tools are needed for use by practitioners and to validate hydrography and modeling advances. Streamflow Duration Assessment Methods (SDAMs) are rapid, reach-scale indices or models that use physical and biological indicators to predict flow duration class. We review the scientific basis for indicators and present conceptual and operational frameworks for SDAM development. Indicators can be responses to or controls of flow duration. Aquatic and terrestrial responses can be integrated into SDAMs, reflecting concurrent increases and decreases along the flow duration gradient. The conceptual framework for data-driven SDAM development shows interrelationships among the key components: study reaches, hydrologic data, and indicators. We present a generalized operational framework for SDAM development that integrates the data-driven components through five process steps: preparation, data collection, data analysis, evaluation, and implementation. We highlight priorities for the advancement of SDAMs, including expansion of gauging of nonperennial reaches, use of citizen science data, adjusting for stressor gradients, and statistical and monitoring advances to improve indicator effectiveness.

United States Supreme Court rulings have created uncertainty regarding U.S. Clean Water Act (CWA) authority over certain waters, and established new data and analytical requirements for determining CWA jurisdiction. Thus, rapid assessment methods are needed that can differentiate between ephemeral, intermittent, and perennial streams. We report on the validation of several methods. The first (Interim Method) was developed through best professional judgment (BPJ); an alternative (Revised Method) resulted from statistical analysis. We tested the Interim Method on 178 study reaches in Oregon, and constructed the Revised Method based on statistical analysis of the Oregon data. Next, we evaluated the regional applicability of the methods on 86 study reaches across a variety of hydrologic landscapes in Washington and Idaho. During the second phase, we also compared the Revised Method with a similar approach (Combined Method) based on combined field data from Oregon, Washington, and Idaho. We further compared field-based methods with a GIS-based approach (GIS Method) that used the National Hydrography Dataset and a synthetic stream network. Evaluations of all methods compared results with actual streamflow duration classes. The Revised Method correctly determined known streamflow duration 83.9% of the time, versus 62.3% accuracy of the Interim Method and 43.6% accuracy for the GIS-based approach. The Combined Method did not significantly outperform the Revised Method. Analysis showed biological indicators most accurately discriminate streamflow duration classes. While BPJ established a testable hypothesis, this study illustrates the importance of quantitative field testing of rapid assessment methods. Results support a consistent method applicable across the Pacific Northwest.

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