Unveiling WOTUS: How NOHWM and SDAM Define Our Nation’s Waterways

Establishing Jurisdictional Waters of the United States (WOTUS) Using the US Army Corps of Engineers National Ordinary High Water Mark Manual (NOHWM) and Stream Duration Assessment Method (SDAM)

For environmental scientists, the determination of whether a wetland or waterway qualifies as a jurisdictional Water of the United States (WOTUS) is not just a regulatory necessity but a vital step in preserving our nation’s aquatic ecosystems. At the heart of this process are two pivotal tools provided by the US Army Corps of Engineers (USACE): the National Ordinary High Water Mark (NOHWM) Manual and the Stream Duration Assessment Method (SDAM). Together, these methodologies offer a comprehensive, scientific approach to delineating WOTUS, ensuring that critical water resources are effectively protected.

Diving into the National Ordinary High Water Mark (NOHWM) Manual

The National Ordinary High Water Mark (NOHWM) Manual serves as a detailed guide for environmental professionals to accurately identify the Ordinary High Water Mark (OHWM) in various landscapes. The OHWM represents the boundary between jurisdictional waters and upland areas, making it a crucial concept for environmental assessments.

Key Components of NOHWM:

1. Physical Characteristics: The manual outlines physical indicators that signify the presence of an OHWM. These include:

• Clear Natural Line: Look for a visible line on the bank formed by the regular presence of water.
• Soil Changes: Notice differences in soil color, texture, or composition that indicate historical water presence.
• Vegetation Patterns: Identify areas where terrestrial vegetation is absent or altered due to frequent inundation.
• Litter and Debris: Examine the accumulation of organic and inorganic materials deposited by water flow.

1. Regional Adaptations: The NOHWM Manual acknowledges the diversity of the US landscape, providing region-specific guidelines to account for varying hydrological and geomorphological conditions.
2. Field Procedures: The manual details systematic field procedures, ensuring consistency and accuracy across assessments. This includes standardized data collection techniques, field forms, and documentation practices.

The Weight of Evidence Approach in NOHWM

A critical concept in the NOHWM approach is the “weight of evidence” (WOE) methodology. This approach ensures that multiple lines of evidence are considered when determining the OHWM, providing a more robust and defensible delineation.

Weight of Evidence Approach:

1. Multiple Indicators: Instead of relying on a single indicator, the WOE approach integrates various physical, hydrological, and biological indicators. This multi-faceted analysis helps in cross-verifying the presence and extent of OHWM.
2. Corroborative Evidence: Indicators such as changes in soil, vegetation patterns, and physical markings on the bank are examined together. Consistency among these indicators strengthens the determination of OHWM.
3. Contextual Analysis: The WOE approach considers the broader landscape and hydrological context, including historical data and regional characteristics, ensuring that the delineation is not only accurate but also contextually relevant.
4. Documentation and Transparency: Detailed documentation of all evidence and indicators used in the assessment is crucial. This transparency enhances the defensibility of the OHWM determination.

Stream Duration Assessment Method (SDAM): A Closer Look

For scientists engaged in stream assessments, the Stream Duration Assessment Method (SDAM) is a game-changer. It allows for precise classification of streams based on their flow duration, which is essential for determining their jurisdictional status.

Stream Classifications:

1. Ephemeral Streams: These streams flow only during and immediately after precipitation events. As a scientist, identifying ephemeral streams involves recognizing temporary water flow patterns and minimal biological activity.
2. Intermittent Streams: These streams flow during certain times of the year, such as the wet season, but not continuously. Intermittent streams show clear signs of seasonal aquatic life and sediment patterns.
3. Perennial Streams: Perennial streams flow year-round and are typically considered jurisdictional. These streams support continuous aquatic habitats and show persistent hydrological and biological indicators.

SDAM Assessment Process:

1. Field Observations: Conduct site visits to observe physical characteristics like bed and bank features, sediment deposits, and biological indicators such as the presence of aquatic organisms and specific plant species.
2. Hydrological Data Review: Examine historical flow records, precipitation data, and stream gauge information to understand the stream’s flow regime over time.
3. Biological Indicators: Identify the presence of certain aquatic organisms and plant species that thrive in continuous or seasonal water flow, offering insights into the stream’s duration and consistency.

Integrating NOHWM and SDAM for WOTUS Determinations

The integration of the NOHWM and SDAM methodologies provides a robust framework for determining whether a water body qualifies as WOTUS. For environmental scientists, this integration involves a meticulous blend of physical, hydrological, and biological assessments.

Determination Process:

1. Preliminary Assessment: Begin with a thorough desktop review, using maps, aerial photos, and existing hydrological data to identify potential jurisdictional waters.
2. Field Verification: Conduct on-site inspections to validate preliminary findings. Observe physical indicators of OHWM and perform SDAM assessments to classify stream types accurately.
3. Data Integration: Combine field observations with historical and current hydrological data. This comprehensive dataset aids in making well-informed determinations of a water body’s jurisdictional status.
4. Documentation and Reporting: Prepare detailed reports that document findings, methodologies, and justifications for the jurisdictional status. These reports provide transparency and serve as a basis for regulatory decisions.

The Environmental Significance of NOHWM and SDAM

For environmental scientists, the accurate identification of jurisdictional waters is paramount. Here’s why:

1. Protecting Ecosystems: Correctly delineating WOTUS ensures the preservation of vital aquatic ecosystems, protecting them from pollution and degradation.
2. Ensuring Compliance: Clear identification helps landowners, developers, and other stakeholders understand their obligations under the Clean Water Act, preventing unauthorized activities that could harm protected waters.
3. Resource Management: Informed decision-making for water resource management, conservation planning, and habitat restoration efforts becomes possible, benefiting both the environment and the communities that rely on these resources.

Conclusion

The National Ordinary High Water Mark Manual (NOHWM) and the Stream Duration Assessment Method (SDAM) are indispensable tools for environmental scientists working to delineate jurisdictional Waters of the United States (WOTUS). By applying these methodologies, the US Army Corps of Engineers ensures that water bodies deserving of protection under the Clean Water Act are accurately identified and managed. For environmental professionals, mastering these methods is not just about regulatory compliance—it’s about playing a critical role in safeguarding the health and integrity of our nation’s precious water resources. Through diligent application of NOHWM and SDAM, we can continue to protect and sustain our aquatic ecosystems for future generations.