The Portland District, U.S. Army Corps of Engineers (USACE), completed a six-year study on the Willamette Valley system of dams and published the Environmental Impact Statement (EIS) in April 2025. The EIS provides USACE with a comprehensive understanding of how its actions/operations may affect the environment, communities, and surrounding ecosystems.

Shortly before the EIS was finalized, two significant developments introduced new requirements—prompting the need for a Supplemental Environmental Impact Statement (SEIS) to address them. We have issued a Record of Decision, outlining how we will operate the Willamette Valley System of dams while we address the new requirements.

SEIS - New Requirements

USACE prepares a Supplemental EIS—or SEIS—when new information or requirements come up that weren’t part of the original study.

This SEIS will address two new requirements:

1. Studying a permanent end to hydropower production at Willamette Valley dams
In December 2024, Congress passed the Water Resources Development Act (WRDA) of 2024, which directed USACE to look at a new option, or “alternative,” that would remove hydropower as an authorized function at certain Willamette Valley dams. This requirement prevents us from completing our overall review of operations and maintenance of the system and consultation with federal agencies, until we prepare and formally analyze an alternative that ends federal hydropower operations in the Willamette Valley.

2. Implementing a deeper fall drawdown of Detroit Reservoir
The National Marine Fisheries Service published a Biological Opinion. That opinion directed USACE to implement a deeper fall drawdown at Detroit Reservoir to support the protection of endangered fish species. We do not anticipate a deep reservoir drawdown at Detroit Dam until fall of 2026. Portland District will first analyze the effects of the drawdown in the SEIS while continuing to carry out water quality and downstream fish passage operations like we have done over the past few years. We are working with NMFS to develop the timing and duration of the deep fall drawdown at Detroit Dam and considering potential impacts to communities and water quality.

Because these new requirements came late in the EIS development process, the Portland District published the existing EIS while beginning additional work on the SEIS to evaluate and address the new requirements.

We completed our last environmental review of the entire system in 1980. Since then, we've made many changes to help endangered fish, made improvements to the dams, and optimized operations for fish passage and improved water quality. Until now, we reviewed these changes one dam at a time—not as a complete system. We also have new information about how our operations effect the environment. For these reasons, we prepared an Environmental Impact Statement (EIS) to investigate how we'll continue to operate and maintain the Willamette Valley System under the changed conditions.

This EIS examines different ways we could adjust our operations and how these changes would affect people and the environment. We call these different approaches "alternatives."

We've only considered alternatives that allow us to continue all our Congressionally authorized purposes:

Flood risk management  |  Water supply  |   Water quality  |  Fish and wildlife protection  |  Recreation  |  Hydropower generation

Each alternative must protect endangered species listed under the Endangered Species Act (ESA). After analyzing a large range of options, we identified our preferred alternative for future operations. The public reviewed and commented on the Draft EIS and our preferred alternative in 2022, during an official comment period. We addressed these comments and updated the EIS based on feedback from the public, Tribes, and federal and state agencies.

Complying with the Endangered Species Act

Before we can decide to implement the preferred alternative proposed in the EIS, we must ensure our plan protects endangered species and won't put them at greater risk of extinction. To do this, we must consult with two federal agencies: the National Marine Fisheries Service (NMFS) and the U.S. Fish and Wildlife Service (USFWS), together known as the Services. The ESA consultation process is not a public process like NEPA.

Under the ESA, the process is called Section 7 consultation and entails the following:

1. NMFS and USFWS review our proposed action, based on our preferred alternative in the Draft EIS, and provide Biological Opinions that states whether our plan is safe for endangered species.
2. If the Services determine our plan might harm endangered species, they'll require alternative actions to take to protect them.
3. We then update the Final EIS to include these protective actions.
4. We issue a NEPA Record of Decision that documents our decisions on the preferred alternative in the Final EIS and describes how our action complies with the ESA.

We received NMFS’ final Biological Opinion on December 26, 2024. Consultation with USFWS is ongoing.