The 1980 eruption of Mount St. Helens left a legacy of sediment that continues to impact the landscape. Billions of cubic yards of sediment flowed into the Toutle and Cowlitz Rivers, threatening communities downstream.

The U.S. Army Corps of Engineers (USACE) took swift action, implementing short-term and long-term solutions to manage this ongoing challenge.

Short-term: USACE focused on reducing flood risk by:

• Increased the crest height on the Sediment Retention Structure. 
• After two authorized crest raises have been implemented, USACE will construct grade building structures. 
• If all other measures are exhausted, USACE can dredge the Cowlitz River to remove sediment build-up when annual monitoring indicates it is required to keep the authorized level of protection.

As a mitigation measure, a Fish Collection Facility was built to capture fish and transport them upstream, ensuring their survival.

USACE continues to work with relevant stakeholders and other agencies to manage sediment flow from Mount St. Helens. This includes developing a long-term plan to ensure the health of the river system while protecting communities and fish populations.

The 1985 Long-Term Plan recognized that additional actions would be needed in the future to maintain the authorized levels of flood risk reduction. USACE is currently evaluating how to best manage sediment to provide authorized levels of flood risk reduction through the year 2035, in a manner that does not jeopardize the continued existence of any endangered or threatened species or result in the destruction or adverse modification of designated critical habitat.

USACE has conducted a limited re-evaluation of sediment management in the North Fork Toutle River and is proposing to implement updated sediment management measures to manage flood risk to established levels for the Washington cities of Castle Rock, Lexington, Kelso, and Longview through the year 2035. In addition to a No Action alternative, three sediment management alternatives were identified and evaluated that would address the flood risks associated with sediment build up through 2035. They are:

• Cowlitz River dredging only
• SRS spillway and embankment raise
• A phased construction plan (preferred alternative) consisting of:
  • SRS spillway crest raises
  • Grade-building structures
  • Dredging, as needed

 The FSEIS also includes an evaluation of the fish conservation measures that were developed to ensure the proposed sediment management plan will not jeopardize the continued existence of any endangered or threatened species or adversely modify or destroy the species’ designated critical habitat. Fish conservation measures evaluated include:

• Replace the fish collection facility on the North Fork Toutle River operated and maintained by the Washington Department of Fish Wildlife (WDFW) and establish a fish release site on Deer Creek
• Modify the fish collection facility on the North Fork Toutle River and establish a fish release site on Deer Creek

Environmental impacts of the no action, sediment management alternatives and fish conservation measures are evaluated in the FSEIS.

USACE's preferred alternative for managing sediment and flood risk includes phased construction of spillway crest raises at the SRS; grade building structures on the sediment plain upstream from the SRS; and, dredging as needed to maintain the authorized levels of protection. The preferred alternative for improving fish passage includes modify the existing FCF and constructing one additional fish release site.

USACE is committed to finding innovative ways to manage sediment from Mount St. Helens. We're not just planning for the future — we're putting ideas into action on the ground right now.
Here's a look at some of the work we've done:

Building Structures that Work with Nature: Upstream from the Sediment Retention Structure (SRS), we're testing out different types of structures that use the river's natural power to our advantage. It's like giving the river a helping hand to manage the sediment it carries. For example:

• Cross Valley Structure: Imagine a series of steps and barriers that act like a temporary dam during high flows. This structure slows the water down, giving sediment a chance to settle out before it flows downstream.
• Island Forming Structures: These "engineered log jams" mimic the effects of natural logjams, creating calmer areas behind them where sediment can collect. These structures also help to create more stable channels for the river to flow through.
• Diversion Berm: This long, sediment-filled tube acts like a gentle guide, directing the river's flow in a specific direction. This helps us to protect certain areas and test how the river responds to these changes.

A Scientist's Toolkit: We're using cutting-edge technology to understand exactly how these structures are impacting the river. Here's a peek inside our toolkit:

• Sediment Sleuths: We're taking regular sediment samples and analyzing them to see what types of sediment are being trapped and how effective our structures are.
• Eyes in the Sky: Light Detection and Ranging (LiDAR) technology allows us to create incredibly detailed 3D maps of the riverbed. This helps us track changes in sediment buildup over time and measure how much sediment our structures are trapping.
• Cameras on the Ground: We're using cameras to continuously monitor the river and our structures, giving us a real-time view of how the system is working.

What We've Learned:

It's Working! The data we're collecting shows that these structures are effectively trapping sediment and helping to reshape the river in a positive way.
Valuable Lessons: This pilot project is giving us critical information about how the river behaves, which will help us design the most effective long-term plan.

Buying Time: In 2012, we raised the height of the SRS spillway, increasing its capacity to hold sediment. This buys us valuable time to develop sustainable solutions while continuing to protect communities downstream.

This work demonstrates our ongoing dedication to protecting communities and restoring the natural environment around Mount St. Helens. We're using ingenuity, a hands-on approach, and cutting-edge technology to create a safer and more resilient future for everyone

Mount Saint Helens monitoring program: Data collection and analysis summary, water years 2013 to 2022

This report covers the water years between 2013 and 2022 and can be found here: Monitoring activities of the Mount St. Helens (MSH) long-term sediment management plan have been ongoing since 1985 and are used to inform the adaptive management of the sediment and flood hazards that originated with the eruption of MSH back in May 1980. The main recommendations of this report are focused on continuing all data collection efforts and analyses on an annual basis and on filling in the temporal resolution of the monitoring activities’ ability to detect the outcomes of large events shortly after they occur.

For water year 2023, click here.