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: The Corps 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.

However, the SRS also blocked the natural migration of salmon and steelhead. To address this, 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 current approved plan is an adaptive management plan that involves several methods of managing flood risk on the Toutle and Cowlitz River.  Based on the results for annual monitoring, there is a sequence of methods that would be considered.  These options would be Raising the Crest raise at least another two times and then building grade building structures and in that sequence.  Dredging is used only as a last resort measure if all other measures have been implemented and the monitoring of the river indicates that dredging is needed.

Protecting Fish is a Top Priority

USACE is currently designing to upgrade the existing Fish Collection facility to improve the fish collection of the listed ESA species (Winter Steelhead and Coho Salmon) and to install a new fish release site at Deer Creek).

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