The Tongass National Forest is packed densely with salmon-producing streams and rivers that connect the ocean to critical freshwater spawning and rearing habitat. Communities throughout Southeast Alaska rely on salmon as a food resource, a staple of sport and commercial fishing livelihoods, and a cornerstone to the Indigenous ways-of-life that have existed in this region for thousands of years. Intact salmon habitat means household freezers full of fish, resilience for fishing and processing jobs, and healthy, nutrient-rich ecosystems.

On the Tongass, there are nearly 700 identified sites of culverts and bridges from past logging roads that may be blocking hundreds of miles of salmon spawning streams. When this road infrastructure deteriorates, fish traveling upstream can be blocked from reaching their spawning habitat, natural nutrient and sediment cycles of the stream are cut off, and the road can become damaged by erosive waters having nowhere to flow. To solve this, there are generally two options: either pull out the culvert to allow the stream to flow free and demolish the road crossing permanently, or preserve the road access to our public lands by inserting a better passage under the road, known as an aquatic organism passage (AOP) culvert. This is a structure that allows the stream to dynamically pass under the road, remaining passable for fish while adapting to the streams changing conditions throughout the year.

Sitka Conservation Society (SCS) works with the USDA Forest Service to find solutions on how to manage resources on public lands seeking the best benefit to ways-of-life, public access, intact ecosystems, and salmon streams. Aquatic Organism Passage projects balance the essential need for fish to travel upstream while keeping road access intact for public use on the land, all while creating more opportunities for road builders, heavy-equipment operators, engineers, fisheries and hydrology staff of partner agencies. Donate to SCS here so we can continue to support the work of bettering the future of the Tongass National Forest.


Donate to Sitka Conservation Society


Video and screenshots by Pioneer Studios. Project commissioned in partnership between the Sitka Conservation Society and National Forest Foundation. The National Forest Foundation works on behalf of the American public to inspire personal and meaningful connections to our National Forests. By directly engaging Americans and leveraging private and public funding, the NFF leads forest conservation efforts and promotes responsible recreation. Each year the NFF restores fish and wildlife habitat, facilitates common ground, plants trees in areas affect by fires, insects and disease and improves recreational opportunities. The NFF believes our National Forests and all they offer are an American treasure and are vital to the health of our communities. Learn more at

If you work in road construction, installing, fixing, and replacing stream culverts is an everyday job. In fact, in the US alone, it's estimated that we install more than 12 million feet of culverts on a road system each year. For generations, we've been building these culverts with one simple goal. Move water from one side of the road to the other as cheaply and efficiently as possible. In practice, this looks like building a rigid metal, plastic, or concrete corridor directly in the middle of an ever-changing and dynamic stream.

Static and dynamic don't mix well, and eventually, things break. Culverts like these commonly get clogged, which can damage the road. They fragment habitat and block fish passage and they cut off rivers from the nutrient and sediment cycles that keep these systems healthy. 

The good news is, there's a better way to build culverts, especially in places where fish passage is critical. But it takes looking to nature and rethinking what a stream crossing should look like. This is an aquatic organism passage, or AOP culvert. On the top and sides, it resembles a traditional culvert, but on the bottom, it looks just like a creek.

For years, engineers have been working to develop a better approach to culverts. One that can work both for nature and the road. The result of that hard work is now called stream simulation, as it embraces the idea that mimicking a natural stream is the best way to create a sustainable crossing. When an AOP restoration project is done right, the results are amazing: Salmon returning to spawning grounds cut off for generations, roads that can handle 100-year flood events and provide climate resilience, communities with reliable infrastructure, cleaner water, and healthier fisheries. 

Because of all these benefits, AOP construction has taken off in recent years, becoming one of the most sought-after and impactful types of restoration projects, especially across the West and in public lands like the Tongass National Forest, where wild salmon still fill streams by the millions. 

But building an AOP construction using stream simulation is no simple task. Crews have to coordinate across multiple disciplines and have to recreate what nature has constructed over thousands of years in just a few days. So if you're a contractor, engineer, or designer interested in AOP, we want to help you understand exactly how these projects work, what to avoid, and how to ensure your culvert lasts a lifetime. AOP is all about embracing change and looking to nature for inspiration. So buckle up and let's get building.


Every AOP restoration project begins with a problem. A culvert that's blocked, a collapsed road, salmon unable to move upstream. Once the problem is established, a design team comes in and works to create a plan for how to get back to a healthy natural stream that flows through the road. 

These plan sets detail every step of the construction process and call out important reminders as well as areas for flexibility. Depending on your site, you'll see one of two types of designs on your AOP structure. The first is called an open bottom culvert. This design requires crews to remove the old culvert and road and then reconstruct a simulated stream directly on the ground. The second type of structure is called a closed bottom culvert, where after removal, the stream is simulated inside a rigid structure. In this video, we'll be taking you through the full construction process on an open bottom culvert because it's easier to see the techniques required without a top on. 

Understanding a Stream

Now that you know exactly what type of structure you're working on, the next major step is to understand what elements of a stream you'll be creating. Streams are incredibly complex. They're 3D ecosystems that take thousands of years to develop and after that they find a sweet spot between constant change and stability that allows plants and animals to thrive. To achieve this, you'll have to work to simulate the following elements of a natural stream. First, the stream bed. This is the foundation, the glue and the core habitat of your stream. Second are the banks. These are the guardians of your structure. And third are key features like cascades, boulders, riffles, and steps. These provide great control and roughness to the stream. And the result is a variety of habitats that fish and other aquatic organisms rely on. All of these elements are defined in detail in your plan set. But to understand them thoroughly, it's recommended to just go look at the stream itself. 

Reference Reach

Design teams for AOP projects always find what's called a reference reach that has a similar grade and shape to the crossing. And they use it as a basis for the design. In a perfect world, your build will feel as natural as this reference. So spending time at the reference reach helps set the bar for the work that's about to be done. In this video, we want to show you exactly how stream simulations are built and the specific techniques required to make a successful AOP project. But before we build the stream, let's quickly go through all the steps that happened prior so you understand the entire build from beginning to end. 


First, most projects begin with a survey that marks all key locations and elevations defined in the plan set. This ensures that the simulated stream aligns with the existing stream channel and matches grade. 

Fish Removal

Second, biologists put up fish nets to protect the stream and net any fish found inside the construction zone and move them safely outside. It's also important to schedule these projects around fish timing windows to avoid impacting major runs and spawning events. 


Third, the site is de-watered. De-watering ensures a clean and dry foundation for construction and limits the amount of sediment runoff for the construction downstream. Effective de-watering keeps a stream clear and protects habitat and channel structure below the culvert. Some projects include de-watering plans like these that define how to set up dams, bypass pipes, and where to place pumps. If your plan set doesn't include de-watering plans, they'll need to be submitted for approval prior to construction. The important thing here is to remember that the entire point of these projects is to provide passage and protect life in the stream. So take your time to make sure that you're limiting sediment runoff and that the channel below stays clear and healthy for fish. It's also important to keep extra pumps on hand throughout the build for de-watering, as heavy rain and mechanical failure can easily flood your site and cause runoff. 


Fourth, the site is excavated and the old culvert is removed. This process is fairly straightforward, but it's key to excavate all the way down to the necessary grade and plan your site access as these builds can get steep and dangerous quickly. Many projects don't have foundation investigations prior to construction, so be prepared to deal with bedrock and large boulders. Since footers often need to be placed below grade and should never be in contact with large rocks and outcroppings, it's critical to speak with your CO or engineer that these features can't be removed. One final note is that as you excavate below your old culvert, you may encounter native streambed material. This is gold for your project, so make sure to sort and save this for later use in the streambed installation. 

Grading and Compaction

Fifth, with the road excavated, crews now work to precisely grade and level the site to prepare for the structure. This is a slow process that involves laying down leveling course base and compacting, then checking grades with the survey rod. If this is not done correctly, the structure can have alignment issues later, can sag, and even fail, so take your time to get it right. 

Setting the Structure

Sixth, the foundation for the structure is set. In an open bottom culvert, this involves lowering precast footers onto the site or pouring them into place, and on closed bottom culverts, now is the time when the tube will be set in the excavated channel.

The most important thing to consider during structure placement is that the footers are aligned properly and are the correct distance apart. This requires constant measuring and grade checks with the survey rod, but it's worth it so your top fits on properly when it's installed later.

Once footers are set, crews work to seal the gaps and join the segments according to the specifications and designs. 

Top Assembly

Seventh, the top is assembled. In some open bottom culverts, a metal arch sits as a cap on top of the footers. Metal arches like these can be delivered fully built or in sections that need to be assembled according to the manufacturer's guidelines. Some crews build these at the end of the project directly on top of the structure piece by piece, but often tops are assembled prior to the final installation so they can be easily lifted as a single piece onto the structure before backfilling and completing the road. With both approaches, it's important that the top is handled carefully to ensure that there's no damage during transport and install. Lastly, as you build your top, it's important to ensure that joints are watertight and there's no leaking as this can result in structural failure down the road. 

Backfill and Compaction

Eighth, all culverts are backfilled and compacted to ensure their structural integrity. The timing of backfilling can vary on projects, but it's important that you backfill as you construct your screen bed or just prior so that it applies even loads to both sides of the structure. Also, if you're backfilling just before the stream bed construction, you'll have better side access and this is safer than creating a dangerous corridor between your structure and the excavated road. When backfilling, lay down material in thin lifts according to the specification in your plans and make sure to compact with hand tools to prevent damage to the structure. Laying down too much material at this stage before compacting can lead to the road sagging and settling over time and can even encourage the stream to move outside the stream structure.

Mixing the Streambed

Now that your site is graded and compacted, it's time to prepare your stream bed mix.

A healthy stream bed mix seals the stream and operates as a shallow aquifer that retains water during dry periods. It provides critical habitat for the millions of microorganisms that break down organic material and support the entire food chain from invertebrates all the way up to large fish like salmon. Finally, the stream bed ensures that the sediment and erosion processes in the watershed can continue to operate, helping maintain water quality and limit barriers to fish in other life. Your building plans provide a detailed breakdown of what your stream bed mix should include. Whether you source your stream bed mix from a pit or have to mix it on site yourself, depends mostly on access. But if you are mixing on site, ensure that you short your piles prior to mixing to make sure that the final stream bed meets the specifications in the plans.

Plan sets often use units instead of actual quantities, so crews can mix by the bucket or scale based on how much they need. Throughout the process, ensure that your CO or inspector checks your mix. And if you're unsure if it's correct, make sure to stop and ask questions.

Once the stream bed mix is ready, crews begin work on the actual stream construction.


Building banks is fairly straightforward, but it requires a few key steps. First, crews measure the correct channel width and depth according to the plans.

Once marked, crews lay down a single layer of rocks and then work to fill in any gaps with finer soil and stream bed material. After, they use water pressure and tamping rods and other hand operated tools to force this material into every gap and ensure that banks are firm and locked together. This process has been repeated until the banks reach the correct height.

It's important not to damage your structure when placing rocks on the banks, as this can spell disaster if you have to replace or recast a footer at this stage in the build.

Don't make the mistake of stacking too many rocks before washing in finer soils. If gaps aren't filled at the bottom of the banks, the main channel can divert, blocking fish passage, and eroding the footers. 

Stream bed riffles 

On most plan sets, areas between key features like ribs, steps, and pools are defined as riffle sections. These make up the majority of the stream bed channel, and most of the time these sections are simply made of stream bed material and large isolated rocks if the plans call for them. To construct these sections, crews start by laying down a layer of stream bed material. 

Then, similar to the banks, they use water pressure, tamping rods, or similar hand-operated equipment to force fine material into every surface void. This process is continued in layers until the channel reaches the correct width and height defined in the plan set. The final stream channel should be dense and interlocked with low permeability. 


Steps, often called rock weirs, are one of the more technical elements of the stream bed to construct. Since they require the precise placement of one or more layers of large rocks that need to be interlocked and tied into the banks, steps help control gradient and provide flow resistance to maintain the diverse range of water depths and velocities needed for fish and other aquatic species. 

Crews begin steps by excavating and establishing a center footer rock at the specified height in the plans. They then continue to place additional footer rocks, creating a v-shape, ensuring there are no gaps between each. As the v grows and reaches the banks, these footer rocks are tied into the banks, ensuring there is no play. This creates a strong and seamless foundation for the step. 

After that, the process for the second layer begins by placing rocks on the upstream side, ensuring that each is braced by both the footer rock below and the rocks to the sides. Although most rocks are locked together, some plans call for small gaps in the surface layer of the rocks to allow passage of the step by juvenile fish, so make sure to check your plans for these. 

Crews then backfill the upstream side of the step with native stream bed material, so the channel makes a natural gradation. Once all this is done, it's critical to water in and compact finds so the entire structure is locked and sealed together. 

Finishing the Build

Once crews understand how to build the specific features on their side, it's a fairly simple process to repeat these methods until the entire stream is fully integrated into the natural one.

After that, all that's left to do is cap the structure, backfill and compact around it, add a riprap to protect the road from erosion, and finally remove the dam so the stream can run wild again.

Final Thoughts

Once a project is complete, it's important to take time to reflect and learn for the next project. AOP design and construction techniques continue to develop yearly, so be open to incorporating new ideas as the practice evolves. Also, always remember to take the opportunity to ask questions if you have them, and don't be afraid to reach out to teams outside of your own to ensure that these projects are success. 

AOP projects are some of the most rewarding construction contracts that crews can be a part of, and their demand is growing as communities and public lands work to restore their native watersheds and repair data infrastructure. There's no better time to get involved, so let's all work together to provide passage for a better tomorrow.

This video is intended as an informative yet simple tool to describe AOP work, which is very technical with its own specific handbook. This video is not meant to replace the handbook and is meant as one tool of many to help communicate the work, how it is done, and why it is done. Access more in-depth AOP resources here:

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