Skip to main content

A distributary channel created by The Nature Conservancy as a shortcut for juvenile Chinook as they move from freshwater to saltwater environments. Photo: Sylvia Kantor/PSI

Ask a scientist: What is a distributary channel?

Distributary channels are offshoots of rivers that send water — and fish and sediments — to more areas in a delta floodplain. In ideal circumstances, they occur naturally, but some scientists hope to carve new distributary channels to enhance estuary restoration projects in Puget Sound. They say it is a novel approach that could pay big dividends for salmon.

In a natural state, a river passing through a floodplain will have many side channels and tendrils that distribute the water, fanning it out across the delta. However, these channels are often lost when rivers are diverted to make room for farmland or other human development. In many cases, even after the dikes and levees are breached and the water returns, they don’t come back easily on their own. 

Now, scientists are re-creating a distributary channel at Port Susan Bay stemming from the Stillaguamish River. The Port Susan Bay site, led by The Nature Conservancy, will eventually connect with the Stillaguamish Tribe’s neighboring zis a ba restoration project. The new channel, scientists hope, will make it much easier for young salmon to move from the river to restored tidal marshes on the edge of the bay to the north. 

Dr. Greg Hood is a senior research scientist with the Skagit River System Cooperative, and one of the architects of the project. He says this is the first time a distributary channel has been created for an estuary restoration project in Puget Sound, and scientists will be studying it closely to gauge its benefits for threatened Chinook salmon and the ecology of the delta in general.

We asked him to explain how these channels work and why they might be important for other projects in Puget Sound. He said there are two key areas to consider: the distribution of fish and sediments. The following interview has been edited for length and clarity. 

Puget Sound Institute (PSI): Why are distributary channels important for sediment delivery?

Greg Hood (GH): Distributaries have a lot of functions. They’re distributing water, but they’re also distributing sediments.

When you build levees along the sides of the river, and when you cut off the distributaries, it’s like taking a garden hose and pressing it together with your finger and thumb and creating a jet. You keep all that energy of the river in the mainstem channel, rather than letting it disperse over distributaries and disperse over flood plains. As a result, that sediment jets into the bay and bypasses the tidal marshes.

There are areas where historical distributaries have been cut off or blocked up and the marshes in front of them to which they used to communicate are now sediment starved. That results in marsh erosion. The idea with distributary channels is to diversify the distribution of water, fish, and sediments, and allow more of it to go where it needs to go.

PSI:  How does this affect fish?

GH: It’s the same thing with fish. Everything coming down that river gets distributed, including fish. Fish are spread out over more habitats. Because they’re not focused in one place, there’s lower density, right?

At higher densities, the fish are competing with each other. They spend less time in the Delta because of this competition. They leave at a smaller size, and earlier in the season, when they’re more vulnerable.

When they’re distributed over a broader area, there’s a lower density of fish. They reside longer because there’s less competition and more food for them, and they grow bigger. They leave the delta at a larger size, and later in the season. They are now less vulnerable to predation when they head out to Puget Sound. As a result, they have higher survivorship. That’s why distributaries are important because they spread the fish over more habitat and lower their densities.

PSI: This seems like a simple idea, but you’re saying that distributaries are not that common in Puget Sound restoration.

GH: I’m not aware of any other distributary restoration projects in Puget Sound currently [other than Port Susan Bay or zis a ba].

PSI:  Why is that? Is this a new thing?

GH: People have known about distributary channels for a long time. And they’ve studied how estuaries work. I would say that distributary channels have been of interest to geomorphologists, but I suspect that it’s still a relatively new idea for many ecologists.

Often, when people look at restoration, they look at acreage — how many acres in a restoration? And that is really important, of course. I’m not denying that. But it’s easy to overlook the functionality of the system, and distributaries are important to that functionality.

PSI:  Don’t distributary channels happen naturally? If you remove dikes and levees, won’t nature just take over and create these channels?

GH: Yes, but it can take a long time for nature to create new distributaries. I mean, you’d probably wait decades, if not centuries, for distributaries to restore themselves on their own. And part of the reason is that the land has been damaged to a certain degree.

When you dike off an area, the soils consolidate and as a result they can form a clay layer. In cases where an area has been converted to agriculture, that is exacerbated by plowing, because when you pull up land, you’re basically re-sorting the sediments.

Take a cereal box or potato chip bag. If you shake it up, all the fine pieces of potato chip, or cereal, or whatever, will settle to the bottom of the bag. And all the larger chips will be near the top. And that happens with sediments when you plow. All those fine sediments will be near the bottom of the plowing horizon. And because they’re fine, those sediments form a clay, especially when they’re compacted by the tractors.

But even without plowing, coastal marshes that are drained and consolidated and subsided, you can get clay layers that are many feet deep. That makes erosion of tidal channels really challenging. And you see restoration sites where they didn’t excavate the tidal channels. They just counted on tidal fluxes to carve channels for them. As a result, you get tidal channels, but they’re only a half foot deep, or a foot deep, rather than six feet deep, because they can’t cut below the clay.

That’s why I’m advocating to everyone who listens that you excavate your tidal channels because it’s going to take forever for them to cut through clay.

PSI:  It sounds like distributary channels are still catching on. Are we on the cusp of other new scientific advances in estuary restoration?

GH: People think sometimes that science has discovered everything there is to discover, that we know everything we need to know to restore habitat. And that’s just not true. Science is continually learning new things, and we’re continually improving, and we’ve only been restoring tidal marshes for about 20 or 30 years. Not very long. So, you know, people have been building bridges for 2000 years, and they still have bridge failures. Even in that case, for an engineering practice that has been around for 2000 years, we’re still learning how to do things better. That’s why I think monitoring and adaptive management are really important. We can always improve.