Hazards from volcanic eruptions continue long after the mountain stops exploding. Take Mount St. Helens: 35 years after her catastrophic morning, the landscape is still continuously affected. It’s not just that trees take a long time to grow up. It also takes a long time for all that loose debris to settle.

For a dramatic example, all you have to do is type “Castle Rock, WA” into Google maps and have a look at the satellite view. Zoom in on the confluence of the Toutle and Cowlitz Rivers, and you see a stark example of a volcanic eruption’s long reach.

Wow, right? When I saw this on Google Earth whilst looking for interesting geological features in and around Castle Rock, I knew I had to go see it in person. Keep in mind, the sedimentation is that extreme even after repeated dredgings, and a Sediment Retention Structure built upstream on the North Fork Toutle River to capture as much sediment as possible before it arrives at the Cowlitz. And this is thirty-five years after Mount St. Helens erupted. Not to mention, Mount St. Helens is 70 kilometers (43.5 miles) upstream.

B and I spent the last day of our recent trip seeking out the confluence. We went up Old Pacific Highway, crossed the bridge over the Toutle River, and stopped by Caldwell Road, where there’s a glimpse of the Cowlitz River before the two rivers meet. Here, you can see how broad and blue it is.

This is a super-dry summer, so it’s not as high as it could be. You can see some modest gravel bars there along the banks.

Now, let’s return to the Toutle. There’s a great spot to park beside the bridge, and nice shoulders even on the bridge, so you can safely walk out and inspect the river from a height. See how different it is from the Cowlitz?

I’m not sure what those bits of brick wall and concrete are from – this bridge appears to have survived the May 18th, 1980 lahar that roared through here. We’ll ignore those chunks for now and focus on the river itself. Dang, right? So different from the Cowlitz. That gravel bar is bloody enormous. And the river is carrying a humongous load of suspended sediment. One website I’ve looked at claims that the Toutle River “carries one of the highest sediment loads of any US river.” I haven’t verified that claim, but I can just about believe it. I mean, this river’s about as far from in flood as you can get, and this isn’t a steep stretch or anything, but it’s practically the River Ankh.

If you walk down the short dirt road from the bridge, you begin to see why, even here, even with the SRS upstream, the Toutle’s carrying so much sediment.

Before the Army Corps of Engineers built the SRS in 1987, they spent the years between the cataclysmic eruption and the completion of the sediment-trapping dam dredging the Toutle, Cowlitz, and Columbia Rivers. As we walked along the tracks to the convergence, we passed a freaking ginormous dredge pile.

They had no choice. They had to prevent excessive flooding somehow, and one thing a sediment-choked river is prone to do is flood copiously – where else is the water going to go when the river channel is already full of stuff? Also, ships use the Columbia River, but they can’t do it when its bed is dramatically raised by all the volcanic debris spilling into it from the Cowlitz, which is just spitting out what the Toutle dumped in it. Mount St. Helens dropped roughly 2.5 cubic kilometers (.6 cubic miles) of volcanic ash, rock, and landslide debris into the Toutle River’s upper reaches. The Toutle River’s sediment load jumped to a hundred times what it had been pre-eruption, and remained that high for years. Some of it got deposited along the lower reaches of the Toutle, but huge amounts were dropped into the Cowlitz, which carried some along to the Columbia.

You can watch it happening before your own astounded eyes at the confluence. We stood on the bank where the two rivers meet. Upstream from the convergence, the Cowlitz is its relatively-clear, serene self.

Then you see where the Toutle River is flowing into it, coursing a little ways up the Cowlitz from its mouth. There’s no mistaking which currents belong to which river – the Toutle’s color is quite different.

Then you look downstream, at the mouth, and you see great big gravel bars dropped by the Toutle as it loses carrying power. There’s so much stuff that the Cowlitz can’t deal with it all. But plenty gets picked up and carried along.

Amazing, innit?

Sediment is going to be a serious issue for a long time to come. The Army Corps of Engineers expected the original SRS to keep working until 2035: instead, it had filled to capacity by 2012, allowing large amounts of sediment to overflow and sail on down the river. They hadn’t factored in the havoc storms can wreak on unconsolidated, poorly-anchored riverbanks, and all of the debris flows Mount St. Helens had in reserve. Volcanoes are fairly messy things at the best of times. In the decades after a major eruption, they are basically just sediment factories, shedding loose debris all over the place. We weren’t prepared for it, and we can barely cope. The Corps has raised the height of the spillway by 2.1 meters (7 feet), but a more long-term solution will have to be found.

Despite the issues all this debris causes for plant, animal, and human life, with all the flooding and choking of river channels, I still find it beautiful. This is a process that has been happening on Earth ever since it developed volcanoes and rivers. This intricate dance between two rivers and volcanic deposits is fascinating. I loved watching the patterns as the rivers flowed together. I even took a short video so you could watch them, too!

We learned a lot from Mount St. Helens when she erupted, and she still has much to teach us. It’s her gift, and her curse.