But Why Mud Mountain?

Usually, when I go to Mount Rainier, there’s no time for diversions. Which is sad, because I’ve wanted to see Mud Mountain Dam ever since I found out about it a few years ago. Every time, I plan to swing by, and every time, it just never works out. Until now.


Image shows the upstream side of an earthen dam in a forested canyon. The dam actually looks like a small, triangular hill with the base at the top. A dirt road zig-zags up the middle. There is a concrete tower visible on the right.
Mud Mountain Dam

This trip to Mount Rainier hadn’t looked promising. After scorching heat all week, the weather took a turn for the Pacific Northwest. We’d intended to go down on Sunday, but the forecast changed to snow showers. Snow. I’m not doing snow on a volcano in a Honda Civic. So Merideth and I switched our plans, moved the trip up to Saturday, and decided we’d play it by ear. We didn’t think we’d see any volcano, but there are plenty of things to do around the base of Mount Rainier. And hey, since the National Park Service was celebrating its centennial, we were going to get in free anyway, so if the trip turned out to be a bust, no big deal.

I had Mud Mountain Dam on the wish list again, but since we got a leisurely start and stopped for a proper lunch, I’d changed my mind. I wanted to get Merideth to Longmire for some hawt hot springs action, and since we were coming in from the north, it was going to take quite some time. But as we passed the sign for Mud Mountain Dam, I decided to hell with it. We’d try for it. I couldn’t remember how far off the highway it was, but figured we’d turn around if it was too far.

It’s not. It’s only 2.1 miles. And it’s so much more neato than expected: there is a HUGE recreation area complete with playground, picnic areas, and a wading pool with a mushroom shower. And it’s all completely free!

Image shows an expanse of browning lawn with a tree to the left. In the distance, there's a children's play area with slides and swings, and to the center right is a wading pool with a tall green mushroom-shaped structure raining water down.
Play area and wading pool at Mud Mountain Dam.

This would be a great place to bring a picnic lunch and all the kids and fuel up before Rainier. But you could also easily spend your entire day here. There’s like ten miles of hiking trails in addition to all the play areas for the kids. It’s super great!

We made a beeline for the viewing area for the dam, because that was our entire point in coming. There’s a very nice handicap-accessible structure with a roof, informational signs, and views of both the White River and Mud Mountain Dam. Unfortunately, the canyon and dam were a bit gloomy under the cloud cover, and there were hornets on the trail down to the lower viewing area, so we didn’t get to look at as much as I’d like. I’ll be going back, getting some better views, and then we’ll really go into the geology and engineering of this place. Here, we’ll just hit the highlights.

So, the whole point of having a dam here is flood control. But there’s no reservoir. Here’s the upstream side, with the White River flowing through its valley.

Image shows a low valley with a braided river flowing through the bottom. There are mountains on the horizon. More is described in the text.
The White River.

It sort of looks like the reservoir is filled with water often enough for trees not to grow, but that’s actually not the case. Do you see the landslide scar on the left? I’m hoping to get a closer look at that next time, and we’ll discuss. Before all the trees grew all over it, it revealed a nice slice of local geology. This is where water backs up in flood situations, but it’s only there for a short time. The floodwater goes through the tunnel in the dam in a measured release, and then it’s back to the regular river channel. While it’s ponded, the temporary lake is called Mud Mountain Lake.

On the other side of the observation platform, you get a great view of Mud Mountain Dam.

Image is zoomed out to show the dam and its tower nestled in a canyon, which is barely visible because of the trees and bushes.
Mud Mountain Dam in its cozy canyon.

This humble little earth-fill embankment dam used to be the highest in the world. 432 feet actually sounds like a lot for something that’s just made of compacted sand, clay, gravel, and glacial till, covered by crushed rock and finished off with huge blocks of quarried stone. I mean, it’s basically just a massive pile of rubble. But it’s held up pretty well since its construction. It was started in 1939, but then World War Two happened, and it wasn’t completed until the late 1940s. It needed a concrete a cutoff wall (seepage barrier) to stop erosion in 1990. That’s another world record right there – the cutoff wall is 402.6 feet deep. Two records for a pretty unassuming structure! Of course, my favorite part of it is that road zig-zagging to the top, which makes it look much different from all the other dams I’ve known.

It was placed within a canyon the river had cut through andesite agglomerate. I’m going to have to do some research to determine the geology of that area – agglomerate nowadays refers to rock formed in an explosive eruption, but they might be using an older definition, considering it was drilled in the 1930s. No matter if they were formed in eruptions or something rather calmer like debris flows, they’re still super neato. Calyx drill cores of the stuff are displayed by the entrance to the viewpoint. They look like big blobs of concrete until you take a closer look.

Image shows a short, wide, dark-gray cylinder sitting on the ground beside a paved path. Its mossy and weathered, but if you look closely, you can see some very large clasts along with many smaller ones.
Andesite agglomerate Calyx drill core.

So, here’s the thing: those were drilled out so they could stuff a geologist with a lamp down the holes to map the geology of the rock unit, and look for any faults or other defects that might compromise the dam. Those things look sorta big, yeah, but they’re only 34 inches in diameter! That’s a tight fit when you’ve got work to do.

But just look at the textures. Yum.

Image shows tightly-packed clasts within a coarse matrix of volcanic fragments. Everything is a dark gray andesite. There are groove marks from the drill.
Close up shot of one of the Calyx cores.

Those, my friends, are very probably volcanic bombs. Delicious! I can’t wait to learn more, and hopefully next time, get my nose against some bedrock down in the river canyon.

So that’s pretty much Mud Mountain Dam. But why Mud Mountain? It’s such a silly little name. And I wish I had a photo of Mud Mountain for you, because it’s actually a fine-looking topographic feature and doesn’t look like a big sloppy mud mound at all. The informational signs at the viewpoint tell us that it’s named for the buildup of mudflows from Mount Rainier, and that it was formed by glaciers that re-worked the deposits, mounding and compacting them into a respectable mountain. I’ll have to look into that for us to see if there’s much more to the story – if it is formed by glacial activity, it’s probably a Cordilleran Ice Sheet story, and I’ll bet there’s more at its core than a bunch of lahars. That’s actually not bad material to work with, though – that stuff can be like concrete.

All that, and we haven’t even gotten into the National Park yet! This is such a great place to go if you don’t have the money to drop on fees. There’s plenty of stories you can tell about the geology and hydrology of Mount Rainier without ever setting foot on it. This is one place where those stories are easily accessible to everyone. And if it’s super hot, you can think them over whilst sitting in a very delightful little pool with a mushroom waterfall!

And don’t forget to stop and admire the fuschias, too.

Image shows red fuschias with pale pink outer petals.
But Why Mud Mountain?