Mardon – the dam built with dirt

May, 2022

The Mardon Resort, where my friend and I were staying, is on the southern shore of the Potholes Reservoir, a human-made lake, created when an “earthfill dam” was built there, a dam built with dirt, basically, and probably built with rocks and such too, but it’s called an “earthfill dam”.

Now, I’m very familiar with concrete dams, huge concrete walled dams. There are lots of them in western USA. The Washington State Department of Ecology says there are 1,226 dams in Washington State alone, most of them earth dams, but still, wow, that’s a lot of dams. I don’t remember seeing a dirt dam before (or I didn’t know what I was looking at), especially one as big as this one that creates the Potholes Reservoir. So I looked forward to seeing what this really big dirt dam was all about.

Here’s a map of the State of Washington. The RED circle is the Potholes Reservoir.

Below is a closer view of the area around the Potholes Reservoir and the Potholes Wildlife Area. LOTS of water hereabouts since the Columbia River (fourth largest river in the USA) reportedly used to run right through here instead of just a few miles to the west. The RED arrow in the photo below points to the Mardon Resort that sits right on the edge of the huge Potholes Reservoir.

The two closest towns in the area are Moses Lake (to the north) and Othello (to the southeast). Because of the presence of those green watering circles, you can surmise that people here grow numerous crops for commercial sale.

So let’s go take a look at the earthfill dam that blocks and creates the Potholes Reservoir. It’s name is O’Sullivan Dam, completed in 1949 (one year younger than I am). It is one of the largest earthfill dams in the USA. It is 200 feet high and 19,000 feet long. That’s not a typo, the dam really is almost 3 miles long!

In the photo below, we were heading out in the big white truck to explore the dam and had just pulled off to the right in order to snap the photo as we were starting to drive out onto the earthfill dam. Notice the very dark sign, just ahead, on the right side of the road.

But also, open the photo below as large as you can to see the water level to the left (the reservoir), and the land to the right that is 200 feet lower than the reservoir water.

I hope they refresh and repaint that sign soon. It’s a bit sad.

There are probably 70 quadrillion tons of water in that reservoir (my estimate so it’s not based on any facts at all). This earth dam holds all of it up there in that reservoir. I was amazed by the roads down there to the right of the dam.

After we had driven about a mile along the top of the dam, there was another spot where we could pull over and get the photos below. There is LOTS of water hereabouts. The water in these two photos is below the reservoir, to the right of the reservoir from our perspective, so not part of the reservoir, but the water here is part of the run-off and part of the natural “potholes” underground water system in this area. (Ahhh, there’s a clue about why this area is called “potholes”.)

 

A gravel road off to the right tempted us, so we drove down to the lake in the photos above. We would be 200 feet (a 20-story building) lower than the water level of the 70 quadrillion tons of water in that reservoir. Down we went. In truth, it felt safe down there, although I did keep thinking about that 70 quadrillion tons of water above me.

 

After that brief excursion, we popped back up to the top of the dam and continued on along the road around the top of the earthfill dam. We drove over the small concrete dam and control house that allowed water runoff (photo below) from the reservoir and into a canal, but there was no place to pull over. The parking lots were gated. The photo below was taken off the internet.

Over the entire length of this earth dam, all three miles of it, there is just this one very small concrete dam that can let water through.

The photo below shows the waterway as the water comes down from the reservoir (from the left in the photo below) and then into the Potholes Canal (to the right).

200 feet or so further along the road, past that small concrete dam, we took the first dirt road off to the right that we could find. The photo below shows the sign on that dirt/gravel road.

The RED arrow in the photo below points to the road that’s on top of the earth dam. The YELLOW arrow points to the Potholes Canal. The Potholes Reservoir is at the bottom of the photo below. It flows down hill through the canal towards the south, away from the reservoir … out into the miles and miles of lakes (“potholes”) that you see in the photo below.

Right about where that YELLOW arrow is, we were able to find another dirt/gravel road that took us over to the Potholes Canal. We found this odd metal staircase on one side of the canal, with a pulley/cable system that connected to the other side of the canal.

But the wire/cable didn’t wind around the pulley such that the cable could be pulled back and forth … there was no extra cable at either end. What?

And … you might notice that on the top platform of that platform, neither of the two smaller barrier chains is connected that might prevent someone from falling off into the canal. The chains were just hanging off to the side. That upper platform was 10 feet above the gravel … with another 10 foot drop into the canal. Why wasn’t that platform secured?

Maybe no one enters this gravel road very often, but if we did, then surely others do.

We drove back to the paved road on top of the earth dam, then over to a parking lot on the other side of the road, still with no access even by foot to the small concrete dam.

But we did find this handsome person sitting on the stopsign.

 

A black-billed magpie.

And now it was time to head back to camp for lunch! More exploring soon.