Home Community Sansei Journal SANSEI JOURNAL: Mounds of Mystery

SANSEI JOURNAL: Mounds of Mystery

Mima Mounds. Photo: David Yamaguchi

AS THE NIGHTS grow long and the days of 2019 dwindle, it is time for this aging boomer to tell a tall tale to mark the year’s end. The mathematics behind today’s story make it exactly 2.76 feet (0.84 meters) tall. But let me start from the beginning.

As readers know, of late I have been spending a fair amount of time guiding travelers around greater Puget Sound. While most of the clients are from Japan, I occasionally drive ordinary Americans. One such group in mid-September was comprised of retired Asian Americans, from Hawaii, who had come to Seattle for the UH-Huskies game. I was tasked with their post-game travel. Our first day together began at Snoqualmie Falls, continued to the Puyallup Fair and state capitol buildings, and ended at a native casino south of Olympia.

As we approached the vicinity of the casino early, I asked my riders if I could show them a point of interest nearby. It is place I have long had a fondness for, yet had not visited since 1981, when I was brought there as a student on a UW forestry field trip.

The place I write of is the Mima Mounds.

“What?” even Seattle natives ask when I mention the place. It is a setting where the natural prairie, between the towns of Littlerock and Mima, undulates with immense mounds. Described in print since the 1840s, the origin of the mounds has been debated ever since. Explanations have included rodent burrowing, native burials, wind and water erosion, and earthquakes.

On our September revisit, I was stunned to find that signage remains scant to the cult-like nature preserve. One has to nearly be upon it before encountering small signs to its parking area and trailhead.

I like the Mima Mounds, among various Northwest destinations, as they are among the last to retain an aura of mystery. They had defied the quest to understand landscapes through science that drove my classmates and me to the ends of the earth and back.

Yet in reading further on the Mima Mounds to pen this essay, I learned that the natural history exhibit signs on display onsite are out of date. For in 2014 the origin of the mounds yielded to the power of computer modeling.

A central question had been could gophers have built the mounds? Here, we are not talking about theoretical “giant” gophers of the past, which my classmates and I debated on our field trip. Rather, the mathematical challenge was to quantify the cumulative work done by modern pocket gophers tunneling across long time periods.

These solitary creatures burrow continuously to eat below-ground plant parts. Adults have a body length of 5-6 inches and weigh only 2.6 ounces. For comparison, an ordinary can of tuna fish weighs 5 ounces. The internet abounds with photos of the cute furry creatures held in gloved hands.

To model pocket gopher behavior, they first needed to be carefully observed. It is here where two key findings were reported in 1987. Tracer studies, using metal pellets that can be tracked with metal detectors, revealed that during the course of the gophers’ toil on extant mound fields, they push soil uphill. Here, separate sketches posted by pest-control companies suggest this happens during vertical tunneling from below. Additionally, the gophers direct soil toward the centers of existing mounds. As they are below ground, probably they are responding to gradients in soil moisture.

These first measurements demonstrated how pocket gophers can maintain preexisting mounds. But can they also build them from scratch?

This question was answered in 2012, when new incipient mound fields were described at a California ranch that had been abandoned for agriculture in the late 1980s. The mounds were being raised from the formerly plowed fields by pocket gophers!

In this way, scientists learned from incremental observations made across 25 years that gophers actually create “mountains” from mole hills. In doing so, they gradually raise their underground labyrinths and nests out of the dampness of deeper soils.

Applying computer modeling to measurements of the work of active gophers in turn has allowed scientists to visualize the slow work continuing to the dimensions of completed mounds, which average 13.8 cubic meters (45 cubic feet) in volume. These specifications are dictated by the distances over which gophers will move soil uphill against gravity. At a height of 0.84 meters, they cease doing so. It turns out that the entire mound-building process, from start to finish, takes 500 years.

An artists depiction of the pimpled prairie on display in its mound shaped exhibit building

To put this in human terms, the best example I can think of locally is the landscape-altering work done by Fujitaro Kubota and his two Nisei sons in building Kubota Garden in south Seattle. Across this 20-acre terrain, the handiwork of these three—in moving boulders and in planting non-native trees and shrubs—is evident throughout. The garden is maintained today by the city parks department.

Kubota Garden is the best physical example I know of for illustrating what can be accomplished in 1-2 human lifespans. If a person has a single dream, and focuses on it without wavering, a garden like this is what remains. It is remarkable, yet not limitless. Its breadth can be traversed by visitors in 20 minutes.

The Kubotas molded their land over six decades. By contrast, pocket gophers live for only five years. Thus, the Mima Mounds represent the work of 100 generations of gophers. Untold later generations have been needed to maintain the original mound fields. Their timing of first building remains unknown, but began sometime after glacial ice withdrew from the Olympia landscape, 17,000 years ago, leaving behind the rock cobbles that underlie the prairie.

In any case, consider stopping by the Mima Mounds (exit 95) the next time you are headed down I-5 past Olympia, and have an extra hour.

I close with a key sentence in the original technical paper:

“We conclude that, scaled by body mass, Mima mounds are the largest structures built by nonhuman mammals and may provide a rare example of an evolutionary coupling between landforms and the organisms that create them.” [Google Scholar: Emmanuel J. Gabet and others, 2014, Biotic origin for Mima mounds supported by numerical modeling, Geomorphology 206, 58-66; pdf available online].