November 22, 2022
Since last week’s bitter cold, it’s warmed up gradually. This afternoon at Missoula’s Greenough Park, it’s actually a few degrees above freezing. Where we’re standing near the DNRC streamflow gauge, Rattlesnake Creek is about two thirds covered with a milky layer of ice. We can see the water slipping and swirling under the ice. An occasional air bubble or cottonwood leaf drifts past in the current. Among the cobbles of the stream bottom, several shadowy trout are resting, maintaining their spots with an occasional flick of a tail.
It’s easy to notice the wintry stream, burbling and gushing icily past the cottonwoods and the red-osier dogwoods. But what’s practically impossible to see or even imagine is what’s happening underground. The water doesn’t end at the streambanks. We’re standing on it, too. Just a few feet below us is the aquifer. It’s a slow-moving river of water within the ground, moving much slower than the stream, water percolating invisibly through layers of sands and gravels, its temperature stabilized by the embrace of the earth.
I’m here today with the Watershed Education Network (WEN) to get a glimpse into this invisible, underground, watery world. Throughout Greenough Park, there are 15 groundwater monitoring wells that WEN has been checking on since 2015. Each well gives us a narrow glimpse into the aquifer. We already know the precise elevation of the ground surface at each monitoring location. And today we’ll be measuring the water level in each well. By combining these two pieces of information, we’ll get a three-dimensional map of the groundwater surface.
Groundwater and the stream connection
Water flows downhill – and that’s true for water within the earth, too. So by seeing where the groundwater surface is higher and where it’s lower, we can understand which way it’s flowing.
In Greenough Park, Rattlesnake Creek is like an interstate highway for water. The flow is so fast that we can hear its rush. In comparison, the aquifer under our feet is like a web of unmaintained Forest Service roads. It’s very slow going for the water. As the stream sweeps past, the groundwater shifts slowly.
But the water under our feet and the water in the stream are intimately connected. At times, the flow is from the stream to the aquifer. At other times, the aquifer flows into the stream, buffering the creek’s inhabitants from extreme temperature fluctuations.
Today, besides checking the groundwater wells, we’ll measure the surface elevation of Rattlesnake Creek in several places.
“That gives us a really good idea of how groundwater and surface water interact,” says Brook Bauer, WEN’s citizen science coordinator.
And in this section of Rattlesnake Creek, there’s a lot of interaction between the aquifer and the stream. During high water in the spring, the flow is generally from Rattlesnake Creek into the surrounding aquifer, rehydrating the sands and gravels under our feet. In the fall and winter, when the stream is lower, groundwater is usually flowing back out of the aquifer into the creek – helping support all of the trout, mayflies, and caddisflies that depend on the in-stream flows.
Checking the wells
This afternoon we split into two teams to check all 15 wells. Brook Bauer goes south with Kat Leister, WEN’s communications coordinator, and Rita Goerlich, a frequent WEN volunteer. Stephie Novak, WEN’s Stream Team coordinator, heads north with me and Kevin Mobley, a University of Montana student completing his degree in environmental studies and history.
Our first well proves easy to find, the metal plate exposed in an area where the snow has melted. We have more trouble finding some of the others on this wintry day. We know approximately where all of them are – but searching for a saucer-sized metal cover under a few inches of snow makes for a frosty sort of scavenger hunt.
Besides tools for opening the well covers, we’re carrying a well tape with us. It’s a $600 electrified tape measure on a reel. When the sensor at the tip makes contact with the groundwater in the well, it completes a circuit, triggering a piercing electronic squeal. Then we carefully note the measurement where the tape crosses the top of the well casing. This gives us our groundwater elevation.
“It’s a lot better than dropping a stick down a well,” Stephie says.
The warmth of the earth
The wells are scattered across Greenough Park, some near the stream and others farther away.
“Do you ever have a problem with the groundwater freezing?” Kevin asks.
Stephie tells us that, for most of the wells in the park, that shouldn’t ever be a problem. The temperature-moderating effect of the earth is incredible. The very top layers of the ground freeze during the winter, but that’s it.
“For it to get multiple meters down to the groundwater is pretty unlikely,” says Stephie.
Closer to the creek, where the ground elevation dips and the aquifer is much closer to the surface, it might be possible for some of the wells to freeze during a cold snap. But the temperature-buffering properties of the earth extend to the stream, too. As temperature-stabilized groundwater seeps into Rattlesnake Creek, it reduces the stream’s tendency to freeze in the winter and to overheat in the summer.
Floods and base flow
I ask Stephie what happens to the groundwater during rainy, fast-warming springs when the streams run bank-full and the rivers flood. But I’m surprised to learn that flood years have only minor impacts on groundwater.
“We had a massive flood year in 2018,” Stephie tells us.
The floods made a big difference in the stream itself.
“We got to see huge changes in channel shape.”
But, once again, groundwater moves slowly. A flood on the Rattlesnake Creek “interstate” passes in the blink of an eye compared to the pace of the groundwater. The torrents sweep past in a rush, without much time for the waters to seep in and recharge the aquifer.
We’re checking our last well now. This one is just a few feet from the edge of Rattlesnake Creek – and during spring floods, it can actually be submerged by the stream itself. It’s just another demonstration of how intimately groundwater and streams are linked.
The riffles of Rattlesnake Creek rush and foam soothingly behind us as we cap the well and pack up the well tape. According to the DNRC stream gauge, the flow of Rattlesnake Creek has more than doubled since last week, no doubt as our sunny afternoons have melted some of the snow in the watershed. But during the low flows of September, when the snowpack is long-gone and the rain showers are scarce, Rattlesnake Creek keeps flowing, providing homes for bull trout, whitefish, and a diversity of insects. Why?
Groundwater. It’s the base flow that keeps the stream going during the dry season. It percolates at its gentle pace from rocks, gravels, and sand lenses throughout the Rattlesnake Creek drainage.
The world of groundwater
Rattlesnake Creek, it turns out, is much larger than I ever would have imagined. It’s not just the visible expressway of the stream itself, but also the slow, hidden underground flow we walk on. Together, they form one complex, dynamic system. Sometimes, the groundwater feeds the stream; sometimes, the stream recharges the groundwater. And what goes on in the Rattlesnake Creek system is a microcosm of the relationship between groundwater and surface flows within and underneath Missoula itself. The surface streams – including Rattlesnake Creek, the Clark Fork River, Miller Creek, and the Bitterroot River – are all connected to vast, slow, complex underground flows, seeping through the earth underneath our roads and houses.
These underground flows form the aquifer that we drink from.
“We get all of our water from the aquifer,” Stephie tells us.
The world of groundwater is hard to imagine – but it surrounds us every day. And afternoons like this one, mapping out the groundwater surface and its relationship with a stream, can help us imagine and understand what our eyes can’t see.
So next time you walk by a stream, turn on the tap, or drink a beer brewed in Missoula, take a moment to imagine the aquifer beneath us. It may be unseen, unsung, and nearly impossible to picture. But we wouldn’t be here without it.