Geology Lessons in our Watershed

It’s been a good year for strawberries. And now the raspberries are demanding attention. The ruby red flashes among the leaves remind us we need to get serious about picking them. And the blueberries are beginning to live up to their name, so it won’t be long before they’re ready for harvest. But this morning, before harvesting our backyard bounty, I have a different kind of harvesting to do, our semi-weekly trip into the woods to collect data on the health of our streams.

Today my sampling partner is Mark Lake, one of our newer members and a retired geophysicist. As we wait for the results of our turbidity test at Wilfred Creek I stare at the washed-out hill face that overlooks this particular sampling site. The different sized sand, gravel, and rock bits, which geologists call sedimentary units, form long layers known as beds. They give the cliff face a striped look but they’re tilted at a steep angle. I always assumed this was a place where the ground had tilted up under a large tectonic force but Mark explains that the layers are too new for that. Instead, the angled beds are simply a function of how the river deposited its suspended materials over thousands of years. Our conversation quickly morphs into a basic lesson on how aquifers work and how one goes about measuring them. An aquifer, for our purposes, can be thought of as a large container. The beds of gravel at Wilfred Creek are really good at holding water. They have high porosity and permeability. On the other hand, the exposed shale at Buckley Bay, where the road goes up to the new highway, acts as a barrier to the easy flow of water. It has low porosity and permeability. These kinds of features define an aquifer.

But determining the features of an aquifer is only part of the story. That is why we regularly harvest data from several local wells, monitoring their water level during the course of the year. This, ultimately, gives us a glimpse into how full the “container”, (our aquifer) might be.

As we make our way along the trail to Cowie Creek, the next stop on our sampling route, I’m again reminded of my unpicked berries back home: the trail is dotted with huckleberry bushes, their branches drooping under the weight of a bumper crop. And me without a bucket! The sight and taste of the huckleberries reminds me of my grandma’s pies, one of the joys of my childhood. But it also reminds me we need to be alert for bears. There are other creatures out here intent on harvesting too.