Nature ænigmatic: September 2011

The Pacific Northwest is defined by its rivers. They are large enough to divide one state from another, and small enough to be followed to the very spot where they spring out of the ground. They irrigate agriculture sent all over the world, and juggle the abundant surface water within the temperate rainforest. While my connection to land predominates, rivers have consistently served as landmarks. One such river flowing through decades of my life is the Sandy River. It has flowed next to me through years of learning about the natural world and through subsequent years of teaching others. While the presence of the river has been constant, the river itself is in constant state of transformation. A recent walk along an old, favorite swimming hole demonstrated the point clearly, and showed the birth of a natural wetland system.

The Sandy River runs a sinuous 55 miles from its glacial origins on Mt. Hood to its delta on the Columbia River. This google map shows some general features of its path. Zooming into the river valley, you can see oxbows and old river channels carve paths like un-braided rope. It has acted as a trading route for first-nation peoples, held wagons of settlers as part of the Oregon Trail, and has seen the land around it morph from wildlands, to agriculture to suburban sprawl. With the headwaters of Reid Glacier, the Sandy River carries large amounts of sediment down a relatively short path. When Lewis and Clark expeditions wrote about the area in 1805 there had been a recent eruption and the mouth consisted entirely of a quicksand bar. The eponymous substrate still moves freely through the river system every year with winter and spring flood events. Occasionally there is a new gravel or sand bed laid down; sometimes entire swaths of land can be cut away.

When I was last familiar with my favorite swimming hole, the river looked something like this:

East of the river runs a well-traveled, two-lane, curvy road. Just before the distance between them is at their least, there was a small place to pull off, a small path to walk down, and the perfect place to jump in the river on a Friday afternoon to escape the long week, summer heat, and the crowds on the west bank of the river enjoying the amazing Oxbow Park.

In August, I made my way down to this swimming hole again. The path is still well worn, but the swimming hole was gone – not surprising with the intervening years. What I did find was an unexpected treat. The river has laid down a gravel bed – deep, maybe 7 feet thick. The rocks that make up the top layer are the size of bowling balls and must have taken some force to move so many into place. On the downstream side of the bar is a steep bank to a hollow that once must have been a channel itself. Now cut off from the main current, the only water source is groundwater and it has become a most convincing wetland. It seemed to be thriving, yet young. How old could this wetland be, and how does a wetland simply pop up on the edge of a river?

The same stretch of the river now looks something like this:

Sand bars are in constant motion along the Sandy River, but the intervening 14 years has seen an entire, well-forested bank entirely shorn away.

Thanks to the fantastic website Oregon Imagery Explorer, we can see this progression over time: the represented years below are 1995, 2000, 2005, and 2009.

In the years between the first two images, the Sandy River experienced two of its three record crests: February of 1996 and November of 1999. This accounts for the first large change with much of the finger of land being swept away. The next large change comes between 2005 and 2009 when two more historical crests were observed on the river.

Only two years since the latest aerial photograph, the area has changed yet again. A new deposit of sand has cut off the large, shallow channel that flowed east towards the smaller side channel. This deposit has cut off the area from constant flows, and the wetland has taken hold. The aerial photograph from 2009 makes it a little difficult to determine if it is simply water flowing along the area in question, or if there was any plant growth. The infrared from this date gives us the answer. Plant growth can be seen in red, and water in blue. The area of the new wetland is roughly outlined with black.

Perhaps an acre and a half in size, the wetland is well populated with common wetland plants: spikerush (Eleocharis palustris), tapertip rush (Juncus acuminatus), and daggerleaf rush (Juncus ensifolius). Underlain sand is covered by a thin layer of greasy organic matter, brown and shiny, it squishes quite satisfyingly under foot. The wetland seemed healthy, but young. Having grown many of the plants there within a nursery setting, they seemed to be as young as first year plants. They did seem to be short on nutrients, not surprising for a substrate made entirely of sand. But they covered the area with abundance. I would estimate the coverage to be at least 60%, and the species diversity was notable.

If the above photo was taken in 2009, and winter storms of 2010-2011 cut off the channel from main flow, that would leave spring 2011 for population by wetland seeds, first growth of plants and laying down of organic matter. It doesn't seem like nearly enough time. And what will become of our burgeoning wetland? Will the main river hold back its floodwaters long enough for the wetland to sink in its roots, lay down its organic matter? Or will the plants be simply washed away with the next winter's storm? There are many questions posed by a single 20-minute visit, and many that can only be answered with time.

Here are a couple photos of the site. Unfortunately with my Iphone, they are not the quality that I would hope in order to remember some of the site details.