By Ezra Cohen
A stark change has marked the transition from spring to summer in Oaks Bottom this year. The lake is drying up. Mud has crept outward from the edges of the basin, leaving dirtied, rivulets of water in its wake. Visitors to the refuge have noted this change, often worriedly, and theories abound: a “blockage,” drought, carp, beaver dams. Yet despite the less-than-picturesque appearance of the lake and wetland, all is well: the fluctuations in water-level are perfectly natural. Indeed, the mud and lack of water is actually an encouraging indicator for the long term health of the refuge.
The current situation is best understood by looking back at Oaks Bottom’s natural state – as a floodplain of the Willamette River. Long before the city of Portland existed, Oaks Bottom was symbiotically connected to the Willamette. Water flowed freely in and out. In spring, the ice melt raised the waters of the Willamette, which overflowed into the marshes along its banks, replenishing the water. In summer, these areas would dry out, leaving extensive nutrient-rich mudflats, which sustained thousands of migrant shorebirds moving south from their tundra breeding-grounds. In fall, rains would raise the Willamette, rejuvenating the parched floodplains. Migrating salmon utilized these areas like Oaks Bottom to rest on their upstream journeys. And throughout the winter, waterfowl found sanctuary in these shallow, marshy floodplains, rich with nutrients. Darian Santner, an ecologist with the Bureau of the Environmental Services, emphasizes the importance of these historic seasonal changes, explaining that, “Entire plant communities evolved to specialize in occupying areas that have this seasonal variation, and along with that, many fish and wildlife species have life histories that depend on these areas at some point in their lifecycle.”
These processes were stanched when Portland sprung up in the early 19th century; the river was diked into its channel, and floodplains mostly ceased to exist in their natural form. Angry neighbors, fed up with hordes of mosquitoes breeding in the wetland, eventually prompted the city to exert control over the water levels in Oaks Bottom – a decision that ultimately was a mixed bag for the health of the refuge. In fact, this effort wasn’t wholly driven by mosquitoes. Mike Houck, naturalist and Director at the Urban Greenspaces Institute, explains that this project was also focused on “manag[ing] vegetation, the immediate concern being willows were dominating the ‘pond.’” In 1988, with these issues in mind, Portland Parks and Recreation installed a small dam, called a weir, in the channel, connecting the wetland to the river. A series of boards were added or removed from the weir seasonally in order to keep the water relatively high. This prevented mosquitoes from propagating, but resulted in several unintended consequences: it stagnated the water, stemmed the flow of nutrients in and out of the area, and led to the proliferation of invasive species. The posterchild of this latter change was purple loosestrife, which infested the lake area during this time. Summers in Oaks Bottom were heralded by a wave of purple blooms that swept over lake and marsh. In each subsequent year, less open water remained; loosestrife was taking over. This “monocrop” of invasive vegetation drowned out most traces of native flora from the lake area. Wapato, formerly in abundance in the area, and one of the namesakes of the marsh (“Wapato Marsh”) became conspicuously absent.
In July of 2018, the U.S. Army Corps of Engineers began a five-month project to return Oaks Bottom to a semblance of its original form and function as a floodplain of the Willamette. Throughout the summer and fall workers extracted the weir, draining the marsh almost completely in the process. They cut down a number of cottonwood trees, installed a large, concrete culvert under the Springwater, and dug a wide, shallow, meandering channel to connect it to the wetland. The newly opened area was replanted with native trees and plants, and the felled trees were stacked into brush piles which would house various animal species. By the time winter of 2018 rolled around, the northern part of the Oaks Bottom marsh had been transformed. Water could now flow in and out seasonally and return Oaks Bottom to its critical role as a floodplain…or so it was thought.
As it turned out, a family of beavers scuppered those plans soon after the completion of the channel. The beavers constructed a dam exactly where the channel enters the marsh area, almost completely eliminating any back-and-forth flow. Hidden from view in an inaccessible part of the refuge, these beavers thrived for seven years. Freshly chewed willows were evidence of their presence, along with the steady water levels in the lake. Thus, this multimillion dollar project was rendered mostly inconsequential.
But in the past few months, circumstances have changed. Multiple reports from neighbors and biologists suggest the dam has fallen into disrepair. Santner says that, “It’s possible that some of the beavers living in the refuge died.” Now, for the first time in seven years, Oaks Bottom is fully and functionally reconnected to the Willamette. And with the proverbial “floodgates” now open, much of the water has drained out into the river.
Ultimately, this is a healthier state for the refuge. But Santner cautions that beavers are mobile, and it might not stay like this for long. “As families of beavers move around, or live and die, sometimes ponds become “derelict” until a new group of beavers moves into the area.” For now, though, Oaks Bottom has returned to a more natural existence. The water looks muddy, the carp thrash about in the shallows, and the ducks are farther from the keen eyes of many observers. But the water levels are exactly where they need to be. With a little luck, the waters will rise again come fall, and next summer they will fall. For the moment anyway, the natural cycles of this ecosystem have returned.
