If you live in western Washington, you are probably aware that the biggest dam removal ever done in the US is happening right in our back yard. Nearly one hundred years ago, developers on the Olympic Peninsula wanted to harness the hydroelectric power of the Elwha River. They built two dams: the Elwha Dam, eight kilometers upstream from the ocean, and the Glines Canyon Dam an additional thirteen kilometers further up. Unfortunately, the builders ignored permitting requirements, and did not make any accommodation for fish passage. The dams failed a safety review in the 1970s and in 1992 Congress passed the Elwha River Fisheries and Restoration Act. This eventually led to the decision to remove the dams entirely rather than to attempt the difficult and expensive task of bringing them up to code.
Together, the two dams blocked a full 90% of the watershed’s fish habitat, and were holding back enough sediment to fill more than 10,000 Olympic-size swimming pools. Planning their removal was not a trivial task. It’s sort of a goldilocks scenario: take them out too quickly and there’s a danger of flooding. But take them out slowly, and it can be bad for fish because of prolonged turbid conditions. After much careful deliberation, a 2-year period of dam removal was deemed just right.
Emily Eidam is a 2nd year graduate student at the University of Washington, in the Marine Geology and Geophysics option within the School of Oceanography. She is looking specifically at what is going to happen to the mud that leaves the reservoirs during and shortly after dam removal. How will that sediment be transported and deposited in coastal waters? At this point, the lower dam has been completely removed, and all that’s left of the upper dam is a short chunk due to be removed with explosives this summer. But what a last portion it is: piled behind what remains of the Glines Canyon Dam is the mother lode of sediment. Once it goes, Emily and her colleagues expect a large flux of muddy sediment to flow seaward.
Emily, a geologist and civil engineer, says that this is what she’s waiting for: the big pulse of muddy sediment. When river water flows into the ocean, it is usually less dense than the salty seawater, forming a plume over the surface. But sometimes, as a result of a big storm or a landslide, the river water takes on an unusually high volume of sediment and briefly becomes denser than seawater, plunging down instead of floating at the surface. Geologists know that this happens, but don’t fully understand how it works because it’s incredibly difficult to capture these super dense, or hyperpycnal flows. They don’t happen often, and when they do they only last for a few hours to a few days.
In a way, the Elwha River dam removal provides the perfect opportunity to study hyperpycnal flows, because it’s creating ideal and predictable conditions for such flows to happen. To ensure that they capture an event, Emily and her lab group have rigged up special tripods that sit on the seafloor and measure, well, just about everything a marine geologist could possibly want. Video cameras record suspended sediment particles. A variety of oceanographic sensors measure everything from temperature and salinity to current speed throughout the water column. The “murkiness” of the water is estimated using instruments that send out a burst of light, and measure how much is scattered back by suspended particulate matter. Sediment collected in traps provides data on size distributions of settling material.
In addition to the data collected on the tripods, Emily and her lab group conduct ship-based surveys several times per year, taking additional oceanographic measurements, and also collecting sediment samples from the seabed.
Because of the difficulty in predicting when and where they’re going to happen, real-time measurements of hyperpycnal flows are sparse (at best) – so capturing one will be amazing! But in the meantime, they are collecting an enormous dataset that can be used to understand where sediment leaving the river gets deposited, and how it gets there. Most of us don’t spend much time thinking about how sediment from a river becomes part of the seabed, but understanding how it works can help scientists interpret geological records that contain clues about ancient coastlines and rivers, changes in sea level, and even past climate.
Learn more about Elwha River history and dam removal: