#aggradation

Mountain Moment: White River

The White River is a classic “braided” river, which are created by glaciers. The river channel often divides, weaving across a wide, rocky river bed. Glaciers generate a lot of rocky debris, more than the river can carry away. As each channel fills up with rocks, the river is forced to change course or divert into new channels, which sometimes can be nearby roads. You may notice along the bank of the White River evidence of dikes and engineered log jams. These structures attempt to direct the river away from the White River Road.

Landscape Language

Serpentine (adj) – to wind like a serpent, meander

Mount Rainier’s glacier-fed rivers are often described as “braided”, but the Nisqually River is looking very serpentine in this photo! This curved shape is created through a process called aggradation. More rocky debris is entering the river system than it can carry away, so as the channels fill up, the river is forced to change direction, meandering back and forth across its riverbed.

As glaciers retreat, they deposit massive amounts of rocky debris into river systems. This debris fills up river channels when the rivers can’t support the amount of sediment flowing in the water, a process known as aggradation. The river water is forced to change direction and create new channels. This is why Mount Rainier’s rivers have wide, rocky, braided riverbeds. Climate change is accelerating glacial recession and thus increasing the sediment available to rivers to transport downstream. This can lead to new river channels in adjacent floodplains that are in the path of buildings and roads. In addition to melting glaciers, climate change is increasing rain and flooding events. Warmer temperatures means that precipitation falls as rain instead of snow. This can result in rapid aggradation and debris flows.

Hotter weather in the summer can also cause melting glacier water to build up quickly, and if it gets trapped in the ice, it could release suddenly in the form of outburst flooding, another cause of debris flows. Glacial outburst floods are the primary cause of glacially-originated debris flows in the park. Mount Rainier scientists are studying glaciers and river systems to better understand these dynamic forces and have developed a debris flow hazard system to determine conditions when debris flows may occur.

Remember, ANY river in the park is at risk of a debris flow. Rising water level, shaking ground and a rumbling noise may signal a debris flow or lahar. If you are near a river and notice a rapid rise in water level, feel a prolonged shaking of the ground, and hear a roaring sound coming from up valley (often described as the sound made by a fast–moving freight train), then move quickly to higher ground! A location 160 feet (50 m) or more above river level should be safe. Learn more about geohazards.

Landscape Language

Braided (adj) – an interconnected network of channels

Rivers on Mount Rainier do not flow in single, well-defined channels. Instead, the river water splits and rejoins, moves from one channel to another, and leaves behind a network of braided channels. Called “braided” rivers, they behave in this way because of a process called aggradation. Basically the opposite of erosion, aggradation is when rocks and sediment carried by the water is deposited faster than it is washed away.  As the river channels fill up with rocky debris, the water is forced into new pathways. Where does all that rock come from? [Hint: you can see its summit in this photo].

Living, working, and playing on an active volcano means learning new words pretty often. One that I learned early on was aggradation; the rate at which a river deposits material. With five good size rivers (and numerous creeks) in the park, Mount Rainier National Park has a lot of rock filling up the glacier-fed riverbeds. In some rivers, the amount of rock coming into the riverbed could equal the amount moving out. Not so at Mount Rainier. In the park, the riverbeds have been piling up about 3 feet of rock per decade (data from 1997-2010). It doesn’t sound like a lot until that there are places like Longmire, Carbon River entrance, and the White River entrance that are lower than the river beside them. Longmire sits 29 feet below the Nisqually River. These lower elevations can be very enticing places for rivers to make new riverbeds.

As one of the year-round accessible park places the Carbon River entrance, in the park’s northwest corner, can be popular in its own quiet way. Its open for your hiking, biking, nature appreciation, and camping needs almost all the time. The big challenge here, and at other park locations, is that the riverbed is so full of rock that now the Carbon River likes to run outside the riverbed, into the forest and quite often onto the Carbon River Road trail. How do we keep access open to these areas when a big, strong river runs over the roads and trails? Definitely a question that park visitors and staff will have to wrestle with now and in the future. ~ams

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Why are river beds in the park so wide and rocky? Aggradation is the process of river beds filling up with rocks and sediment faster then it can be eroded. Mount Rainier's glaciers generate so much material as they carve through the mountain that the park's rivers quickly fill up. During times of high river flow, such as during recent storms, this process can be accelerated.

Learn more about aggradation and the effect this process has on the mountain's landscape - and its human inhabitants - at http://www.nps.gov/mora/learn/nature/aggradation.htm