Condit Dam

        The history of earth is a story full of the massive, brief events. Most of the time, we only catch a glimpse of the mark they left behind such as a turbidite from an underwater avalanche, or a ignimbrite deposit from an immense volcanic eruption. The geologist is left to imagine the power and sheer awesomeness of these processes that only once in a great while manifest themselves. However, with the rise of the internet and dissemination of technology more and more moments of amazing natural phenomena are being captured and shared. In this case of this blog post, its’ the removal of the largest dam ever in the United States.

       The Condit Dam is located on the White Salmon River in Washington state. It was dismantled on October 26th, 2011. Fortunately, a man by the name of Andy Maser captured this process with time lapse photography.

       While the impressive torrent of water released downstream is cool to watch, the more interesting thing to me is what happens upstream of the dam. If you watch carefully, in the last 30 seconds or so there are three periods where the entire surface of the river bed appears to move. If you missed them, I made three gifs showcasing the movements. 

         What is triggering the mass wasting? What interrelated factors are changing such that the river bed is oscillating from a stationary state to a state of motion? I am unsure of the real answer to this as the makeup of the stream bank is unknown to me, and even if it was the mechanics are complex. However, if I were to make an educated guess I would say that a high angle of incision may have caused the mass wasting of the river bed.

figure 1. the dam before the removal

figure 2. Steep incisions, or drops in river beds are transmitted upstream through river erosion. Before the introduced hole into the dam, this process was stuck in limbo due to the structural integrity of the dam. Once the dam was bypassed, the steep incision was free to migrate upstream. This figure simplifies the reality of the situation by assuming the vertical drop created by the dam is accommodated by one large incision when in reality several probably occurred, each migrating upstream independently. This could explain the multiple mass wasting events.

figure 3. At some point the incision migrating upstream reached a point where the hanging wall of the incision was structurally weak, or an influx of water from upstream pulsed. Either way, a large block of sediment detached from the ground and slid downstream. It is interesting to note that when the mass wasting occurs the block of loose sediment acts essentially as a solid and the internal geometry is mostly preserved. That is, the whole thing moves, but it still looks the same afterwards.

        Again the process going on here is probably much more complicated than I am presenting it as. For example, the first gif/movement appears to fit my model and the third gif/movement appears to be a rotational variation of it. However, the second gif/movement may have something to do with a wave from upstream dislodging the sediment (notice how the sediment rises before it moves).  Watch the videos and gifs for yourself and see what you think!

Until next time -TB

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