The Marcy Dam Breach and Its Effects

 

While hiking in the Adirondack High Peaks on July 13, 2021, we came across what's left of the Marcy Dam that breached during Hurricane Irene in August 2011.  The top view looks downstream and the bottom view looks upstream.  There used to be a bridge that spanned across the top of the dam that served as a route for hikers on their way to Mt. Colden, Mt. Marcy and other High Peaks. It turned out that as my son, Tim, shared his photos on social media that he learned a good friend of our family's had crossed over the top of this dam with his children hours before it failed.  The New York State Department of Environmental Conservation (NYSDEC) has decided wisely, I believe, to not rebuild the dam. They instead have built a new stone crib bridge to replace the function that the dam provided.  We crossed over that bridge as we made our way to Mt. Colden.  The dam itself, constructed of timber cribs filled with boulders and faced wtih timber planks was not well suited to such a harsh environment and aggressive stream conditions.  A replacement dam, constructed to NYSDEC dam standards would cost $10M or more and would still be a safety hazard for people and infrastructure located downstream of the dam.  

As we approached the dam from upstream on our way back from Mt. Colden, I noticed that the banks of Marcy Brook were severly eroding and many trees had fallen into the brook.  The breach of Marcy Dam also triggered a channel avulsion that changed the brook's geomorphology in a dynamic manner that can be explained by the relationship developed by E. W. Lane (Lane, 1955).  In his emperical relationship, there's a balance between stream energy (represented by the product of stream discharge, Q and channel slope, S) and sediment yield (represented by the product of sediment discharge Qs and median sediment size, ds) as shown below:   

Q_s * d_{s ~} Q * S

Prior to the dam's breaching the channel slope, S, upstream of the dam had flattened and sediment had been deposited in the upstream zone.  When the dam breached, the slope began to re-steepen by increasing the sediment discharge, Qs from in front of the dam, but that erosion also extended upstream as I observed from the severely cut banks and downed trees.  A change to any one variable in Lane's relationship on one side of the equation requires a change to one of the variables on the other side of the equation.  In this case, channel slope, S increased while stream discharge, Q, remained unchanged, thus the sediment discharge, Qs must increase since median particle diameter would be unchanged by the dam breach.  Streams in a pristine environment, such as the Adirondacks, don't typically display this dynamic unless there's a significant and dynamic change.  Eventually the stream will return to a quasi-equilibrium and the banks upstream of the dam will eventually cease to erode.