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Constructed in 1887, the Great Works Dam spanned the Penobscot River from Old

2019-03-14

Town to Bradley. At a height of about 19 ft and a length of about 1,000 ft the structure posed a major hurdle to sea-run fish on the river. -In 2010 the Penobscot River Restoration Trust purchased the Great Works dam and powerhouse facilities as part of an agreement that allowed them to purchase two other dams on the river. This agreement was a major step in the process of improving access for sea-run fish to nearly 1000 miles of habitat in the Penobscot River watershed. -In June 2012, an opening ceremony on the banks of the river celebrated the first step in this multi-year restoration of Maine's largest river. The ceremony brought together tribal, state and federal partners, along with the Penobscot River Restoration Trust, to not only celebrate the breaching of the dam, but also the significant private and public collaboration they had achieved. -By November 2012, after removal of approximately 10,000 cubic yards of material by the contractor, the dam removal was largely complete. -The cooperation between the Trust and its partners went beyond the removal of the dam and included monitoring, testing and observations of the river before and after the dam was removed. -The USGS is involved in one piece of this monitoring, which documents the shape and geological character of the river channel at 18 cross-sections within the area of the Great Works and Veazie dams. Before dam removal, baseline data was collected at each cross-section, consisting of channel bathymetry, sediment characteristics and bank conditions. -After the dams are removed, the same survey will be conducted at each location. Six cross-sections are focused around the Great Works dam, both upstream and downstream of the old dam location. -The USGS collected bathymetric data by surveying ground points using a laser survey gun and rod, while data in the channel is collected using an acoustic profiler, something we will discuss later. Once the channel and ground points are output and meshed together, we are able to produce a plot of channel depth as a function of distance across the channel. -This plot shows the results of various surveys collected before the dam was removed at one cross section. Data post-dam removal has not been collected at this location. Final results of the study will be made public once complete. -This work is a collaboration between the University of Maine and the USGS, and funded by the Penobscot River Restoration Trust through the NOAA Restoration Center and the American Recovery and Reinvestment Act. -In September 2013 we set out to document the channel conditions now that the Great Works dam has been removed. One cross section, located on the downstream end of Treat-Webster Island, also known as French Island, presented a challenge when we returned to collect our data. -This image shows the cross section at that location before the dam removal. Shore and bank elevations were surveyed using a laser survey gun and rod, while bathymetric data was collected using a manned boat and ADCP. The ADCP, shown here on the bow of the manned boat, uses acoustics to measure the depth and speed of the channel. -When we arrived at this same cross section in 2013 with our manned boat, we were surprised to find a 2-4 ft hydraulic drop just downstream of the cross section. This drop was underwater prior to the dam removal as the dam impoundment extended over 1.5 miles upstream, previously covering this feature. -Our challenge was safely and accurately documenting the channel characteristics here given this newly exposed feature. -We decided to deploy a smaller manned boat and string a fixed Kevlar line across the river. We tethered our boat to the line as a safety and pulled ourselves, and the ADCP, across to collect our bathymetry data. -Once the bathymetric data was collected, we setup our survey equipment on the shore and surveyed the ground points. Given the shallow water and exposed ledge on the edges, we also had to survey points in the water to meet up with the ADCP data we collected by boat. -Once data collection was complete, we removed our line from the river, packed up our gear and moved onto the next cross section downstream.