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Dams provide the opportunity to study river depositional processes in systems with known initial and boundary conditions. The deltas deposited in the still water behind dams can be viewed as analogues for offshore sedimentary systems, which host petroleum reserves, at a scale intermediate between laboratory flume experiments and full-scale outcrop or offshore studies. This project studies the deposits associated with dams on rivers in Maine and New Hampshire. Through field-based investigations of floodplain and lacustrine sedimentology, the project will provide new insight into the dynamics and architecture of sediment accumulation after impoundment (an analogy for transgression) and redistribution after dam removal (regression), at a spatial and time scale not often studied.

During the second year of the project, we worked on two rivers: the Souhegan River in southern New Hampshire, and the Sheepscot River in Maine.

The lowermost barrier on the Souhegan River was the ~4-m high Merrimack Village Dam (MVD, ~500 m upstream of the confluence with the Merrimack River), which was breached and removed in August 2008. The MVD was built in 1906 at a location where various dams have existed since the 18^th century. Based on a pre-removal sediment-thickness survey, the MVD impounded at least 62,000 m³ of sediment, mostly sand. The removal of the dam presents a “natural” experiment for our project, because it exposed this reservoir delta (formed during the dam-induced transgression) to rapid incision (regression).

At beginning of the project in 2008, two months before the dam removal, we conducted a ground penetrating radar (GPR) survey of the impoundment. We are interpreting and comparing them to the stratigraphy exposed by the incising channel (see the 2009 “nugget” associated with this project). This research was the focus of undergraduate student David Santaniello’s senior honors thesis, which was completed in April 2010. This summer and fall, he has been re-working his thesis into an article called “Using ground penetrating radar to determine the quantity of sediment stored behind the Merrimack Village Dam, Souhegan River, New Hampshire,” for a GSA special volume called The Challenges of Dam Removals and River Restorations. He also presented his research at the Northeastern Section GSA meeting in Baltimore in March, and the SAGEEP conference in Colorado in April.

We also use historical maps and aerial photographs to estimate the possible extent of dam-influenced deposition at the site, and study 12 monumented cross sections, longitudinal profiles, repeat photography, and sediment samples to document the response of the Souhegan River to the removal of the MVD. This was the focus of graduate student Adam Pearson’s M.S. thesis, which was completed in April 2010. Pearson submitted a manuscript (“River response to dam removal: the Souhegan River and the Merrimack Village Dam, Merrimack, New Hampshire”) on this work to Water Resources Research in July. He also presented a poster on the work at the AGU Fall Meeting in San Francisco in December 2009, and will give an invited talk on the final findings at the same meeting this year.

During the summers of 2009 and 2010, graduate student Stephanie Strouse (with several undergraduate field assistants) studied two former two dam sites on the Sheepscot River in Maine extensively. This research will become her M.S. thesis, to be completed in early 2011. Both sites have well-exposed deposits associated with the dams, which were breached in the mid-20^th century. We analyzed the geomorphology of the study sites using aerial photographs, maps and lidar data. We investigated the stratigraphy using riverbank exposures and soil pits, and collected samples for grain size, bulk density and radiocarbon analysis. Preliminary findings (see the 2010 “nugget” associated with this project) suggest that bank stratigraphy was deposited in the millpond setting, underscoring the large influence dams had on river morphology in the study area. Strouse presented preliminary findings at the Northeastern Section GSA meeting in Baltimore and the Maine Water Conference in Augusta, both in March. She is also presenting her research at the AGU Fall Meeting in San Francisco in December 2010.

In summary, the project is going very well, with considerable student participation on two separate field sites. So far, the project has produced six presentations by my students at scientific conferences, with two more scheduled for December. It has resulted in two completed theses, and a third in progress. We have submitted one article for peer review, and have another in preparation. This project has added great breadth to my research program. I expect to wrap up the project and associated article submissions by the summer of 2011.