Reports: GB8
47858-GB8 Dam-Controlled River Deltas as Meso-Scale Analogues for Offshore Sedimentary Systems
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 first year of the project, our focus has been on the Souhegan River in southern New Hampshire. 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 18th 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 used a 100 MHz instrument deployed in a canoe. The instrument belongs to our collaborator, Prof. Allen Gontz from UMass Boston. He and one of his graduate students accompanied us during the one-day survey, as did Prof. Ilya Buynevich from WHOI. We worked with Prof. Gontz to process the GPR images. We are now interpreting and comparing them to the stratigraphy exposed by the incising channel (see the “nugget” associated with this project). This research is the focus of undergraduate student David Santaniello’s senior honors thesis, which will be completed in April 2010. His goal is to determine the internal stratigraphy and thickness of the reservoir deposit.
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 is the focus of graduate student Adam Pearson’s M.S. thesis. Prior to dam removal, in August 2007 and June 2008, we surveyed the cross sections and longitudinal profile. We conducted re-surveys after removal in August and October 2008, and again in July and August 2009. All of my undergraduate and graduate students have participated in these field campaigns over the past years.
By comparing pre- and post-removal surveys Pearson has found that, in a 495-m reach upstream of the former location of the MVD, the Souhegan River eroded a net 38,100 m3 (48,000 metric tons) of sediment. This response began with rapid (hours to days) incision of a narrow channel, exhuming in some places bedrock and boulders that likely formed the pre-dam riverbed. Over the year since dam removal, the channel has widened by bank erosion but this process is limited by root strength and recruitment of large woody debris in the riparian zone of the former impoundment. Downstream of the former dam location, during the first days after removal, a sand deposit up to 1.0 to 3.5 m thick prograded almost to the confluence with the Merrimack River. We presented preliminary results from the research by Pearson and Santaniello at the American Geophysical Union meeting in San Francisco in December 2008. We are planning to present the sediment budget and GPR interpretations at the AGU meeting in December 2009. Pearson plans to finish his thesis and submit it to a journal by the summer of 2010.
We have also begun working in Maine. The proposal called for research on the Narraguagus River, but after conducting preliminary fieldwork there in August 2007, we made a decision to shift some attention the Sheepscot River, which has similar attributes, but is more easily accessible and has better exposures of impounded sediments. We collected some field data from the Narraguagus River site, and will compare this to our findings at the Sheepscot River sites. We have all of the same basic data for both rivers (e.g., historical and current maps and aerial photographs, an October 2007 lidar elevation survey).
During the summer of 2009, graduate student Stephanie Strouse studied two former two dam sites on the Sheepscot River extensively. This research will become her M.S. thesis, to be completed in late 2010 or early 2011. Both sites have well-exposed deposits associated with the dams, which were breached in the mid-20th century. Strouse worked with a field crew that included two new students. We analyzed the geomorphology of the study sites using aerial photographs, maps and lidar data. We analyzed the stratigraphy using riverbank exposures and soil pits, and collected samples for grain size and bulk density analysis. In at least one of the sites, we were able to identify a surface we interpret as the pre-dam floodplain, which is now buried under ~1.5 m of impoundment sediments. Strouse collected samples for radiocarbon dating to test our interpretations and calculate sedimentation rates. At present she is processing these samples, and will send them out for laboratory analysis (to be paid by Boston College internal funds) this fall. She plans to present her preliminary findings at the Northeastern Section of the Geological Society of America meeting in March 2010.
In summary, the project is off to an exciting start, with considerable student participation on two separate field sites. We anticipate that the project will eventually produce three theses and at least two peer-reviewed journal articles, in addition to at least five presentations at scientific conferences. This project adds breadth to my research program.
