Campus

Environmental Impact of the Removal of the Dam


November 27, 2016

The end has come for the Great Dam of Exeter, a historical landmark constructed by Englishmen in 1638 to power a grist mill, enabling the mass production of a large variety of products which brought wealth to Exeter. While it has not only proven commercially useful but also historically important, the dam has been adversely affecting Exeter’s environment since it was erected.

         For example, Dr. Samuel Tenny, who noticed in 1795 that fish were already being blocked by the dam, wrote in an account, ‘there was formerly at the falls in this town, an alewife fishery, which afforded an abundant supply of that kind of fish, for the inhabitants of the town and vicinity. But for want of sluices in the dams, by which they might ascend the fresh riven, and gain proper places for spawning, they have, for many years, almost disappeared.” In short, the dam has been obstructing fish spawning patterns since the 18th century and changing many ecosystem dynamics.

The dismantling of the great dam was ordered not because of its environmental damage, but rather its weakening stability and potential to be a safety hazard. In 2000, The New Hampshire Department of Environmental Services’ inspection lead to the conclusion that the dam would not be able to withstand another 50 years of storms.

The deconstruction of the dam would result in a great number of benefits for the environment and act as a restoration process. A main issue that has persisted since the creation of the dam is the obstruction of many fish, like atlantic salmon, rainbow smelt, and alewives, from their spawning grounds. In 1795, Dr. Samuel Tenney noted in his written account that, “there was formerly at the falls in this town, an alewife fishery, which afforded an abundant supply of that kind of fish, for the inhabitants of the town and vicinity. But for want of sluices in the dams, by which they might ascend the fresh river, and gain proper places for spawning, they have, for many years, almost disappeared.” The addition of the fish ladder in the 1950s has had negligible to no effect on this issue. Hopefully with the dam gone, these fish species will return to the rivers to breed again.

The river will also undergo an increase in dissolved oxygen levels and sediment transport. The construction of a dam lowers the ability for a river to reaerate, preventing many microorganisms and plants from thriving. This exacerbates dissolved oxygen decrease, as photosynthesis becomes limited in plants and microorganisms. Once the river is further oxygenated, water quality will increase substantially. With increased sediment transportation, the depth of the river should also return to normal as well.

With the dam gone, flood risk should also largely decrease. The past floods have not only affected the environment, but Exeter’s community. Steve Kaneb and his wife have experienced flooding in their restaurant, the Loaf and Ladle, and look forward to the deconstruction of the dam.

         With these benefits, there are also consequences that may negatively affect the environment. Because the removal of the dam will decrease flooding, some floodplain ecosystems may be put at risk, including a white oak swamp, a rare ecosystem. While it is still uncertain how much these floodplains will be affected, it is clear that they will shrink as plants that inhabit the drier parts of the river will overtake parts of the floodplain. The academy will also be affected, as the academy currently withdraws too much water from the river, for heating and irrigation, for the river to support once the dam is removed. It is still unclear how largely the river will be affected as a water source, but the academy will most likely have to find another source of water.

The environmental restoration the removal of the dam will bring clearly outweighs its costs, and as Mr. Kaneb said, “It’s basically putting nature back to the way it was built before humans altered it.”

Sources:

http://exeternh.gov/sites/default/files/fileattachments/2013-10-31_feasibility_report_es_final.pdf

http://www.clf.org/blog/exeter-removes-great-dam/

http://www.unionleader.com/Funding-flows-in-for-1.8m-removal-of-downtown-Exeters-Great-Dam

http://www.seacoastonline.com/article/20140214/NEWS/402140341