Measuring 20th century fluvial response to 18-19th century anthropogenic activity using two generations of damming in the South River, western Massachusetts
2018
Centuries-long intensive land use change in the
northeastern U.S. provides the opportunity to study the response
timescale of geomorphic processes to anthropogenic perturbations.
In this region, deforestation and the construction of dams
following European settlement drastically altered the landscape,
leading to the impoundment of sediment in mill ponds. This legacy
sediment continues to be released into transport decades after a
dam has been removed or breached. Geochemical tracers can help
distinguish sediment sources and understand how sediment moves
through a watershed. The South River in western MA is located in a
formerly glaciated watershed, and these surficial deposits compose
98% of the area. It experienced two generations of damming,
beginning with smaller mill dams in the 18th-19th centuries,
followed by the construction of the Conway Electric Dam (CED), a 17
m tall hydroelectric dam in the early 20th century. Legacy sediment
deposits from sediment stored behind mill dams cover 1.5% of the
watershed area. The CED is located near the outlet of the river,
providing a century-long depositional record for the watershed,
during reforestation. I hypothesize that sediment mobilized from
human activity will contain a different geochemical signature than
glacial material, that recent erosion in the watershed is primarily
from anthropogenic legacy deposits rather than from glacial age
landforms, and channel widening is occurring in reaches of the
channel composed of legacy sediment, rather than in glacially
confined reaches. These hypotheses were tested through a two part
investigation, consisting of a sediment tracing study using Hg, and
a Geographic Information Systems (GIS) analysis of channel changes
using aerial photographs from 1940 and 2014. Samples were collected
from river bank exposures of 11 glacial deposits and four mill pond
legacy sites. Two vibracores measuring 476 and 500 cm were
collected in reservoir sediment stored behind the CED in 2013 and
2017, respectively. Hg concentrations range from 1-4 ppb in glacial
sediment, 3-380 ppb in legacy sediment, and 2-18 ppb and 7-50 ppb
in the two CED cores. I used Hg as a tracer to estimate percent
contributions to the CED reservoir from each watershed source
during the 20th century. Results from a sediment mixing model
suggest glacial sources contributed 32 ± 15%, and legacy sediment
deposits contributed 68 ± 15% during the 20th century. Based on
137Cs dates on the cores, high amounts of legacy sediment filled in
behind the CED prior to 1953 (74 ± 35 %), and background erosion
from glacial deposits dominated from 1953 until the reservoir was
filled in the 1980s (63 ± 14%). GIS analyses using aerial
photographs from 1940 and 2014 indicate that the channel did not
significantly widen along any section of the river, however,
increases in sinuosity (up to 12%) occurred in the legacy sediment
dominated reaches of the channel, and minor increases (1-2%)
occurred in the glacial reaches. Overall, these analyses show an
increase in the amount of sediment released in…
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