dam-removal project in history has increased river water
cloudiness caused by suspended particles, known as
turbidity, a process that could affect aquatic life.
The dismantling of
two large dams on the Elwha River in Washington began in
September 2011 and has increased river turbidity
significantly, even though most of the sediment trapped
behind the dams has yet to erode into the river.
appear in the first published report
sediment response to the Department of Interiorís Elwha
River Restoration Project.
of dams can cause major disruptions to natural processes
on riverways, and we can expect their removal to also
have anomalous effects for some time until rivers regain
their equilibrium," said USGS Director Marcia McNutt.
"What we are learning from the response of the Elwha
River ecology to increased sedimentation during dam
removal will help guide other large dam removal and
river restoration projects in the future."
Geological Survey authors examined sediment and
water-flow data from the first six months of dam removal
and found that dam-removal activities and natural
processes both affected the riverís turbidity. Breaching
of several temporary earthen dams built to assist with
deconstruction caused sustained increases in downstream
turbidity. High river flows associated with rainfall
also increased turbidity, at measuring stations both
above and below the dams.
"During the first
six months of dam removal, most of the sediment released
was silt and clay, which caused substantial -- but not
unexpected -- turbidity in the river and coastal waters.
As dam removal progresses we expect more and more sand
and gravel to be released into the river, which will
likely help build river bars and slow coastal erosion
near the Elwha River mouth," said Jonathan Warrick, lead
author of the report and research geologist for the USGS.
"Although the river has been quite turbid since dam
removal began, most of the sediment-transport action is
yet to come."
levels can reduce the amount of light penetrating river
and coastal waters, which can inhibit aquatic plant
growth and affect wildlife that rely on sight to find
food and avoid predators.
Removal of the
dams is exposing more than 24 million cubic yards of
sediment stored in the reservoirs, enough to fill the
Seattle Seahawks' football stadium eight times. Using a
combination of measurements from a station downstream of
the dams, the authors estimated that less than 1 percent
of the 24 million cubic yards of sediment stored behind
the dams had eroded and moved downstream. Thus, the
authors conclude that completion of dam removal in
2012Ė2013 will expose much more sediment to erosion,
resulting in continued turbidity downstream as well as
changes in the shape and sedimentary makeup of the
riverbed and the coastal landforms around the mouth of
the river into the Strait of Juan de Fuca, which is only
5 miles below Elwha Dam.
changes will be important to assessing their effects on
habitat for fish and other wildlife in what historically
was one of the most productive salmon rivers in Puget
Sound," said Warrick. Scientists expect dam removal to
cause short-term adverse effects on aquatic life,
followed by large-scale ecosystem resurgence once the
riverís sediment load returns to a more normal and
findings are contained in a brief report in the Oct. 23
issue of Eos, Transactions, American Geophysical Union.
decommissioning has become an important means for
removing unsafe or obsolete dams and for restoring
rivers to a more natural state. Two dams are being
removed incrementally during this 2-year project: the
105-foot-high Elwha Dam impounding Lake Aldwell and the
210-foot-high Glines Canyon Dam impounding Lake Mills.
Only part of the
total sediment stored behind the dams -- 9 to 10 million
cubic yards -- is expected to erode into the river and
move downstream to coastal areas. The methods and
schedule of dam deconstruction are largely governed by
management of this sediment, with controlled drawdowns
of the reservoir levels to prevent deleterious impacts
of an abrupt release.