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Ashton Powerhouse and Dam

The Ashton Hydroelectric Project is located on the Henry’s Fork of the Snake River (“Henry’s Fork”) in Idaho. PacifiCorp owns two hydroelectric developments on the Henry’s Fork: Ashton and St. Anthony. Both projects are licensed by FERC as Project No. 2381. This application for Low Impact Hydropower Certification pertains to the Ashton development. The St. Anthony development is not currently operational due to an outage of the generating unit in 2003. PacifiCorp is exploring decommission or sale of the St. Anthony’s development in consultation with FERC.

Project Name Ashton
LIHI Certificate Number 61
LIHI Effective and
Expiration Dates
December 31, 2014
December 31, 2019
Owner PacifiCorp Energy
State Idaho
Location Located on the Henry’s Fork of the Snake River in Idaho.
Installed Capacity 6.85 MW
Average Annual Generation 36.9 GWh (based on past 30 years)
Facility Type Run-of-river
FERC No. 2381

The Henry’s Fork watershed in eastern Idaho and western Wyoming encompasses 1.7 million acres and over 3,000 miles of rivers, streams and canals. The river originates from the outlet of Henry’s Lake, located in the Continental Divide Mountains. The Upper Henry’s Fork subbasin, located in eastern Idaho, encompasses 1,068 square miles, including 30 square miles in Wyoming and 60 square miles in Yellowstone National Park. The northern extent of the subbasin is bounded by the continental divide, which also delineates the boundary between Idaho and Montana. The subbasin is located within the Greater Yellowstone Ecosystem and possesses many of the unique geological, scenic, recreational, and wildlife attributes for which Yellowstone National Park is valued. The majority of the subbasin is managed by the U.S. Forest Service.

The Ashton dam, located at river mile 45 of the Henry’s Fork of the Snake River (Henry’s Fork), forms the southern boundary of the Upper Henry’s Fork subbasin. After exiting the subbasin, the Henry’s Fork continues in a southwesterly direction for 79 miles through the Lower Henry’s Fork subbasin before reaching its confluence with the South Fork of the Snake River. The Ashton dam and powerhouse are situated in a sparsely populated, semi-arid area in which the dominant land uses are irrigated agriculture and outdoor recreation, particularly trout angling and hunting. The Idaho Department of Fish and Game lists the Henry’s Fork as Value Class I, the highest class possible for fishery resources. The area’s topography is flat to gently rolling, and its climate is characterized by warm, dry summers and cold, snowy winters.

The Ashton development is comprised of a 56.6-foot-high, 226-foot-long, earth and rock-filled dam that has a downstream slope covered with roller compacted concrete and an upstream slope stabilized by additional rock fill. The crest elevation of the dam is 5156.6 msl. There are two- foot-high flashboards on the dam crest to prevent spillage from reservoir wave action and an 82- foot-long reinforced concrete spillway surmounted by six 10-foot-high radial gates. The reservoir has a surface area of 404 acres, with a gross storage capacity of 9,800 acre-feet and a usable storage capacity of 3,988 acre-feet at normal water surface elevation (5156.6 feet msl). The development features a reinforced-concrete powerhouse located at the right bank, with integral intakes controlled by vertical slide gates and containing two generating units, each with a nameplate rating at 2,000 kW, and one generating unit rated at 2,850 kW.

In consultation with FERC, PacifiCorp plans to rehabilitate Ashton Dam in 2010-2011 to mitigate seepage and piping (i.e., internal erosion) risks posed by a deteriorating upstream silt core within the dam. The rehabilitation will involve excavating and reconstructing a portion of the upstream embankment. Other features of the project include replacing the headrace retaining wall, replacing the concrete crest structure, and adding a concrete overlay to an unprotected portion of rockfill between the spillway and the powerhouse.

PacifiCorp operates the Ashton development in an instantaneous run-of-river mode for the protection of fish and wildlife resources in the Henry’s Fork. The average annual generation of the facility is 36,916 Mwh. The facility has a 46/2.3-kV step-up transformer and electricity is conveyed to the substation via a 133-foot-long, 46-kV transmission line. Run-of river operations will be maintained during the planned rehabilitation of Ashton Dam except during drawdown and refill periods. A low-level outlet tunnel will be installed to provide river diversion during construction. The outlet tunnel will be constructed through the right abutment bedrock and will include a vertical shaft housing slide gates for flow control.

Bald Eagles on Artificial Perch

Bald Eagles on Artificial Perch

Ashton Spillway looking upstream

Ashton Spillway looking upstream

Certification History

November 8, 2015: LIHI Executive Director Michael J. Sale, using authority delegated from the LIHI Governing Board, has certified that the Ashton Hydroelectric Project (FERC No. P-2381) continues to satisfy the LIHI Certification Criteria. A Preliminary Decision was announced on November 9, 2015, and a 30-day appeal period was open through December 8, 2015. No requests for appeal were received.

The effective certification date for the Ashton Project is December 31, 2014 for a five (5) year term, which will expire on December 31, 2019.

February 3, 2015: The Low Impact Hydropower Institute has received a complete and timely application for a new term of Low Impact Certification for the Ashton Hydroelectric project.  Please see the application files below.

October 22, 2010: The Ashton Hydroelectric Project has been certified as low impact for a five year term, effective December 31, 2009 and expiring December 31, 2014.

March 1, 2010: LIHI received a comment letter from the Henry Fork’s foundation. A copy of the letter can be read or downloaded in the “Files” section below (Ashton LIHI on ltrhd.doc).

December 31, 2009: PacifiCorp Energy has submitted an application for the certification of the Ashton Hydroelectric Project. The public comment period will remain open for 60 days.

Ashton Reservoir north shore

Ashton Reservoir north shore


Files: