25-MW River Mill Facility is Legacy Power for Portland

By John Esler

In 1905, Thomas Edison visited Portland, Ore., and the Lewis & Clark Exposition. A big spectacle of the fair – championed by Portland’s power company – was not a Space Needle or an Eiffel Tower.

It was a city lit up by electricity.

The fair helped fuel the growth of Portland and brought in more people wanting power. Between 1900 and 1910, the city doubled in population, reaching 207,000 residents.

To help provide for the growing city, Portland’s power company, today known as Portland General Electric, built a series of small thermal and hydroelectric generation facilities, using a letter of the alphabet to sequentially name each plant. In 1910, the company began construction on the dam for its 12th generation facility – Station M, which years later was renamed River Mill and currently is at 25 MW net capacity.

Completed in 1911 on the Clackamas River, located on the western slope of the Cascade Mountain Range, River Mill is the furthest downstream element of PGE’s 191-MW West Side hydroelectric project. West Side hydro consists of five hydro facilities, four on the Clackamas and one on Willamette River.

The newly completed River Mill Dam, circa 1911, is surrounded by felled timber at the dam’s construction site.
The newly completed River Mill Dam, circa 1911, is surrounded by felled timber at the dam’s construction site.

The Jan. 4, 1913, edition of the Journal of Electricity, Power and Gas suggested that right from the start the project was unusual and noteworthy “… for the rapidity of its construction, considering the exceedingly substantial manner in which it is built, and for the fact that it employs the first Ambursen type of dam for power plant work on the Pacific Coast.”

“Our region’s rich resources and the great facilities our predecessors built have left us with a strong legacy,” said Maria Pope, PGE’s senior vice president of power supply and operations and resource strategy. “We don’t think of River Mill as an old project. We consider it a heritage project. It just shows that if you put the right work into a project, the project will last.”

Today the dam is listed on the National Register of Historic Places for its rare design and the role its construction played in the development of the town of Estacada and it’s contribution to the growing electrification needs of the greater Portland area.

In 2015, River Mill was inducted into the Hydro Hall of Fame during HydroVision International in Portland. This marks the second PGE hydro facility to be included in the Hall of Fame – the 18-MW T.W. Sullivan Plant, which began commercial operation on the Willamette River in 1895, was inducted in 1995.

The River Mill site is considered so interesting and historically important, in 2010, the Estacada Area Arts Commission collaborated with PGE on a multi-year project to explore these dams and powerhouses through the arts. Artists visited the site and created works showing the River Mill area in photography, paint and wood.

Plant design and construction

Nils F. Ambursen, a Norwegian-American civil engineer and inventor who is known for his influential dam designs in the early 20th century, designed the dam at River Mill.

River Mill is unique because of its construction. It was the first Ambursen-type dam on the West Coast and today is one of the few original Ambursen-designed dams still operating in the country.

Also known as buttress dams, Ambursen dams use relatively thin slabs of concrete to hold back the water, supported by buttresses along the length of the dam. This design marked a turning point toward more efficient use of raw materials in dam construction. This design required less concrete, which was heavy and expensive to transport to dam sites. These types of dams were also quicker to build and used about 50% less concrete than an equivalent concrete gravity dam as the structure allowed an interior hollow under the spillway.

River Mill Dam was apparently one of only three documented Ambursen-type dams built west of the Mississippi, and evidently the only one to have survived.

River Mill Dam, pictured during its construction circa 1910, was an enormous investment at the time in terms of money and technology for the local community. It has since become a stalwart facility.
River Mill Dam, pictured during its construction circa 1910, was an enormous investment at the time in terms of money and technology for the local community. It has since become a stalwart facility.

Labor of the pick and shovel type was required. Workmen lived at the site in temporary housing; a sawmill on site provided lumber for construction purposes. Rail lines built to carry workers and materials to the site were also important because they brought tourism to the region, which ultimately led to the birth of the city of Estacada, 35 miles from Portland.

“River Mill Dam is one of the more important projects in PGE’s hydroelectric fleet from a control perspective because it allows us to regulate the flow on the Clackamas River,” said Nick Loos, plant manager for PGE’s hydroelectric projects west of the Cascade Mountains. “It took a commitment from PGE and combined efforts from many agencies to make this facility outlast others.”

“It’s not large in the sense of comparing it to some of our other projects, but 20 MW is 20 MW, according to Loos. It’s also the furthest downstream dam in the series of PGE’s Clackamas River dams, and provides an underappreciated value regulating the flows from the projects above.

Main dam

Located on the east bank of the Clackamas River, the main dam (which PGE calls the powerhouse dam) is about 173 ft in length, supporting the powerhouse and gatehouse. Overall height at the downstream face is about 86 ft.

The hollow interior of the structure is composed of a series of poured-in-place buttresses spaced on 14- to 18-ft intervals, with horizontal supporting members of concrete slabs forming the upstream and downstream faces. Interior finishes still show the rough concrete form marks remaining from the original construction and the interior is accessed via a narrow suspended walkway that runs the length of the dam.

Slab and buttress dams enjoyed popularity beginning in the early 1900s and continuing through the 1950s when skilled labor costs were low compared with material costs. Methods for the determination of design loadings for earthquakes were still at the early stages of their development. As a consequence, the structural capacity of buttress dams may be seriously compromised if assessed using today’s standards. In 2000 and 2001, as an element of the Federal Energy Regulatory Commission-mandated seismic remediation, the hollow interior bays of the powerhouse dam were filled with concrete to rectify limitations of the original design.


The River Mill facility currently contains five turbine-generator units: Three of these units were up and running by the end of 1911. Peak capacity in January 1924 was 13.8 MW. The fourth unit was installed in 1927 and the fifth in 1952. Those later additions helped raise the plant to its current peak capacity of 25 MW.

The powerhouse is a large cast concrete structure built above the dam. A three-story rectilinear structure sits perpendicularly across the river channel. The power floor, holding the power generation equipment, is lit by two banks of original steel fixed and pivot sash multi-pane windows.

Spillway dam

Located between the powerhouse dam and the west bank of the Clackamas River, the spillway dam is 406 ft long and about 73 ft high. A 54-ft-wide non-overflow section divides the spillway dam from the powerhouse dam.

The River Mill Dam spillway consists of 22 cast concrete buttresses, varying in width from 15 to 72 in and depending on their location, the buttresses are set parallel to the river channel with most intervals 18 ft center-to-center. Cast concrete slab sections form both faces of the spillway, with interior cast concrete corbels and lateral cast concrete struts providing additional support.

Other structures

Located on the east bank, the outdoor River Mill substation consists of transformers and early metal latticework supports along two 57-kV lines.

The forebay for the River Mill powerhouse, Estacada Lake/River Mill reservoir, is a 3-mile-long, 150-acre portion of the Clackamas River channel.


Modernizing control of the generating units in 1954 made the River Mill hydro plant semiautomatic.

A flood in December 1964 pointed out the need for the dam to pass greater magnitudes of water compared to previous events. Thus, in 1966, River Mill Dam’s spillway capacity was increased 50%, from 75,000 to 150,000 cubic feet per second by increasing the existing project freeboard through the construction of a flood wall on the right bank and a “levee” type dike atop the left bank. The spillway capacity now matches the spillway capacity at North Fork Dam, a few miles upstream.

Due to the excellence of the construction methods and materials originally used in the dam, dam safety officials determined the reinforced concrete and strength of the sections were adequate to withstand the additional pressure from a flood greater than the 1964 event. Because of higher flood elevations, abutments and wing walls were topped with 8 ft of concrete and PGE constructed a compacted-earth dike across the lowlands at the south end of the dam.

Helping fish

Because it is the first structure encountered by salmon migrating up the Clackamas River, River Mill’s fish ladders play an important role in upstream migration.

The first ladder was built from 1911 to 1913 and was serially modified to improve its function. When first built, the ladder was a square-sided concrete structure rising from the downstream elevation through a series of “switchback” turns to allow upstream migrants to bypass River Mill Dam. The ladder is box-shaped, with an open, wire-mesh-protected top.

There is still a piece of the original fish ladder from when the project was built – and that fish ladder was literally a ladder. It’s impressive that fish got up it, but they did.

This photo shows River Mill Dam as it is today, with calm waters and the upstream fish collector visible at left. The collector was completed and put into operation in 2012.
This photo shows River Mill Dam as it is today, with calm waters and the upstream fish collector visible at left. The collector was completed and put into operation in 2012.

Recognizing the need to improve the performance of this antiquated fish ladder, PGE proposed to replace the ladder before the existing FERC operating license expired in late 2006. The replacement fish ladder was constructed in 2005 and 2006. It is a modern design called a “Half Ice Harbor” ladder, which is an adaptation of the Ice Harbor design that consists of one weir, one orifice and a non-overflow wall between fishway pools. Migrating fish can use this facility easily.

A recent improvement on the upriver side of the powerhouse includes a state-of-the-art juvenile salmon surface collector as part of the new Federal Energy Regulatory Commission operating license requirements. It was constructed by J.R. Merit and completed in 2012. This system moves fish safely to the tailrace of the dam so they can continue migrating downstream.

“Replacing the fish ladder before we relicensed the project demonstrated PGE’s commitment to River Mill Dam,” Esler said. “Retrofitting existing projects is never easy. You are so constrained on where you can build. But putting in a new fish ladder with the best modern design was our way of saying this dam is important to us and worth the investment to keep it operating into the future.”

The lamprey is one fish reaping benefits from the retrofit. “The cultural significance of lamprey, especially to the tribes, is great,” said Esler. “Pacific lamprey passage needs are unique. They are not strong swimmers. You need to really manage flows in terms of velocity and turbulence. A lot of effort went into the design and construction of the ladder to ensure the concrete surfaces of the ladder are to their liking. This ladder appears to have some of the highest passage success in the Northwest for Pacific lamprey.”

Looking forward

Between 2005 and 2010, PGE obtained new operating licenses from FERC for all three of its hydropower facilities.

PGE is continuing to invest in its legacy facilities, with a US$35 million slated for seismic and related upgrades at the Clackamas and Willamette hydropower facilities and more than $25 million in new hydro IT and communications systems. The seismic updates to the River Mill Dam powerhouse will be completed in late 2015.

River Mill’s powerhouse has five turbines that use water from the Clackamas River to provide a total capacity of 25 MW of electricity to power homes and industry in the area, including the city of Estacada, Ore,
River Mill’s powerhouse has five turbines that use water from the Clackamas River to provide a total capacity of 25 MW of electricity to power homes and industry in the area, including the city of Estacada, Ore,

“Over the past decade, with our new FERC licenses in hand, PGE has invested well over $200 million dollars in fish passage and water quality improvements,” Pope said. “And we’re not alone in this. Other hydro operators and government agencies in the Northwest are collectively spending billions of dollars on programs and improvements to reduce the environmental impact of hydro facilities and ensure they have a viable future. We’re excited about the coming decades to meet our customers’ need for reliable, affordable, clean energy.”

John Esler is Portland General Electric’s environmental compliance and licensing project manager for the company’s hydro plants on the Clackamas River.

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