The Mount Hood Railway and Power Company (MHR&P), also known as the Mount Hood Company, began the project in 1906, building the Little Sandy Dam to divert water through a wooden flume, about 3.2 miles (5.1 km) long, to Roslyn Lake.
 The dam reduced streamflows on the lower 1.7 miles (2.7 km) of the Little Sandy River and blocked all salmon and steelhead access to the upper 6.5 miles (10.5 km) of the river.
Roslyn Lake was at 656 feet (200 m) above sea level, about 400 feet (120 m) higher than the mouth of the Bull Run River, at 242 feet (74 m).
The 140-acre (57 ha) lake acted as a reservoir for the powerhouse, which was completed and put into operation in 1912, the same year that the MHR&P merged with Portland Railway, Light and Power Company (PRL&P), the predecessor of PGE.

In 1913 PRL&P built a dam on the Sandy River to supplement the Little Sandy Dam.
The Marmot Dam, 45 feet (14 m) high, diverted water from the Sandy to the Little Sandy by canal and tunnel, the longest of which ran 4,690 feet (1,430 m) under the ridge between the two rivers.
The new dam supplied up to 600 cubic feet per second (17 m³/s) to the Little Sandy above its diversion dam, and the Little Sandy provided up to 200 cubic feet per second (5.7 m³/s), all of which could be diverted through the flume to Roslyn Lake.

The Marmot Dam included a fish ladder to allow migration of salmon and steelhead; however it performed poorly at first and required frequent upgrades and maintenance, which continued into the 1990s.
To prevent fish from being swept into the diversion canal, PGE installed fish screens in 1951 and later added a bypass system to rescue fish trapped in the canal.

In 1989 the original timber crib Marmot Dam was replaced with a concrete structure.
The Mount Hood Railway and Power Company (MHR&P), also known as the Mount Hood Company, began the project in 1906, building the Little Sandy Dam to divert water through a wooden flume, about 3.2 miles (5.1 km) long, to Roslyn Lake.  The dam reduced streamflows on the lower 1.7 miles (2.7 km) of the Little Sandy River and blocked all salmon and steelhead access to the upper 6.5 miles (10.5 km) of the river. Roslyn Lake was at 656 feet (200 m) above sea level, about 400 feet (120 m) higher than the mouth of the Bull Run River, at 242 feet (74 m). The 140-acre (57 ha) lake acted as a reservoir for the powerhouse, which was completed and put into operation in 1912, the same year that the MHR&P merged with Portland Railway, Light and Power Company (PRL&P), the predecessor of PGE.
In 1913 PRL&P built a dam on the Sandy River to supplement the Little Sandy Dam. The Marmot Dam, 45 feet (14 m) high, diverted water from the Sandy to the Little Sandy by canal and tunnel, the longest of which ran 4,690 feet (1,430 m) under the ridge between the two rivers. The new dam supplied up to 600 cubic feet per second (17 m³/s) to the Little Sandy above its diversion dam, and the Little Sandy provided up to 200 cubic feet per second (5.7 m³/s), all of which could be diverted through the flume to Roslyn Lake.
The Marmot Dam included a fish ladder to allow migration of salmon and steelhead; however it performed poorly at first and required frequent upgrades and maintenance, which continued into the 1990s. To prevent fish from being swept into the diversion canal, PGE installed fish screens in 1951 and later added a bypass system to rescue fish trapped in the canal.
In 1989 the original timber crib Marmot Dam was replaced with a concrete structure.
Bull Run Hydroelectric Project