Learning Center Juvenile Fish Bypass

Fish bypass system

An innovative juvenile fish bypass system helps move young salmon and steelhead quickly and safely past the Rocky Reach Hydro Project.

The bypass includes two main parts. The first is a collector system  consisting of two channels, 22 feet wide, 60 feet deep, and 120 feet long in the forebay of the dam. The channel walls are made of fine, non-abrasive, stainless steel screen. Along the outside of the channels, 29 large submerged pumps work to create a strong current that appeals to the natural instincts of juvenile salmon and steelhead to attract them into the collector channels in the dam’s forebay.  

Once the fish are in the collector, water moves them into the second part of the system -- a steel tube , 9 feet in diameter for most of its length. The tube passes through the dam and extends a total of 4,600 feet around the back side of the powerhouse, across the face of the spillway and then about one-third mile down the east side of the Columbia River before the fish are returned to the river. The entire trip takes young fish about six to eight minutes.

Construction of the bypass system in 2002 was carefully planned so that all work in the river would be done between fish migrations. Some of the large components of the collector system, including the pump house, were built at a peninsula upriver from the dam during the summer of 2002, and then floated into place once salmon migrations were finished.

In addition to making the down-river trip easier for juvenile salmon and steelhead, the bypass system will also reduce the need to spill water over the dam. That water can then be used to generate electricity more than offsetting the $107 million cost of the bypass.

Bypass system backgroundPhoto of construction of the pipe continued on the east bank of the river.

Starting in 1985, the PUD developed laboratory models and tested prototype fish bypass systems for intercepting and moving juvenile fish around Rocky Reach as they travel downriver to the ocean. Screens were designed to steer young salmon and steelhead away from the turbines and into a bypass channel. None of the prototype screening systems tested achieved the results experienced at other Columbia River hydro projects. So a new approach was taken in 1995 -- a surface bypass and collection system that appeals to the young fish's natural instinct to migrate downriver near the surface, following the water flow. This differs from conventional turbine intake screens, which require fish to dive into the turbine intakes before they are intercepted by the screens. Also, after the prototype surface collector was added in 1995, the fish guidance effectiveness improved for the screen systems left in two units to enhance interim protection. Because of the improvement in the screens' performance, they have been incorporated into the final design of the fish bypass system.

Sonar studies determined that fish generally travel in the upper 60 feet of the river. The prototype was designed to use natural and turbine-induced surface currents in the upper 60 feet of the flow to give fish an alternative to diving into the turbine intakes -- entering the bypass system instead. Attractive features of this concept include the minimal volume of flow that's lost, minimizing power losses, as well as the relatively low installation cost.

Flows through the bypass pipe were occasionally diverted to an evaluation facility, where the juvenile fish were examined to identify species and condition. A 24-hour videotaping system counted the number of fish using the bypass system.

To measure the collector's effectiveness in moving fish, a few of the young salmon and steelhead were implanted with electronic tags and released upstream of the hydro project during the spring and summer testing period. The movements of other test fish outfitted with acoustic tags were monitored as they moved through the forebay to determine how they reacted to the surface collector. These studies allowed biologists to view 3-dimensional movement of fish in the forebay as they approached the fish bypass system and dam. This technique was used for this reason first at Rocky Reach.

Photo of original CAD diagram providing an overview of the Rocky Reach fish bypass system including pump station (left, in forebay) and fish return pipe (lower, right).The prototype surface collection system was modified each year, based upon test results from the previous year. Increased flows into and through the collector, plus improvements to the diversion screen/gatewell collection system in certain units, provided very encouraging study results in 1997. A second entrance was added to the surface collector in 1998, but results in attracting fish were not as good as anticipated. So in 1999, the second entrance to the surface collector was modified to allow biologists to vary the entrance width from a minimum of 22 feet to a maximum of 44 feet. This allowed biologists to analyze how water flows affected, and which water flows were better at attracting fish.

By 2000 and 2001, the District, in coordination with the fisheries agencies and native American Tribes determined that the configuration of the fish bypass system had been tested satisfactorily and that installation of a permanent system was warranted.

After final testing, the permanent bypass system was installed in 7 months between fall 2002 and the beginning of the 2003 migration of juvenile salmon in April towards the ocean. Each day from April 1 through August 31 during bypass operations,  the flow in the bypass pipe is temporarily diverted for short periods to an evaluation facility, where the juvenile fish are counted and examined to identify species and condition.

The District has conducted nine additional years of project passage and survival  studies following permanent construction, confirming its efficiency and that survival of young fish using the bypass system is nearly 100 percent.