Project Description:

Supported by the U.S. Fish and Wildlife Service and the Bureau of Reclamation, Anadromous Fish Screen Program. The presented study addresses critical questions regarding unscreened diversions and assesses their threats to juvenile Chinook salmon, such as:

» How do hydraulic fields vary with diversion intake size, diversion rate, and river flow conditions?

» How do juvenile Chinook salmon respond behaviorally to varying hydraulic fields associated with unscreened diversions?

» How does the behavior of juvenile Chinook salmon vary under different hydraulic and environmental conditions and with different ages/sizes of fish?

» How is the entrainment rate of juvenile Chinook salmon at unscreened diversions affected by varying hydraulic and environmental conditions, and how is the entrainment rate affected by non-positive barrier fish deterring devices?

Fish screens have proven to be a reliable technology at preventing fish entrainment into agricultural water diversions. Fabrication and installation of costs such screens limit their use. Additional they are problematic when they become covered with river debris, requiring regular monitoring and cleaning. These factors can discourage private land owners from installing screens on the inlets of their agricultural water diversion pipes. Because juvenile Chinook salmon could pass through the openings in these devices, they are considered behavioral deterrents and not positive-barrier screens. The devices were constructed for ca. $3500 to $1500 each, and therefore offered relatively low-cost alternatives for reducing fish entrainment without the use of fish-screens.

Experiments testing fish-deterring devices were conducted at night and in turbid water during the day, to determine their effectiveness in environmental conditions with reduced visual cues. Juvenile Chinook salmon migrate out of the river systems at night. The Sacramento and San Joaquin Rivers and the Sacramento-San Joaquin Delta are turbid. Turbidity may limit fishes’ ability to visually detect water-diversion pipes until fish are within close proximity. Therefore, each device was tested in night experiments and in turbid-water experiments during the day.

Key Findings:

Findings of the final year, of the three-year study, are presented. Juvenile Chinook salmon swimming trials were performed with actual fish in the large flume (representing a model river) containing a diversion pipe that withdrew controlled amounts of water from the flume’s channel water flow. Previous years experiments had shown that fewer fish are diverted at lower intake velocities, therefore the devices tested in year three were designed to increase water-intake area, and decrease water-intake velocity.

Key hydraulic findings of the experiment include

» Increase in secondary velocities from the upstream to the location of the diversion pipe, and decrease from the location of the diversion pipe to downstream.

» Highest velocity magnitudes are observed in the vicinity of the diversion pipe, and the deterrent device.

» Magnitudes of the velocities at the downstream of the diversion pipe are smaller than those at the upstream due to the water diversion, the hydraulic energy loss due to the friction between the flume walls and the water, and energy loss due to the obstacle effect of the diversion pipe and the deterrent device.

Key biological findings of the experiment include

» Significantly different numbers of fish became entrained during the day, night, and turbid-water experiments among the different guidance-device treatments.

» During day control experiments, 64% of juvenile Chinook salmon were entrained into the open diversion pipe inlet. Significantly fewer fish became entrained through the widened trash-rack box (50% reduction in entrainment). Very few fish were entrained in the louver box and perforated cylinder experiments (97% reduction and 93% reduction respectively).

» 6 additional control experiments were run after completion of the day experiments to determine if fish growth during the 14-d period caused a reduction in entrainment susceptibility. No significant difference was found, providing that differences in fish-entrainment rates among treatments were due to the presence of behavioral guidance devices, and not due to fish growth.

» During night experiments, the widened trash-rack box reduced entrainment 48%, the widened louver-array reduced entrainment 62%, and the perforated cylinder reduced entrainment 75% compared to the control.

» Significantly fewer fish were entrained in the control experiments in turbid water, compared to both day and night conditions. Significantly fewer fish were entrained in each of the guidance device treatments compared to those of the control.


We strongly recommend the continued development and field testing of the louver-array box and the perforated cylinder fish-guidance devices. Both devices reduced fish entrainment risk during pipe passage by roughly 99%. Under control conditions, 50% of out-migrating salmon are estimated to be entrained after encountering 5 to 7 unscreened water diversions under the conditions tested in these experiments, but with the louver array box or the perforated cylinder attached to the pipe, it would take well over 350 encounters with water diversions before we expect to lose 50% of fish to entrainment mortality. These devices offer a promising, low-cost method of reducing fish entrainment without reducing water-intake rates, which may provide an alternative to positive barrier screens at small-sized water diversions.