2004 Evaluation of Juvenile Fall Chinook Salmon Entrapment in the Hanford Reach of the Columbia River

The Hanford Reach stretches from Priest Rapids Dam 82 kilometers downstream to Richland, Washington. The topography, river dynamics, and climate of the area create a unique habitat for wildlife and fish populations. The Hanford Reach supports the larger of the only two remaining healthy naturally spawning fall chinook salmon (Oncorhynchus tshawytscha) populations in the Columbia River System (Huntington et al.1996). This population is a primary source of ocean and freshwater sport, commercial, and in-river tribal fisheries (Dauble and Watson 1997) and is a primary component of the Pacific Salmon Treaty between the United States and Canada. River flows for this section of the Columbia River are controlled by discharge from Priest Rapids Dam. Flow fluctuations from Priest Rapids Dam can occur rapidly due to changes in hydroelectric power generation, irrigation, water storage, and flood control. These fluctuations have been observed to cause stranding and entrapment of juvenile fall chinook salmon on gently sloped banks, gravel bars, and in pothole depressions in the Hanford Reach area of the Columbia River (Page 1976, Becker et al. 1981, DeVore 1988, Geist 1989, Wagner 1995, Ocker 1996, Wagner et al. 1999, Nugent et al. 2001a and 2001b).

Stranding of juvenile fall chinook salmon occurs when the fish are trapped on or beneath the unwatered substrate as the river level recedes. Entrapment occurs when the fish are separated from the main river channel in depressions as the river level recedes. Fish mortality in entrapments occurs from stranding, thermal stress, and piscivorous, avian, and mammalian predation.

The impact of river fluctuations due to operation of hydroelectric facilities on rearing salmonids has been assessed on numerous Columbia River tributaries and other river systems (Thompson 1970, Witty and Thompson 1974, Phinney 1974a and 1974b, Bauersfeld 1978, Tipping et al. 1978 and 1979, Becker et al. 1981, Woodin 1984, and Beck 1989) but limited research has been conducted on the Hanford Reach prior to 1997. In 1997, the Washington Department of Fish and Wildlife (WDFW) was contracted through the Bonneville Power Administration (BPA) and the Grant County Public Utility District (GCPUD) to perform an evaluation of juvenile fall chinook salmon stranding on the Hanford Reach. The multi- year study was developed to assess the impacts of water fluctuations from Priest Rapids Dam on rearing juvenile fall chinook salmon, other fishes, and benthic macroinvertebrates of the Hanford Reach and for directing the future management of flows from Priest Rapids Dam.

The Army Corps of Engineers was contracted in August 1998 to collect detailed bathymetry data on 35.1 km² of the Hanford Reach from Rkm 571.3 to Rkm 606.9 using Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS). This data was used in conjunction with the Modular Aquatic Simulation System 1D (MASS1) unsteady flow model to provide information on the Hanford Reach at a range of stage discharges. From this information, the extent of area of shoreline exposed by flow fluctuations and the configuration of the river channel could be determined. A sampling plan was designed by Pacific Northwest National Lab (PNNL) and WDFW prior to the 1999 field season to estimate the total number of juvenile fall chinook salmon killed or placed at risk due to flow fluctuations. The study area was confined to the portion of the Hanford Reach defined by the SHOALS bathymetry data at river elevations corresponding to Priest Rapids discharges from 40 kcfs to 400 kcfs.

The study area was stratified into 40 kcfs flow bands and divided into 3600 ft2 (344.4 m²) plots or sampling cells. The sample plot size was based on the Mean + size of entrapments found in 1998. A list of all cells contained within the study area was compiled and cells were randomly selected to use in daily field sampling activities. Daily sampling targeted random sampling locations within wetted flow bands identified in the previous 48-hour flow history. If entrapments were encountered, an assessment was made to determine the percentage of the entrapment contained within the sample plot. Entrapments with area of 50% or greater within the circle were sampled in their entirety. Entrapments with area of greater than 50% outside of the circle were not surveyed.

Evaluations were conducted within the 21- mile study area for the two following years, 2000 and 2001. In 2002 and 2003, the study area was reduced to an 8-mile section (15.7 km²) of the Hanford Reach from Rkm 584.5 to Rkm 600.2 (Locke Island to Hanford Townsite). Sampling in the reduced study area would continue to provide in-season monitoring of impacts to juvenile fall chinook and a mortality and at risk estimate could be generated using only one two-person crew. Mean mortality and â€�"at risk”¹ estimates generated though the random sampling method ranged from a low of 45,487 mortalities in 2000 to 2,013,638 mortalities in 2001 (Table 1).

Table 1. Estimated annual impacts (mortality and at risk) to juvenile fall chinook in the Hanford Reach, 1999-2003.

Comparisons of loss estimates of juvenile fall chinook by river elevation indicate that flow fluctuations at lower flow bands (<120 kcfs) present the highest risk for stranding and entrapment. Hourly flow fluctuations in low flow years such as 2001 have been shown to produce significant mortality impacts on emerging and rearing fall chinook. Large flats or flood terraces exposed at lower flows (40-120 kcfs) pose the greatest threat of stranding and entrapment to juvenile fall chinook on the Hanford Reach (Figure 2). The highest concentrations of stranding and entrapment of juvenile fall chinook salmon observed within the study area occur at island complex areas such as Locke Island/White Bluffs Slough Area (596-602 Rkm) and 100 F Islands (588-593 Rkm). These areas with their large and varied shorelines and diverse shallow water areas appear to provide excellent rearing habitat as well as high stranding potential.

Figure 2. The area of shoreline exposed within 10 kcfs flow bands for a portion of the Hanford Reach of the Columbia River from Rkm 571.3 to 606.9.

These findings have led to the development of a protection plan for emerging and rearing juvenile fall chinook that imposes tighter restrictions on daily flow fluctuations for Priest Rapids Dam at lower flows and allows increased operational flexibility at higher flows. The combination of very high spawning escapements of fall chinook in 2003 and expected low flows in the Columbia River during emergence and rearing in 2004 provided optimum conditions for evaluating entrapment due to fluctuations in discharge from Priest Rapids Dam.

In 2003, a second study was completed focusing directly on mortality resulting from the formation of entrapments along unwatered shorelines. The entrapment evaluation was able to assess impacts throughout the Hanford Reach. The final report for the 2003 Assessment of Losses of Juvenile Fall Chinook in the Hanford Reach has not been completed as of August 2004 but should be available through Columbia River Inter-Tribal Fish Commission by January, 2005.

1 Juvenile fall chinook found alive in entrapments were categorized as �"at risk” as these entrapments were subject to draining, lethal temperatures, or reflooding.