2003 Field Summary Report: Assessment of Losses of Juvenile Fall Chinook in the Hanford Reach of the Columbia River in Relation to Flow Fluctuations (for Inclusion into Final Report)

The Hanford Reach supports the larger of the only two remaining healthy naturally spawning fall chinook salmon 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 manipulated by discharge from Priest Rapids Dam. Flow fluctuations from Priest Rapids Dam occur rapidly due to changes in hydroelectric power generation (power peaking), 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. Entrapped fish may become stranded when depressions drain completely. Fish mortality occurs from stranding, thermal stress (warming of water in entrapments), and by piscivorous, avian, and mammalian predation in small shallow entrapments.

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 (Oncorhynchus tshawytscha) stranding on the Hanford Reach. The multi-year study, has been 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 for the same area in 2000 and 2001. In 2002 and 2003, the study area was reduced to an 8 mile section of the Reach (RM to RM ). 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 2-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. Comparative impacts (mortality and at risk) to juvenile fall chinook in the Hanford Reach, 1999-2003.

Hourly flow fluctuations in low flow years have been shown to produce relatively significant mortality impacts on emerging and rearing fall chinook. Channel bathymetry at elevations corresponding to discharges of less than 110 kcfs results in large dewatered areas in response to even modest flow fluctuations. The combination of very high spawning escapements of fall chinook in 2002 and expected low flows in the Columbia River during emergence and rearing in 2003 provide optimum conditions for detecting the potential significance of stranding and entrapment due to fluctuations in discharge from Priest Rapids Dam. Large numbers of juvenile salmon emerging in 2003 will provide better identification of location and factors affecting susceptibility of fall chinook to stranding and entrapment. Thus, 2003 sampling might place a reasonable upper bound on impact estimates.

Improvements in sampling design can substantially improve accuracy and precision in estimates of stranded juvenile fall chinook in Hanford Reach rearing areas. Previous estimates based on a random design (among habitat types) produced highly uncertain estimates in part because of the expansion effects of high sample variance. Previous estimates may also have substantially underestimated actual stranding numbers by sampling only a segment of the entire reach, sampling only during daylight hours when some dewatered areas had already been reinundated², and inadequately representing large entrapment pools.

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

2 Reinundation may wash away fish left stranded on dewatered shorelines or entrapped in pools that drain prior to increases in water levels. Underestimation of mortality may result where reinundation occurs prior to sampling and entrapment surveys.