Study Area

Round Butte Dam is a 134-m high and 402-m wide, rockfill embankment dam. At the base of the dam is a powerhouse containing three Francis turbine units capable of generating a total of 324 megawatts (fig. 2). During normal operations, all water passing the dam goes through the powerhouse intake, approximately 200-m upstream from the dam. An emergency spillway with a single radial gate is to the west of the dam but is not used during normal dam operations. The 1,619-hectare (ha) Lake Billy Chinook has a storage capacity of 65,991 hectare-meters (ha-m), but a usable storage capacity of 33,797 ha-m. The forebay elevation typically fluctuates plus or minus (±) 1 m during normal weather and project operations.

The fish collection structure of the SWW (fig. 3) is approximately 27 m wide × 46 m long × 24 m tall in size, extends to a depth of approximately 14 m, and has water drawn from the surface of the reservoir. This water is directed to the power intake via a vertical flow conduit, with a maximum inflow of 6,992 cubic feet per second (ft³/s). Discharge herein is presented in cubic feet per second (ft³/s), according to the local reporting conventions of both the U.S. Geological Survey (USGS) and PGE. In total, the SWW structure is 83 m in height. Fish enter either the north or south collector flume (9.6 m wide × 12.2 m deep), move past dewatering screens, and then pass directly into a pipe that delivers them into the fish transfer facility alongside the SWW. At the fish transfer facility, fish are sorted and identified before being trucked and released into the Deschutes River downstream from the Project. The SWW collector is placed near the north shore of the reservoir and over the turbine intake. In 2022, from March 15 to June 15, the SWW followed a nighttime generation operation protocol, whereby the Project attempted to maintain a nightly minimum flow of 4,500 ft³/s to increase attraction flow during periods of active smolt migration. Additional information on SWW operations and performance may be found in Pyper and others (2017), and Portland General Electric Company and The Confederated Tribes of the Warm Springs Reservation of Oregon (2015, 2018), Smith and Hatton (2022), and Smith and others (2022).

Dam Operations and Environmental Conditions

Project discharge, forebay elevation, and water temperature data were summarized to document the environmental conditions that fish experienced from March 17 to June 15, 2022. Hourly data were collected and provided by PGE. Water temperature data were collected at a water quality monitoring station in the forebay at 2 m in depth and turbidity measurements were collected from the tailrace. Tributary inflow data were collected at the U.S. Geological Survey stream gages on the Deschutes and Crooked rivers near Culver, Oregon (U.S. Geological Survey, 2022a, b), and the Metolius River near Grandview, Oregon (U.S. Geological Survey, 2022c). Data were summarized using hourly observations, but mean daily values were used to increase plot clarity.

Imaging Sonar Deployment and Data Processing

Adaptive resolution imaging sonar systems were used to collect data of fish movements immediately upstream of the SWW entrances. The sonars were operated at 1.8 megahertz (MHz), with an operating range of 2–12 m. Sonars were attached to a pole-mounted platform between the collector entrances, deployed 3.5 m below the water surface at the center of the collector, and aimed perpendicular to each entrance (fig. 4).

The imaging sonars collected data continuously at the SWW from March 17 to June 15, 2022 to coincide with the peak outmigration timing of salmonid smolts. Data collection was interrupted only when the SWW was non-operational because of maintenance or when equipment malfunctioned. All data collected were stored on hard drives for archival and subsequent processing. Over the duration of the study, 91 dates with complete 24-hour sampling periods were included in the analysis.

The image recognition and tracking programs used for identifying fish targets at the entrances used four types of software to prepare and process the collected acoustic signals. Initially, ARISFish software (Sound Metrics, 2018) removed acoustic noise from the collected files. Then, files were parsed to time sequential images using MATLAB software (The Math Works, 2019). Next, computer vision software, OpenCV (2020) was used to locate fish target positions in the sequential images. Finally, target tracking algorithms developed in Python software (Python Core Team, 2020) were applied to the located targets to determine individual fish behavioral and morphometric data.

Imaging sonar technology cannot distinguish individual fish that have entered and exited the field of view multiple times; therefore, the following analysis assumes that each fish track, not the fish, were independent observations. Additionally, it is not possible to definitively identify the species of fish observed; however, if discrete group sizes were present and identifiable through other methods (in other words, collection), informed inferences of species can be made. Despite these limitations, imaging sonar can be used to collect data that would otherwise be unattainable.

Summary statistics of fish targets derived from signal processing (mean length, direction, speed, and location in beams) were imported into SAS software (SAS Institute, 2012), for subsequent proofing and to merge imaging sonar data with the environmental data. Data were proofed to eliminate non-valid records or records that did not provide measurable morphometric or behavioral data. Target datasets were then exported for additional statistical analysis using R software (R Core Team, 2014).

Dam Operations and Environmental Conditions

A combination of upper basin reservoir water storage for irrigation and lower than average precipitation in 2022 resulted in reduced instream flows during March and April compared to normal flows during a typical spring on the Deschutes, Crooked, and Metolius Rivers (fig. 5). However, by May, instream flows returned to above or near-median levels for the remainder of the observation period. The median hourly SWW discharge was 3,235.2 ft³/s (range 118.8–5,778.3 ft³/s; fig. 6; table 1). The spillway was not opened during the study and all water passed through the SWW, with a peak daily mean discharge of 3,972.5 ft³/s occurring on June 12, 2022. The reservoir elevation was maintained at a constant daily elevation of nearly 1,944.4 feet above National Geodetic Vertical Datum of 1929 for the entirety of the study, with a 1.5-foot fluctuation (table 1). Water temperature generally increased through the study period, peaking at 16.5 degrees Celsius on June 9, 2022, whereas the lowest water temperature of 8.8 degrees Celsius occurred on April 20, 2022 (table 1; fig. 6).

Fish Abundance

Data from the imaging sonars indicate that abundances of both smolt and bull trout-size fish near the entrances of the SWW varied while the migration season progressed (fig. 7). Abundance of fish observed on both imaging sonars generally were lowest during late-March, then trended upward to a peak abundance of 7,007 fish per day on May 18, before decreasing to a low of 210 fish per day on June 9. Fish observations were largely dominated by smolt-size fish; however, the daily abundance trends of adult bull trout-size fish were low (range 4–145 daily) and generally corresponded with that of the smolt-size fish.

In total, 128,912 fish observations were recorded, with approximately 12 percent more fish observed at the south entrance (n=72,407) than at the north entrance (n=56,505; table 2). The Chinook and sockeye smolt-size fish group (95–190 mm) was the predominant size class observed, constituting 64 percent of the total observations. The steelhead and kokanee fish-size groups represented 24 and 9 percent of observations, respectively, whereas the adult bull trout-size group constituted 3 percent of total observations.

Direction of Travel

The general direction of travel of the three smolt-size fish groups near the entrance to the SWW depended on fish group, entrance, and sometimes discharge (fig. 8). Near the north entrance, the general directions of travel (either toward or away) of the Chinook and sockeye smolt-size fish group were toward the entrance at all discharge levels through the SWW, with moderate effect values indicated by Cramér’s V at discharge levels greater than 1,000 ft³/s (table 3). The directions for kokanee-size fish were like the Chinook and sockeye smolt-size fish group, with directions toward the entrance at all discharge levels. The general direction of travel became near 50 percent, either toward or away from the entrance when discharge was less than 2,000 ft³/s, with low Cramér’s V scores indicating weak effects for travel direction (table 3). For steelhead smolt-size fish, the general directions of travel at all discharge rates were toward the entrance, with moderate effect values (Cramér’s V>0.29) at all discharge rates (table 3).

Near the south entrance, the general direction of travel of the Chinook and sockeye smolt-size group was toward the entrance when discharge through the SWW was greater than 1,000 ft³/s, with moderate effect values indicated by Cramér’s V (table 3). However, when discharge was between 3,000 and 4,000 ft³/s, the general direction of travel became near 50 percent, either toward or away from the entrance, but with a weak effect indicated by Cramér’s V. The directions for kokanee-size fish were similar to observations at the north entrance, with directions toward the entrance at all discharge levels, and the general direction of travel became near 50 percent, either toward or away from the entrance when discharge was less than 2,000 ft³/s, with low Cramér’s V scores (<0.10) indicating weak effects for travel direction (table 3). For steelhead smolt-size fish, the general directions of travel at all discharge rates were toward the entrance, with moderate effect values (Cramér’s V>0.28) at all discharge rates above 1,000 ft³/s (table 3).

Differences in observations of the three smolt-size fish groups traveling toward the entrance to the SWW before the lead net was installed (2021) and following installation (2022) depended on fish group and discharge (table 4). Travel toward the SWW was significantly increased under all discharge levels for the Chinook and sockeye smolt-size fish group in 2022 (p<0.001). The directions for kokanee-size fish were near 50 percent toward the entrance when discharge was less than 2,000 ft³/s during both sampling years. When discharge was greater than 2,000 ft³/s travel toward the SWW was significantly increased in 2022 (p<0.001). For steelhead smolt-size fish, travel toward the SWW in 2022 was significantly increased under all discharge levels except for when discharge was between 3,000 and 4,000 ft³/s (p=0.418); however, the direction of travel remained predominantly toward the SWW (66.4 percent).

Fish Swimming Velocity

The speed at which fish traveled at the entrances of the SWW generally differed between the fish-size classes, as well as between the two entrances (table 5; fig. 9). For example, at the north entrance, the mean swimming velocity of Chinook and sockeye smolt-size fish was 0.29 meters per second (m/s; interquartile range [IQR]=0.23), 0.35 m/s (IQR=0.26) for kokanee-size fish, 0.33 m/s (IQR=0.23) for steelhead-size fish, and 0.41 m/s (IQR=0.29) for bull trout-size fish. These differences in fish swimming speed were significant between fish sizes (ANOVA; F_{3, 56,501}=537.0, p<0.001) with mean swimming speeds varying irrespective of size groups. Results of the post hoc Tukey HSD test indicate that the swimming speeds of all fish-size groups had significantly different swimming speeds from each other (p<0.01).

At the south entrance, the mean swimming velocity of fish was slower than observed at the north entrance, with Chinook and sockeye smolt-size fish velocities at 0.23 m/s (IQR=0.20), 0.29 m/s (IQR=0.25) for kokanee-size fish, 0.28 m/s (IQR=0.19) for steelhead smolt-size fish, and 0.36 m/s (IQR=0.29) for bull trout-size fish. These differences in fish swimming speed also were significant between fish sizes (ANOVA; F_{3, 72,403}=802.7, p<0.001). Results of the post hoc Tukey HSD test indicate that the swimming speeds of the steelhead smolt-size group and the kokanee-size group were not significantly distinguishable from each other (p=0.053), whereas all other fish size groups had significantly different swimming speeds (p<0.001).

When observations of fish were compared between the north and south entrances, swimming velocities were statistically different for each of the fish-size groups (at the α=0.05 level) but were unlikely to be biologically significantly different for the smolt-size fish groups. For Chinook and sockeye smolt-size fish, mean swimming velocities were 0.06 m/s different between the two entrances but were significantly different (t; _82,128=51.56, p<0.001). Mean swimming velocities for kokanee-size fish were 0.06 m/s different between the two entrances and were significantly different (t; _11,665=15.41, p<0.001). Swimming velocities for steelhead-size fish were slightly faster at the north entrance (0.33 m/s) than at the south entrance (0.28 m/s) and were significantly different (t; _30,918=27.48, p<0.001). Likewise, bull trout-size fish were slightly faster at the north entrance with a speed of 0.41 m/s, and 0.36 m/s at the south entrance, and were significantly different (t; _4,193=8.18, p<0.001).

Timing of Detection

The counts of fish observed with the imaging sonars at the SWW entrances differed by discharge. For all fish size groups observed at the entrances, counts were lowest during the daytime periods when discharge rates were reduced (fig. 10). After approximately 1900 hours, counts of steelhead and Chinook and sockeye smolt-size fish groups began increasing approximately 2 hours after the initial increase in discharge, and generally increased through the night. Maximum observations of steelhead and Chinook and sockeye smolt-size fish groups occurred at 0300 hours, before rapidly decreasing to reduced observations during periods of daytime and low discharge rates. General trends in counts of kokanee-size fish were similar to that of the smolt-size fish groups, with the highest counts occurring at 0400 hours, immediately preceding the decrease in discharge. Bull trout-size fish were most abundant at 0300 hours, during the period when smolt-size fish were most abundant.

Fish Track Density

The spatial distributions of fish positions near the entrances of the SWW indicate some differences in distribution, depending on the fish size group. The fish location point density data include greater than 3.7 million location points from 128,912 individual tracks that were recorded by the imaging sonars in 2022. Point density data indicate that positions for Chinook and sockeye smolt-size fish observed on the north sonar were centered near the south half of the entrance, with lesser concentrations of positions on the outsides of the SWW structure (fig. 11). However, positions for Chinook and sockeye smolt-size fish observed on the south sonar were nearly spread across the entirety of the south entrance. These 2022 observations were similar to previous observations from the 2020 and 2021 studies for both entrances, except for reduced observations primarily near the area directly between the SWW entrances. Positions for the kokanee group differed at the north and south entrances, with positions from the north sonar nearly spread across the north entrance, while positions from the south sonar were primarily in the north half of the south entrance, with few detections on the far south end of the SWW (fig. 12). Compared to the previous studies, the observations from the north entrance were more spread across the entrance, while the south entrance was similar. Positions for the steelhead smolt-size group were similar at the north and south entrances, with positions primarily observed near the area between the SWW entrances and fewer detections on the far north and far south ends of the SWW (fig. 13). Again, these observations were similar to those from previous studies for the north entrance and reduced observations near the area directly between the SWW entrances for the south entrance. The bull trout-size group positions observed on the north sonar were primarily near the area between the SWW entrances, while positions observed on the south sonar were concentrated approximately a quarter of the distance from the center of the SWW (fig. 14). These observations were similar to the previous study years for the north entrance, while more spread out from the center for the south entrance.