To examine the relative impact of decoupled drought conditions of reduced flow and increased water temperature, three study reaches were established: (1) an upstream reference (REF) reach that was unaltered, (2) a middle (Low-flow) reach in which flow was decreased (diverted) to mimic low-flow drought conditions, and (3) a downstream warmed (+Temp) reach in which diverted streamflow was passively warmed in a coil system and reintroduced downstream to elevate stream temperatures and mimic drought temperature conditions (see design diagram).
To create these conditions in a remote landscape, a passive (gravity-fed) flow diversion system (see 'B' in design diagram) was developed. The Low-flow reach was created by placing a temporary plywood barrier across the stream in which there were two 4-inch holes. An irrigation line was placed through one hole (“diversion line”) and one hole was left empty to allow fish to pass through the barrier (“pass-through line”) and so that approximately half of the flow was maintained in the channel (see 'B' in design diagram). The goal was not to fully dewater the channel, but instead to reduce discharge proportionally to flow in the stream. The pass-through line was positioned low enough within the flow diversion barrier to ensure potential fish passage. The diversion line was approximately 100 m in length and redirected approximately 50% of the streamflow (see 'C' in design diagram). The diverted water in this diversion line was reintroduced to the channel 100 m downstream from the plywood barrier and 30 m downstream of the lower end of the focal Low-flow study reach. The outflow from this flow diversion line re-entered the stream 8 m upstream of the start of the +Temp reach (see 'D' in design diagram).
The +Temp reach was created by heating water passively in coiled tubing (see 'A' in design diagram). In addition to the main 4-inch diversion line (which also heated the water slightly relative to the stream), eight ½ inch diversion lines siphoned water from an upstream pool (above the Low-flow reach but below the REF reach) and heated the water for the +Temp reach. The flow in these lines also contributed to flow diversion but was a small portion of volume relative to the 4-inch diversion line. The ½ inch warming lines were arranged in coils that were exposed to direct sunlight during the day by placing them along the side of the USFS 320 Rd that runs parallel to the study reaches in MCTW (see 'A' in design diagram). The warmed water in the coiled ½-inch lines was reintroduced to the stream approximately 8 m above of the upstream end of the +Temp reach in an area where there was a high degree of mixing so the elevated temperature water was incorporated into the flow.
Vertebrates and stream characteristics were measured during the pre and post experimental. The difference in the time period for each treatment reach was then compared to the reference reach to understand the relative impact of each treatment.
Pre-treatment "Before" electrofishing surveys was conducted in all reaches (REF, Low-flow, and +Temp) between July 18 and July 22. In each reach, block nets were set at the upstream and downstream ends to close the system, conducted three passes through each reach, and collected all trout (the sole fish specifies present) and all salamanders that we found in each pass. Trout and smaller salamanders were held in aerated coolers next to each stream. Large salamanders were held separately to avoid predation during holding. All trout were anesthetized with AQUI-S, weighed (to 0.1 g) and measured (total length and fork length to nearest mm). To evaluate fish summer growth rates, every captured trout, larger than 80 mm, received an 9 mm Biomark PIT Tag . Visible Implant Elastomer (VIE) tags were additionally applied to all salmonids, with each reach receiving a different batch color; red for the T reach, yellow for the Q reach, and orange for the R reach. Salamanders were placed in a plastic bag for measurement of both vent and total length (to nearest mm) and weighed (to 0.1g).
Study reaches were resurveyed in early September, after approximately 7 weeks (total of 51 days) of the Low-flow and +Temp treatments. The “post-treatment” surveys were conducted during the final days of the treatment. The +Temp reach was surveyed on 2022-Sep-08, and the REF and Low-flow reaches were both sampled on 2022-Sep-09. Only two passes were conducted on these sites because there was a “red flag” extreme fire danger warning for the western Cascades of Oregon on 2022-Sep-09, which forced field crews to complete work over a shorter period than initially planned (over 1.5 days rather than over 3 days). Given the high depletion rates that were achieved in these small headwaters in the July sampling, two passes adequately estimated abundances in these September sampling events. All fish and salamanders were fully processed (weighed and measured) the same as in July surveys. Elastomer tag recaptures were noted, and tag numbers of recaptured PIT tagged trout were recorded, but no new tags (elastomer or PIT) were applied in the September sampling. The flow and temperature treatments ended on 2022-Sep-09.
Stream cross-sections were surveyed every 5 meters to document stream dimensions before and after the experimental design. Measurements at each cross-section included wetted width, bankfull width, and depths at five evenly spaced points.
Pools were identified and measured in each reach before and after the experimental design, noting the maximum pool depth, depth at the outflow, width, and length.
HOBO temperature sensors were installed in each reach, recording stream temperature every 15 minutes. Sensors were installed on 2022-Jul-18 and pulled from the stream on 2022-Oct-19. Temperature sensor locations were recorded as the distance downstream from the top of each reach.
