Foster Dam on the South Fork Santiam River was outfitted 30 years ago with a spillway weir as a surface outlet for downstream migrating fish passage. The weir sits on top of stop logs inserted into a spill bay gate opening. The stop logs are used to regulate pool elevation and the weir provides a surface passage alternative for fish that has more benign hydraulic conditions than passage through either the turbines or open undershot spillway gates. The modified fish weir opening measures 22 feet in total width, 11 feet on either side of the center wall. Velocities across the weir increase from 11 fps on the leading edge to 37 fps at the trailing edge. In 2013 the USACE, Portland District, documented out-migrating juvenile fish moved through the modified fish weir throughout the year.

However, until recently there has been no way to characterize movement patterns and provide detailed movement data of PIT tagged fish over the weir. In May 2014, a new PIT tag antenna, designed and installed by West Fork Environmental, began detecting PIT tagged fish moving over the weir. For the first time in 30 years fisheries managers are now able to collect detailed data to characterize the timing, number and life stage of winter steelhead, rainbow trout, and spring Chinook salmon moving downstream over the Foster Dam fish weir.

The all mild steel structure of the weir and stop logs and the need to maintain the existing hydraulic conditions over the weir constrained antenna design and construction options. Material choice was constrained by the requirement for a smooth, abrasion resistant weldable thermoplastic readily available in thick large sheets. Two 11-foot by 42-inch antennas were constructed using a jacket of ultra-high-molecular-weight polyethylene (UHMW-PE) with a core material holding both the ferrite shielding and Litz wire coils. The overall thickness of the antennas was limited to 2.5 inches. Each antenna weighs in at over 520 pounds. The antennas were positioned on the fish weir after it had been removed from the spillway by a crane and moved to a nearby work site.

The antennas are each anchored to the mild steel weir deck by multiple stainless steel fasteners. Each component of the installation is mounted within the outside dimensions of the weir limiting the potential for damage to the components during reservoir elevation changes when the crane is used to reposition the weir and stop logs. The antennas are driven by IS1001-ACN’s and controlled by an IS1001-MC MTS. The installation and initial tests of this PIT tag antenna at Foster Dam proves there is great promise for thin body ferrite shielded antennas even at sites with technical challenges that include entirely mild steel structural components and high noise levels. Initial tests showed high tag read efficiencies at a water depth of 24 inches and a velocity range of 11 to 37 fps.