Prepared by Melissa Muradian and Jack McLaren
What happened?
Due to warm water temperatures and low dissolved oxygen in the water at the intake to the Island Park hydroelectric plant, the plant’s aerators have been unable to maintain dissolved oxygen in power plant outflow above the level required by Fall River Electric's operating license. Thus, the plant shut down yesterday morning (July 15), and all outflow from Island Park Reservoir was shifted to the bottom-withdrawal gates on the west side of the dam.
How does this affect the river’s water quality?
Temperature: Water delivered through the dam gates is from the bottom of the reservoir and as long as thermal stratification remains in the reservoir, the gates will deliver cooler water than the power plant. However, the response we saw in temperature was about 1 ˚C (approx. 2 ˚F) at our IP East sonde. We saw no discernable difference in water temperature by the time that water made it to Pinehaven. Thus the cooler water temps that result from total flow coming out of the gates only last until some point upstream of Pinehaven, and most likely only last until part-way through Box Canyon.
Dissolved Oxygen (DO): DO was hovering at and below 7 mg/L when the transfer happened and once all flow was through the dam gates, DO jumped up to 8 mg/L. This is due to the physical turbulence of water through the dam gates structures that easily mix air into the water. Fall River Electric has reported to us that DO at their siphon intake dropped below 4 mg/L over the weekend. At the current warm water temps, their aeration system cannot boost the DO above 7 mg/L, whereas the physics of the dam gates structures are more effective and can achieve higher DO.
Plant material: We went out this morning and took pictures, took water samples, and talked to any anglers we could find from below IP Dam to Pinehaven. There has been a noticeable amount of plant material being conveyed down the river. At this point, we think most of this material has been torn away from the river bottom. This hypothesis is based on our observations: 1) we first heard reports of “salad” in the river as early as Tuesday July 10, the day the gates were opened to accommodate an increase in outflow beyond power plant capacity; 2) the fact that this plant material seems recently detached, it is bright green and fresh; 3) the material is the same species of aquatic plants we have in the river (and not, for instance, what grows in abundance in the west end of the reservoir); and 4) the quantity of uprooted plant material in the river seems to increase with distance downstream of IP dam. For instance, while we witnessed some rafts of aquatic vegetation floating downstream at the Log Jam, we noticed none at the Box Canyon boat ramp but saw many more rafts of larger size at the Pinehaven dock. Based on these, it seems material is certainly being broken off in the channel, whether some material is also coming from the reservoir or not.
Why would flow out of the gates cause plants to break up and move downstream? It’s possible the changed dynamics of flow close to the dam are uprooting material that was formerly in local low-flow areas. Or it is possible that the increase in outflow boosted water velocity high enough to cause tear out, and the opening of the gates is coincidental with these flow increases.
We need to do a little more testing to confirm or deny the origin of the material, and we may not be able to positively confirm the origin before conditions change.
Water clarity: From our sonde data, our own observations, and from angler reports, it seems water clarity—aside from the detached plants—decreased slightly after the initial opening of the dam gates and has since cleared up.
What does HFF think about this?
Maintaining DO above 7 mg/L in their outflow is a condition of Fall River Electric’s FERC license. HFF was one of several stakeholders to the creation of the FERC license. The DO condition protects our wild trout fishery by maintaining sufficiently high levels of DO for adult and juvenile trout during the warm summer months. In other words, this flow transfer and resulting DO increase is by design to benefit the long-term health of the fishery. Without the DO condition in the FERC license we would currently have DO levels below 5 mg/L between the dam and the Buffalo confluence. These levels would be low enough to be physiologically stressful to trout at water temps above 15 ˚C (59˚F). Without the FERC license and cooperation of Fall River Electric and other stakeholders there would effectively be no trout fishery between the dam and the Buffalo confluence during the warmest parts of summer since the trout would move out in search of more appealing habitat.