On Wednesday, September 7th HFF initiated an exciting addition to our water-sampling program designed to answer the following question: How far downstream of Island Park dam do high turbidity levels persist? These results will be important for predicting which reaches of the river will be most affected, and to what extent, during periods of high sediment delivery from Island Park reservoir.
We already test weekly water samples from just below Island Park dam and at Pinehaven for turbidity, nutrient levels, and suspended sediment concentration as part of our water-quality monitoring program, but for this study we focus our attention on the stretch of river between these sites. Therefore, we will frequently collect and test water samples at 6 additional locations between Island Park dam and Osborne Bridge (Figure 1). A brief description of the new sample sites follows: 1) a site just below Island Park dam; 2) a site near the confluence of the Buffalo River with the Henry’s Fork to capture the turbidity of that water coming in from the Buffalo; 3) a site we’re calling Mid Box Canyon, which is mid-way through the canyon and where water flowing out of the dam has mixed with Buffalo water; 4) at the Last Chance boat ramp below Box Canyon; 5) at the Log Jam access site where the Harriman State Park reach begins; and 6) at Osborne Bridge, which is within Harriman State Park.
This investigation was prompted by recent elevated turbidity levels below the dam due to the very low volume of water left in the reservoir. Current reservoir volume is approximately 20,725 acre-feet (as of Sept 9, 2016), which is 15% of capacity and is approximately 4,000 acre-feet lower than the long-term average volume for this date (Figure 2). HFF has been working hard with partners in the Drought Management Planning Committee all summer to maintain Island Park reservoir levels above 20,000 acre-feet while still meeting the demands of downstream water rights. Over the last several years of monitoring water quality in the upper Henry’s Fork, we have found 20,000 acre-feet to be a significant threshold; drawing the reservoir below this volume causes mobilization of high concentrations of sediment that may be harmful to the downstream ecology.
We expect that several factors influence how long suspended material remains in suspension in the river. These factors include flow rates, densities of macrophytes— rooted aquatic plants—in the river channel, and river gradient. Macrophyte density is a factor in this study because they slow water velocity and trap sediment (Kuzniar et al, in press). All things being equal, the amount of material that settles out over a given distance is a function of flow rate. We know the changes in river gradient between sampling sites and we can track the timeline of growth and senescence of macrophytes (abundance peaked about 2–3 weeks ago and is now slowly decaying). Therefore, once we repeat this sampling scheme several times under different rates of flow, we’ll be able to understand the relationship between these drivers and the spatial persistence of turbidity below IP dam.
The following explains what we observed after taking our first set of data earlier this week. First, I want to share some typical turbidity values to put our data (and the units) in perspective. The turbidity of well water is usually between 0.05 and 10 NTU, orange juice is between 300 – 900 NTU (depending on pulpiness), and whole milk has turbidity of around 4000 NTU. Now onto our data… Turbidity right below the dam was a little over 11 NTU on Wednesday afternoon and Buffalo water at the confluence had turbidity around 1 NTU. After 3.5 river miles, turbidity in the Henry’s Fork decreased by nearly half to less than 6 NTU at Mid Box Canyon. Then we observed that turbidity linearly decreased over the next four sample sites to approximately 3 NTU at Osborne Bridge, a distance of 8.5 river miles later. Recall that the Buffalo River confluence occurs between our Island Park dam and Mid Box Canyon sampling sites (Figure 1), so the improvement in clarity of the Henry’s Fork between these sites is also due to mixing with spring-fed Buffalo River water.
We plan to conduct this test over the next 12 months and will report on results as they arrive. Stay tuned!
References
Kuzniar, Z. J., R. W. Van Kirk, and E. B. Snyder. (in press). Seasonal effects of macrophyte growth on Rainbow trout habitat availability and selection in a low-gradient, groundwater-dominated river. Ecology of Freshwater Fish.
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