Turbidity Persistence Test: Conclusions From the 2016 Data
This fall HFF initiated a new water-quality monitoring study designed to answer the following question: How far downstream of Island Park Dam (IP Dam) do high turbidity levels persist? I introduced this study in my blog post made on September 9, 2016. Please find details of the background and motivation for this study, as well as a map of the sampling sites, in the previous blog post. The current blog post summarizes what we found during our 2016 data collection season.
Summary of the Study and Interpretation of Results
Our 2016 Byers intern, Hunter Hill, collected and tested weekly samples from seven locations between IP Dam and Pinehaven from September 7th to October 28th. These data are shown in Figure 1 (black dots), along with the mean turbidity observed at each site over the sampling period (‘x’ character).
Figure 1. Observed turbidity in NTU (black dots) as a function of distance from Island Park Dam (IP Dam) collected during fall of 2016. Each sampling site is labeled. The ‘x’ character displays the mean of each site’s observations over the eight sampling dates, and the dashed curve shows mean turbidity as a continuous function of distance from IP Dam. The curve was fit to the data points using the best statistical model relating turbidity to distance downstream of the dam.
After only eight weeks of testing, some trends emerged. Predictably, turbidity was highest just below the dam compared to the rest of the sample sites, and turbidity values decreased with distance downstream from the dam. This supports what anglers have long observed; Island Park Reservoir is the primary source of downstream turbidity during the fall.
To understand the relationship between turbidity and distance downstream of IP Dam, we modeled turbidity as a continuous function of distance (Figure 1, dashed curve). For those of you interested, this model was created using linear regression on log-transformed data, with log-normally distributed residuals.
To understand how far turbidity persists downstream of the dam, we estimated the rate at which turbidity levels decrease as a function of distance (Figure 2), which is the slope (technically, the derivative, for those of you who know calculus) of the continuous turbidity vs. distance curve shown in Figure 1 (dashed curve).
Figure 2. Average rate of decrease in turbidity (NTU) as a function of distance from Island Park Dam (IP Dam). Each sampling site is labeled, for reference.
We were curious to know whether the rate of decrease in turbidity levels remained constant or changed with distance downstream of the dam. As Figure 2 clearly shows, this rate is not constant, but itself decreases with distance from the dam. We estimated that turbidity decreases at a rate of 3.50 NTU per river mile immediately below the dam and then substantially slows over the next 2 river miles to 0.65 NTU per mile. Recall that the Buffalo River confluence occurs within the first half mile, so the increase in clarity is helped by the addition of clearer, spring-fed water from the Buffalo. Moving downstream, the rate of decrease continues to slow down until becoming nearly zero at Osborne Bridge and Pinehaven. It is important to point out that changes in turbidity of less than 1 NTU are practically imperceptible to the unaided eye. Therefore, the river becomes more clear the farther downstream of the dam you go, but the greatest relative improvement in clarity is found closest to the dam and is negligible downstream of the first 4 or 5 river miles (Figure 2).
To avoid confusion, data collected from the confluence of the Buffalo River are not depicted in Figures 1 and 2. The Buffalo Confluence data mainly reflect turbidity of water flowing from the Buffalo into the Henry’s Fork, which has not yet fully mixed with the main stem river. Mean turbidity at the Buffalo River confluence was 2.54 NTU, with minimum and maximum observed values of 0.87 NTU and 3.98 NTU. Compare these values to those in Figure 1.
Concluding Remarks and Future Work
In-stream processes, such as changes in turbidity, are highly seasonal. Hence, this blog contains preliminary conclusions made after only eight weeks of testing during a single season. Next year we’ll resume testing before spring snowmelt and continue through late fall. Therefore, our conclusions will become more robust after the addition of several years of data.
In addition, further data collection will allow us to identify other factors that potentially influence how long (and how far) turbidity persists by investigating factors that potentially influence how long physical material remains in suspension. Recall that turbidity is measured using the amount of light scattered by suspended particles in the water column. The amount of light scattered in the water is not only a function of suspension density, but is also influenced by characteristics of the suspended particles themselves, such as color, shape, and reflectivity. Therefore, there is a relationship between turbidity and the quantity of material in suspension, but that relationship changes as the composition and type of suspended material change.
Factors that influence turbidity, aside from distance and composition of suspended material, will likely include flow rate or density of rooted aquatic plants. The latter are important because they slow water velocity and trap suspended material (Kuzniar et al., 2016). Results from this study will be used to predict which reaches of the river will be most affected, and to what extent, during periods of high turbidity downstream of Island Park Dam.
Kuzniar, Z. J., R. W. Van Kirk, and E. B. Snyder. 2016. Seasonal effects of macrophyte growth on Rainbow trout habitat availability and selection in a low-gradient, groundwater-dominated river. Ecology of Freshwater Fish. doi:10.1111/eff.12309