Turbidity in Perspective
During our regular water-quality monitoring last week, we observed that recent turbidity levels are above average in all reaches of the river, except below Island Park Dam, and the highest observed turbidity last week is higher than any observed turbidity below Island Park Dam since we started regular water-quality monitoring there in the summer of 2013. The highest turbidity levels we observed last week were in the Warm River-to-Ashton Reservoir reach (18 NTU) and at Pinehaven (9 NTU). The Warm-River-to-Ashton reach is the section of the Henry’s Fork least regulated by dams and diversions, and this reach had the highest turbidity levels last week, due to the process of runoff. Runoff occurs when water from snowmelt runs over the land into the river and conveys soil, sediment, decomposing plant tissue, and nutrients into the river. We find evidence of this process in increased natural flow and elevated turbidity levels, which began in the Henry’s Fork last week.
The reach below IP Dam had turbidity of 2NTU this week, which was the least turbid (or clearest) reach of river in the watershed for the week. Using the maximum and the annual average turbidity of our water quality data over the past three years puts turbidity levels last week in perspective: the turbidity of 2NTU we observed last week below IP Dam is well below that reach’s annual average turbidity of 4NTU, and the highest level of turbidity below IP Dam we have observed since we started water quality sampling in 2013 was 11NTU, which occurred late in the summer of 2013 when the reservoir was drawn down to only about 25% of capacity. Current low turbidity in that reach is due to low outflow and to the presence of the dam: Island Park Reservoir slows the transport time of water above that reach allowing material to settle out of the water and become trapped at the bottom of the reservoir.
Therefore, two important points are learned by putting last week’s turbidity data in perspective: 1) increased turbidity due to natural runoff in the Warm River-to-Ashton reach of the Henry’s Fork is currently higher than any recorded turbidity levels below Island Park Dam since we began our water-quality monitoring program; 2) we see that during natural runoff, when the rest of the watershed is experiencing above average turbidity levels, the reach just below IP Dam exhibits lower than average turbidity.