Excess algae growth in 2018 is unrelated to the wastewater treatment pond leak that occurred at the Yellowstone Club in early March 2016. The treated wastewater spill increased levels of nitrate and ammonia in Second Yellow Mule, South Fork, and West Fork in the days immediately following the spill; however, there were no long-term effects to water quality.
The largest impact to river health after the spill was caused by excess fine sediment carried from the steep hillside beneath the wastewater treatment pond to Second Yellow Mule and the South Fork. Excess fine sediment can cause a cascade of negative impacts to fish and stream insects by covering spawning habitat, clouding water, and impacting their gills. Montana Fish, Wildlife, and Parks reported that fish and stream insect population would recover in Second Yellow Mule Creek and the South Fork in several years.
For more information, the Gallatin Task Force developed a webpage with more information on the incident: https://www.gallatinrivertaskforce.org/yellowstoneclubspill/.