According to email communication with the Montana Department of Environmental Quality (MT DEQ):

The MT DEQ has noticed significant changes in the timing and location of algae blooms on other western Montana rivers, most notably during 2016 and 2017. The MT DEQ is also more aware of algae blooms occurring during the past three to five years in many reservoirs in western Montana. Higher than average snowpack and spring runoff during 2018 provided a low to mid-range stream algae growth season within the region. The Gallatin River seems to be a regional anomaly of especially active growth and we are not sure why. The 2018 Gallatin algae conditions are on MT DEQ’s emerging topics list and the MT DEQ is coordinating with the Gallatin River Task Force and others to a keep pulse on the situation. MT DEQ will coordinate with the Task Force to compile and analyze data this winter.