Fassnacht focuses on snow hydrology and hydrological modeling, as well as how people perceive snowfall and snow.

A professor in the Watershed Science Program, Fassnacht’s work has emphasized how people perceive their environment (in particular snowfall and snow), how this perception compares to what the data says, and what social impacts may be created by any discrepancies between perception and process.

Fassnacht is affiliated with several professional organizations, including the American Geophysical Union, the International Association of Hydrological Sciences and the International Association of Cryospheric Sciences. He was a short-term Fellow of the Japanese Society for the Promotion of Science in October 2022 and a Fulbright Global Scholar to Chile and Spain in 2023-2024. He won the Natural Resources Ecology Laboratory Outstanding Advisor Award in 2013, 2018 and 2021, and the Harry E. Troxell Distinguished Service-to-Students Faculty Award in 2023. He served as the Eastern Snow Conference president from 2008 to 2009.

Fassnacht studied civil engineering at the University of Waterloo in Canada, earning his B.A.Sc in 1992, his M.A.Sc in 1995, and his Ph.D. in 2000.