Preliminary Assessment of Flooding Hazards in the Nooksack River Watershed, Washington State, and its Effect on Water Quality and the Local Shellfish Industry

Preliminary Assessment of Flooding Hazards in the Nooksack River Watershed, Washington State, and its Effect on Water Quality and the Local Shellfish Industry

John Prizzi, MLWS 2017


Flooding in the Nooksack River in northwestern Washington State has predominantly been the product of heavy rains occurring between the months of October and March (Nooksack Indian Tribe, 2016). Future climate change models as well as trends observed in historic data suggest that yearly winter temperatures have increased by about 4º C over the last 50 years will continue to increase by about 3º C by 2100 under current land use conditions and fossil fuel emissions (Kremen, 2007). Increasing winter temperatures would decrease the amount of snowfall experienced along the Nooksack River floodplain and the rest of Whatcom County and replace it with rainfall. By increasing rainfall along the Nooksack River floodplain, the frequency of flood events, which are defined in this study as flow rates equal to or above 565 m3/s, is expected to increase during the months of October to March by about three floods per year. Heavy rainfall and overland flow along the floodplain as the result of flood events has the potential to move sediment, organics, and contaminants from the floodplain to the Nooksack River (NOAA, 2015). Due to the agricultural and industrial development as well as expansive rural environments, bacterial contamination has been of concern in the Nooksack River and its discharge point in Portage Bay since testing began around 1990 (Peterson, 2011). Due to this contamination, shellfish beds in Portage Bay have experienced conditional and permanent closures since 1995 (Peterson, 2011). Shellfish beds in Portage Bay are relied on by residents as sources of food and money and understanding influences on fecal coliform concentrations in the Nooksack River can lead to preventative measures to reduce fecal coliform introduction to the Nooksack River and Portage Bay.

To investigate influences on fecal coliforms concentrations in the Nooksack River, the relationship between flood events and fecal coliforms concentrations was analyzed. Conclusions drawn from this relationship was then taken one step further to analyze how increasing temperatures and precipitation would affect the frequency of floods over the next 50 years and ultimately the potential response of the fecal coliform concentrations in Portage Bay. It is expected that flood frequency will increase over the next 50 years and as a result fecal coliform concentrations in the Nooksack River and Portage Bay will increase as well.