Global change affects the forests of California (USA) through rising temperatures, earlier snowmelt, more rain and less snow, greater vapor pressure deficits, and forest dieback, resulting in an increased wildfire frequency, size, and severity. California has experienced its eight largest wildfires since 1932 in the period from 2017 to 2022. The largest fire to date occurred in 2020 and burned 418,000 ha, a year in which 1.7 million ha or 4% of California’s land area burned. These fires have the potential to cause severe impacts on water quality and aquatic ecosystems. Water quality variables affected by wildfire include temperature, sediment load, turbidity, dissolved oxygen, pH, redox potential, soluble and particulate organic carbon, nutrients, metals, and primary producers. Wildfire and water interact at watershed scales with water quality generally responding linearly with the percentage of the watershed burned and responding exponentially as burn severity increases. Vegetation recovery is a key to the duration of water quality impacts, and short-term, post-fire weather dictates actual water-related impacts. Urban areas are hot spots for water quality pollutants such as sediments, heavy metals, mercury, nutrients and toxic organic compounds. Water treatability challenges after wildfire include short-term smell and taste impacts, increased sediment and turbidity issues, and increased total and dissolved organic matter. Implications of catastrophic wildfire on downstream reservoirs is an important research need as ~80% of California’s water supply passes through reservoirs before use. Notably, there is a crucial need for development and assessment of post-fire, land management practices to mitigate adverse water quality impacts. Finally, continuous and coordinated measurements of water quality are critical for documenting the severity and duration of wildfire effects on aquatic ecosystems.