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Summary Global change is impacting the forests of the western United States through rising temperatures, earlier snowmelt, more rain and less snow, greater vapour pressure deficits in spring and autumn, forest dieback and increasing forest fire frequency and severity. A catastrophic forest fire (Las Conchas fire) occurred in central NM , USA , in 2011 burning c . 634 km 2 with c . 46% of the fire being of severe or moderate intensity. National Oceanic and Atmospheric Administration ( NOAA ) next‐generation radar data ( NEXRAD ) were used to link precipitation events occurring in the burn scar to extreme water quality excursions observed in the Rio Grande. At four sites, in situ sensors captured the response of water temperature, specific conductance, pH , turbidity and dissolved oxygen to flood events following the fire. Runoff from burn scars caused turbidity peaks (to 2500 NTU ), dissolved oxygen sags (to 0.0 mg L ‐1 ), pH sags (up to 0.75 units) and conductivity changes (both increases and decreases). These water quality excursions extended at least 50 km downstream, with significant implications for the ecosystem health of this crucial river that supplies water to cities and agriculture. Sudden, dramatic changes to forested catchments from severe forest fires and forest dieback are very likely to be among the strongest impacts of global change on stream and river ecosystems throughout the western United States.