Effects of catastrophic flooding on stream biogeochemistry in a headwater stream in Shenandoah National Park, USA

This study examined the stream chemistry changes in Staunton River (a second-order headwater stream with an average annual discharge 704 m³ ha^-1 yr^-1, Shenandoah National Park, Virginia) resulting from a catastrophic flood in June 1995. This flood, which followed after 800 mm of rain in a 4-day period, caused large-scale debris flows and complete scouring of riparian soils down to bedrock in the lower 2 km of the stream, and has been estimated to be a 1000-year flood. The flood affected stream chemistry on both short- and long-term time scales. The primary short-term response was elevations in stream concentration of Ca²⁺, Mg²⁺, and K⁺ by 59%, 87%, and 49%, respectively, for 6 months immediately following the flood. The long-term impact of decreased concentration of all base cations and SiO₂ during summer months (8% average) lasted about 2 years. At the episodic time scale, Ca²⁺, Mg²⁺, and K⁺ flushed from soil sources during pre-flood storms while Na⁺ and SiO₂ diluted; these trends generally reversed during post-flood storms for 2 years. Short-term effects are attributed to the leaching of unconsolidated soil and upturned organic matter that clogged the streambed after the flood. The long-term and superimposed episodic impacts may have resulted from the loss of riparian soils and vegetation in the flood.