Streamwater Chemistry and Sediment Responses to Wildfire in the Colorado Front Range

The influence of forest fire on streamwater chemistry depends on the extent and conditions of the burn, the physical and biotic characteristics of the watershed and the flow regime. A monthly streamwater monitoring network initiated in 2001 on the Pike National Forest allows evaluation of fire effects in catchments burned by the 2002 Hayman fire and allows comparison of streams in burned and unburned drainages. In burned watersheds, volume-weighted suspended sediment increased by an order of magnitude during the first post-fire year and by another order of magnitude the following year. Stream nitrate concentrations increased linearly following the burn. The year following the burn, discharge-weighted concentrations of Na⁺, K⁺, Mg⁺² and Ca⁺² differed little between streams in burned and unburned watersheds; however, in year 2 these constituents increased by 51, 13, 30, 35 and 51%, respectively for burned drainages. Similarly, differences in streamwater ANC, NO₃^- and Cl^- between burned and unburned drainages were more pronounced the second year after the burn. We will also assess the influence of watershed size and fire severity on the responses of individual streams.