Threshold events in spring discharge: Evidence from sediment and continuous water level measurement

SummaryIn September 2004, three major hurricanes, Frances, Ivan, and Jeanne, traveled up the eastern United States from the Gulf Coast bringing large amounts of rain to Central Pennsylvania. Monitoring equipment in place at Arch Spring, Blair County, PA captured the effects of these storms on the karstic spring flow. Together these storms revealed a quantitative limit for the carrying capacity of the conduit system. Ivan was a much more devastating storm to the area because rain fell on ground already saturated by Frances, but the net stage increase at the spring was greater during the earlier Frances storm, a 74 cm stage increase versus a 54 cm increase. Storm water not transported through the Arch Spring system was diverted into surface channels during these storms.Suspended sediment collected by an automatic sampler during Frances reveals another threshold crossed. Concurrent with increasing stage and high conductance water, maximum sediment concentrations (933 mg/L) exceed previous fluxes by up to an order of magnitude. The timing of the sediment pulse indicates that high sediment concentrations occur not only when the storm water reaches the spring, but also when stored water is being flushed out of the karst spring system. Sediment previously deposited in the conduit system is flushed only when adequate flows occur, indicating that sediment transport in karst is marked by thresholds and is a strongly non-linear process.