MAXIMUM WALL RETREAT POTENTIAL IN FLUVIOKARST CONDUITS: THE BALANCE OF PHYSICAL AND CHEMICAL PROCESSES

Most prior researchers have examined the development and enlargement of fluviokarst conduits primarily in terms of chemical processes. Calculated rates of wall retreat due to dissolution vary up to 1 mm per year depending on gradients, water chemistry, flow rates, and system maturity. This focus on dissolution is certainly appropriate during early karst aquifer development when conduit diameter is less than 1 cm, and flows are laminar to transitional. However, after conduit diameter exceeds this threshold, discharge can become turbulent and capable of transporting sediment. The sediment in turn can affect the rate of conduit enlargement.

Geomorphologists have directly and indirectly recorded incision rates in insoluble bedrock up to several millimeters per year in active, high gradient systems. The rates increase on recently exposed bedrock surfaces and at sites with particularly weak bedrock. These processes are a complex interplay of gradient, discharge, and sediment load, but sediment plays an important role in the lowering of bedrock channels by acting as a tool or a shield on the bed. Similar physical processes are also operating in fluviokarst conduits, acting in concert or conflict with chemical processes.

We examine the potential interplay of these wall retreat mechanisms at Smullton Sinks, a series of karst windows in Brush Valley, Centre County, PA, USA. The Sinks occur in Ordovician limestones in an anticlinal valley in the Appalachian Valley and Ridge of central Pennsylvania. The ridges surrounding the valley are made up of sandstones and shales. The water flowing through the Sinks ultimately reaches the surface <0.5 km downstream at a spring forming the headwaters of Elk Creek. Following an intense storm, the discharge at the sinks showed variable chemistry, suspended sediment load, and bedload illustrating that wall retreat potential can shift among these contributors across a single storm. The sediment was primarily quartz and other silicates, and likely derived from the surrounding ridges. Earlier work by others indicates that the increased discharge and chemistry shifts associated with storms play an important role in dissolutional enlargement. Our data suggest that increased sediment transport associated with storms may play a similar role in enlargement by mechanical action.