ENTRAINMENT AND TRANSPORT OF COARSE STREAM BED MATERIAL IN A FLUVIOKARST WATERSHED, SOUTH-CENTRAL MISSOURI: A TRACER PARTICLE STUDY

Studies have shown that cave streams have the potential to mobilize large quantities of coarse stream bed material with a magnitude similar to that of surface streams. However, there remains a lack of data confirming these estimates. Commonly the threshold of motion is estimated based on Shields (1936) criteria. In fact, the simple Shields criteria, using typical assumptions, may not accurately describe initial motion for non-uniform gravel-bedded streams. The purpose of this investigation is to examine sediment transport dynamics in the area of Tumbling Creek Cave – a fluviokarst watershed located in the southern Missouri Ozarks. Specifically, we examine two streams (1) Bear Cave Hollow, a headwater ephemeral surface stream draining an area of ~ 1.94 km², and (2) Tumbling Creek, a larger perennial cave stream with a recharge area of ~ 23.4 km², with much of its water coming from discrete locations along losing streams and from nearby sinkholes. Both streams are covered by alluvial beds composed primarily of coarse-grained residual chert eroded from the Ordovician age Cotter dolomite. Approximately 81% of the watershed remains forested, although a heavy ice-storm in the winter of 2008-2009 has effectively removed close to 70% of trees in some stands. The objectives of this study include (1) determining the threshold of motion and short-term sediment transport rates for large (>16 mm) stream bed sediments (bed load fraction), and (2) developing a dataset capable of testing whether or not surface streams function similarly with cave streams in regards to bed load sediment transport dynamics. The primary research tools are 660 painted tracer particles (deployed under low-flow conditions and surveyed after storm events), water gaging stations providing discharge data for the study reaches, and automatic level and total station surveys providing cross-section and bed slope data. To date, four storm events in the surface stream and six storm events in the cave stream have had the competence to mobilize bed load sediments. Further research will use 80+ year precipitation records from nearby weather stations to develop correlations between rainfall and discharge in order to describe and predict the frequency of flow events.