A TEST OF THE DOWNSTREAM PATTERN OF PARTICLE SIZE IN THE PASCAGOULA RIVER SYSTEM, MISSISSIPPI, USA

Downstream fining of alluvium in a river system is a well-known and broadly assumed phenomenon. This assumption is supported by Sternberg’s Law, also known as the abrasion law. Most grain size studies that have investigated the abrasion law have been performed in gravel-bed river systems. Our study examines grain size at 59 sites within the Pascagoula River system. Channel and bank samples were collected on the Leaf, Chunky, Chickasawhay and Pascagoula Rivers. Alluvium samples were air dried and sifted using a sonic sifter. Grain size distribution (i.e. cumulative frequency) curves were created for each sample and Φ5, Φ16, Φ50, Φ84 and Φ95 sizes were determined from the curves. Each Φ value was plotted against the sampling location’s distance from the river’s mouth at Pascagoula Bay. The graphs show no clear pattern of downstream fining. Inclusive graphic standard deviation (ISD) values were calculated for each sub-system, as well as the entire Pascagoula River basin. ISD values reveal moderate sorting throughout the entire Pascagoula River system. Abrasion and selective transport are the most cited factors that may influence sorting and fining within a fluvial system. We postulate that the effects of selective transport and abrasion on downstream fining is minimal within the Pascagoula River system; rather sediment additions through tributary and colluvial sources are keeping the alluvial system moderately sorted with no clear downstream fining pattern.