QUANTIFYING SEDIMENT TRANSPORT IN A SMALL STREAM SETTING

Correlating sediment grain size and discharge rates in small Piedmont streams can be difficult due to highly variable flow conditions and a limited literature base. Sediment from these streams can, however, be a significant source of excess nutrients delivered to nearshore marine environments. Our study focuses on one such stream in the Ronald R. Rhein Environmental Study Area, Kutztown, PA. The study stream flows along a boundary between active farmland and woodland and is ~1m in width with steep, incised (20 – 80cm) banks. We calculated discharge via salt discharge and correlated discharge to stream height with a simple stage gauge. Grab samples of sediment were taken from the stream bed and sieved to develop grain-size distribution curves for the stream. We then compared discharge-based Shield’s stress calculations to the observed grain-size distribution. We determined that the largest particle the stream can move during normal flow is 0.690mm in diameter and at bank full flow is 4.22mm in diameter. Therefore, only 2.83% of the stream’s bed (by weight) is actively moved during normal flow and 37.2% moves at bank full flow. Our calculations agree with observations; samples collected during times of bank full flow indicated that most sediment in transport was suspended load with very limited bedload movement. This finding disagrees with the general assumption that the largest sediment in a stream moves during bank full flow. The implied disequlibrium between available stream power and bedload suggests that the majority of the sediment in this stream system represents a legacy from prior conditions and processes. This is not unusual in the heavily settled Piedmont where anthropologically impounded sediment has been shown to represent a significant portion of unconsolidated material. In this case, however, the relatively coarse sediment suggests more energetic processes and we believe that the gravel bed is a result of periglacial processes and was likely deposited during the last glacial maximum. The stream is, consequently, substantially sediment-starved at high flow levels and it is likely that it will not only continue to erode laterally into the fine sediments of its banks, but also efficiently transport the substantial runoff from the adjacent farmland further into the Delaware River system.