Evaluation of Urban Riparian Buffers on Stream Health in the Tookany Watershed, PA

Abstract

Stream channels and their corresponding riparian zones are composed of complex spatially and temporally dynamic systems. Changing land-use associated with urbanization has resulted in large shifts in riparian assemblages, stream hydraulics, and sediment dynamics leading to the degradation of the world’s waterways. To combat degradation, restoration and management of riparian zones is becoming increasingly common. However, the relationship between flora, especially the influence of invasive species, on sediment dynamics is poorly understood. This relationship must be studied further to ensure the success of management practices. Three methods were used to monitor erosion and turbidity within the Tookany Creek and its tributary Mill Run in the greater Philadelphia, PA region. To evaluate the influence of the invasive species Reynoutria japonica (Japanese knotweed) on erosion, reaches were chosen based on their riparian vegetation and degree of incision. Methods used to estimate sediment erosion included measuring changes in bank pins, repeated total station transects, and monitoring turbidity responses to storm events. While each method has been used in previous studies to monitor sediment flux, the combination of methods in this study allowed their applicability to be compared. Measurements taken with YSI turbidity loggers showed large fluctuations in turbidity based both on riparian conditions and geomorphic positioning, suggesting that future studies need to be careful with logger placement when using sediment calibration curves to estimate sediment yield within streams. There were pros and cons of using both total station and bank pins to estimate bank erosion. Total station has the potential to produce highly accurate measurements but a greater risk of loss of data if the control points used to establish the grid cannot be re-established from one measurement to the next. Bank pins are more likely to influence bank erosion and be affected by freeze-thaw conditions but provide a simple method of monitoring erosion at frequent intervals. Volume calculations based on total station transects along the main stem of the Tookany did not show a consistent relationship between riparian type and erosion rates. However, erosion calculations based on bank pins suggest greater erosion in reaches dominated by knotweed with 4.7x10-1 m3/m and 8.3x10-2 m3/m more erosion than those dominated by trees at Chelten Hills and Mill Run respectively. Turbidity responses to storm events were also higher (76.7 v 54.2 NTU) in reaches with knotweed, although this increase was found when the reach dominated by knotweed was also incised. Thus, this study linked knotweed to increased erosion using multiple methods.