SUSPENDED SEDIMENT TRANSPORT AND THE EFFECTS OF ALGAE AND MOSS ON THE ENTRAINMENT OF SILT AND CLAY IN THE BUSHKILL CREEK WATERSHED, EASTERN PENNSYLVANIA

Accelerated soil erosion associated with land-use change has resulted in an increase in the suspended sediment load and fine-grained sediment deposition in many streams draining agricultural land or land undergoing development. We have been using a combined field monitoring approach coupled with flume experiments to gain an understanding of spatial and temporal patterns of suspended sediment transport and the effects of algae and moss on fine-grained sediment entrainment in the Bushkill Creek watershed in Eastern Pennsylvania. The Bushkill has a drainage area of area of 206 km2 and is located in Northampton County, Pennsylvania. The upper portion of the watershed consists of a mix of agricultural and forested land whereas the lower half of the basin consists of agricultural land undergoing rapid development. The lower 10% of the watershed flows through urban land served by storm sewers. Water and suspended sediment samples are taken using two automated samplers; one deployed near the mouth of the basin and one located in the upper portion of the drainage basin. Discharge is measured using pressure transducers and data loggers at each of the sample locations. In addition suspended sediment samples were collected by hand throughout the drainage basin during individual runoff events. Our data show that, in the lower watershed, peak suspended sediment loads often lag behind peak discharge. This may reflect the lag time between sediment bearing runoff from development and agricultural sites and more sediment free runoff delivered from impervious surfaces via storm sewers. Fine-grained sediment load is also greater in the lower portion of the basin, which suggests that active urban development contributes more sediment to the system than agricultural activity. Flume experiments indicate that algae- and moss-covered rocks retain as much as 68% more fine-grained sediment than algae- or moss-free rocks when subjected to the same flow velocities and shear stress. Elevated nitrate levels in small streams facilitate algal growth and therefore, may significantly affect the behavior of fine-grained sediment entrainment and transport.