Biological assessments of streams that have been adversely impacted by sediment runoff in Idaho, USA
Biological assessments of streams that have been adversely impacted by sediment runoff in Idaho, USA Sediments are the major source of pollution in surface waters of the Pacific Northwest Region of the USA. The purpose of this study is to evaluate the relationship between SMI water quality scores at 76 sampling sites in eight watersheds and the observed soil erosion rates on adjacent landscapes. The water quality SMI scores in streams were obtained using stream macro invertebrates as an indicator of water quality, while soil erosion rates were determined by observation on adjoining landscapes during periods of maximum precipitation. Soil erosion rates of >2, 2-5, 5-15, 15-25 and <25 mt/ha/yr were observed at 9, 20, 45, 14 and 12% of the sampling sites, respectively. Landscapes with erosion rates of less than 5 mt/ha/yr generally resulted in good water quality in adjacent streams; however, when soil erosion rates on adjacent landscapes exceeded 5 mt/ha/yr SMI water quality scores were less than good 86% of the time. Strong significant relationships were observed between SMI water quality rating and observed soil erosion rates. Consequently, land management or rehabilitation practices that reduce soil erosion rates to levels below 5 mt/ha//yr should improve stream water quality.