Long-term sediment loading trends in the Paradise Creek watershed

Abstract

The Northwest Wheat and Range Region is historically known for high soil erosion rates. During the 1920s and 1930s, erosion rates of 200 to 450 t ha⁻¹ (90 to 200 tn ac⁻¹) in a single winter season were observed. Improved soil conservation practices over the last 80 years have significantly reduced soil erosion rates, yet there is scarce evidence of significant reductions in sediment loading delivered by streams in the region. In this paper, detailed monitoring data collected in the Paradise Creek watershed, located in the high precipitation zone of the Northwest Wheat and Range Region in north central Idaho, provided an opportunity to assess the impacts of management practices on sediment loading at the watershed outlet. Both detailed event-based sampling over the last eight years and three day per week grab samples collected over the last 28 years indicate a statistically significant decreasing trend in overall sediment load. This decreasing sediment load can be attributed primarily to conversion from conventional tillage systems to minimum tillage and perennial grasses through the Conservation Reserve Program, practices initiated in the late 1970s and early 1980s. Over the last 10 years (1999 to 2009), management practices have targeted gully erosion and stream bank failures. Upstream and downstream sampling shows a larger than expected increase in sediment load through the urban areas of the watershed. Preliminary modeling results and empirical evidence indicate that delayed reduction in sediment load at the watershed outlet and the increased sediment load through the lower urban portion of the watershed may be caused by sediment storage in the stream channel.