Variations in turbidity in streams of the Bull Run Watershed, Oregon 1989-90

Water-Resources Investigations Report 93-4045



In this study, turbidity is used to help explain spatial and temporal patterns of erosion and sediment transport.

Automated turbidity sampling in streams in the Bull Run watershed during water years 1989 and 1990, showed turbidity levels, in general, are remarkably low, with levels below 1 NTU (nephelometric turbidity unit) about 90 percent of the time. However, ephemeral increases in turbidity in streams of the Bull Run watershed occur in direct response to storms. Turbidity is caused by abundant organic particles as well as by materials eroded from unconsolidated geologic materials located along roads, stream channels, or stream banks. Seasonal and within-storm decreases in turbidity are attributed to depletion of accumulated particle supplies. During winter storms, erosion caused by rainfall intensities greater than 0.25 inches in 3 hours is sufficient to increase stream turbidities from less than 1 NTU to as much as 100 NTUs. Large-scale storms or floods cause persistent effects because mass erosion or scour of channel armor increases available sediment supply.

Spatial variability in turbidity is evident only during storms when erosion and sediment-transport processes are active. Parts of the Rhododendron Formation are particularly prone to channel and mass erosion during large storms. Eroding glacial deposits in sections of Log Creek affected by a 1964 dam-break flood also cause high stream turbidity relative to other streams in the watershed.

Analysis of characteristics of magnetic minerals in sediment sources and deposits was unproductive as a means to identify source areas of suspended sediment because high concentrations of magnetite in all samples of the volcanic rocks masked differences of less magnetic minerals in the samples.

Study Area

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Variations in turbidity in streams of the Bull Run Watershed, Oregon 1989-90
Series title Water-Resources Investigations Report
Series number 93-4045
DOI 10.3133/wri934045
Year Published 1994
Language English
Publisher U.S. Geological Survey
Contributing office(s) Oregon Water Science Center
Description v, 28 p.
Country United States
State Oregon
Other Geospatial Bull Run Watershed
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