Summary and Conclusions


The U.S. Geological Survey investigated the sources and transport of suspended sediment in the Johnson Creek basin, Oregon, during water years 2007–10, in cooperation with the cities of Damascus, Gresham, Happy Valley, Milwaukie; and Portland; Clackamas County Water Environment Services; Multnomah County; and the East Multnomah Soil and Water Conservation District. Suspended-sediment loads are used by these cooperators, as well as other agencies and parties, as an indicator of the health of the watershed. If specific locations are determined responsible for mobilization of suspended sediment, they can be targeted for future remediation efforts. Similarly, if specific months or seasons are responsible for a disproportionate amount of suspended sediment, greater effort can be undertaken to control erosion during those periods of time. Watershed wide sediment issues likely would warrant more systemic changes in management practices. 


Gaging stations at Gresham and Milwaukie provided continuous streamflow and turbidity data. A regression model was created for each station relating suspended-sediment concentration to streamflow and turbidity. These models were used to compute continuous suspended-sediment concentration records and subsequent suspended-sediment loads at both gaging stations. Suspended-sediment loads were evaluated to determine the timing and spatial distribution of suspended sediment transport within the watershed. 


Annual streamflow for the 4 water years encompassing the study period was relatively typical of the average for the 1941–2010 period of record; therefore, the amount of suspended sediment computed at the Gresham and Milwaukie stations should be a reasonable indicator of near-average conditions. Annual suspended-sediment loads for the 4-year study period averaged 1,890 and 4,640 tons for the Gresham and Milwaukie stations, respectively. Because there are no major tributaries of Johnson Creek between the Milwaukie station and the confluence with the Willamette River, the Milwaukie station value provides a measure of sediment loading from Johnson Creek to the Willamette River.


During the study, almost 75 percent of the suspended-sediment load originated during November–January for both stations. Conversely, less than 10 percent of the suspended-sediment load originated during April–October for both stations.


For the wettest months (November–May), when most suspended sediment is transported in Johnson Creek, 40 percent of suspended sediment originates upstream of the Gresham station. The topographical drainage area upstream of the Gresham station is about 30 percent of the size of the topographical drainage area upstream of the Milwaukie station. However, the annual streamflow at the Gresham station is about 40 percent of streamflow at the Milwaukie station, so the amount of suspended sediment originating upstream of the Gresham station does not appear to be disproportionate.


The average annual suspended-sediment yields at the Gresham and Milwaukie stations during the study period were 123 and 87.2 tons per square mile, respectively. The annual streamflow yields at the Gresham and Milwaukie stations were 1.99 and 1.44 cubic feet per second per mile, respectively. The annual ratios of Gresham to Milwaukie for suspended-sediment and streamflow are nearly equal, suggesting that on an annual basis, the increased sediment productivity at the Gresham station primarily is the result of higher streamflow yield.


First posted October 3, 2012