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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5187

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Summary and Conclusions

Annual suspended-sediment loads (SSL) and suspended-sediment yields (SSY) were highest during the relatively high streamflow of water years 1999 and 2000. Three-day SSL and annual SSL totals were highest in the three upper-basin stations that were in operation during one of those years. Although water year 1999 was the higher annual streamflow year, water year 2000 was the higher sediment transport year. Comparisons among the lower-basin monitoring stations were less definitive because SSL totals were available for only four water years (2001–04) that had average to below-average streamflow.

North Santiam River below Boulder Creek, near Detroit

The highest SSL totals at North Santiam were not associated with the highest instream turbidity. SSL was highest at the North Santiam monitoring station during the November 1999 storm. The 3-day SSL was more than 22,000 tons, greater than the annual SSL at the station for water years 2000–04. This storm-related SSL also resulted in the highest percentage of total annual SSL (85 percent) at this station. For the remainder of the period of study, the subbasin upstream of North Santiam produced between 29 and 50 percent of its annual SSL during the respective highest 3-day SSL. In water years 2001 and 2004, the sediment responsible for the highest SSL events originated on the slopes of Mount Jefferson, likely from glacial events or slope failures. Because these very high turbidity events did not occur in association with high streamflow, the relative SSL totals were not as significant.

The total annual SSL calculated for water years 1999 and 2000 were slightly lower than the SSL published for those years in a previous report from the study (Uhrich and Bragg, 2003). The addition of more than 20 samples to the regression analysis resulted in a change to the model-estimated SSC from instream turbidity for North Santiam. Although the R² value was slightly lower (0.89) than that of the previous model (0.91), the difference of the SSL estimates for both water years was less than 10 percent.

Breitenbush River above French Creek, near Detroit

SSL was highest at Breitenbush of all stations during the period of study. The 69,800 tons of sediment transported in 3 days during the November 1999 storm represents more than twice the annual SSL totals at any of the monitoring stations for any of water years 1999–2004. The instream turbidity during this storm did not reach the sensor maximum but was higher and persisted longer than the turbidity at the other upper-basin stations. The sediment causing the high turbidity during this storm was believed to have originated in a small unnamed tributary of one main tributary of the Breitenbush River. Since this storm, the U.S. Forest Service has made efforts to reduce the amount of surface runoff in this area.

The annual SSL calculated for water years 1999 and 2000 at Breitenbush was higher than the SSL published for those years in the previous report from this study (Uhrich and Bragg, 2003). The increase was greater during water year 2000, when the annual SSL was nearly 50 percent greater than the previous estimate. The original regression analysis included many samples with instream turbidity less than 1 Formazin Nephelometric Unit. More than 20 of these samples were excluded from the current analysis. Although many samples, including several high-turbidity samples, were added to the analysis, 10 fewer samples were used for the current model-estimated SSC from instream turbidity. The R² value of the current model (0.92) was slightly lower than that of the previously published model (0.95).

French Creek near Detroit

The total annual and highest 3-day SSL at French were nearly equal for water years 2002–04. The small subbasin (10 mi²) produced 765, 668, and 716 tons of sediment for water years 2002 through 2004, respectively. The highest 3-day SSL (400 tons) resulted from a small debris flow that entered the creek 0.5 mi upstream of the monitoring station during the storm on January 30–February 2, 2003.

Blowout Creek near Detroit

The Blowout Creek subbasin produced a significant SSL during storms in water years 1999–2000. The highest 3-day SSL was during the December 1998 storm. The turbidity during this event did not reach the sensor maximum (1,370 FNU) but was greater than that of the other two upper-basin stations. A slope failure or encroachment by the many earthflows in the subbasin may have occurred during the storm, but the SSL could not be attributed to a specific area. This storm also produced the highest SSY for any of the stations during the period of study.

The sediment source related to the December 2001 storm at Blowout was easily identifiable. Less than 3 mi upstream of the monitoring station, debris flow and a road failure carved a swath through the forest and deposited nearly 5,000 tons of sediment into a tributary of Blowout Creek. As a result of either the size of the slope failure or its close proximity to the monitoring station, the turbidity far exceeded that of the December 1998 storm. However, the higher streamflow and longer duration of the 1998 storm produced the higher 3-day SSL at Blowout.

Like at Breitenbush, the total annual SSL calculated for water years 1999 and 2000 at Blowout were higher than the loads published in the previous report from this study (Uhrich and Bragg, 2003). The current SSL estimates for both water years were 35–40 percent greater than the previous estimates. Although several turbidity samples less than 1 Formazin Nephelometric Unit were excluded from the regression analysis, many more samples were collected at Blowout. Thirty more samples were included in the current analysis, including many with high turbidity.

North Santiam River at Niagara

The highest SSL events in the upper North Santiam River basin had minimal effect on the annual SSL at Niagara. Only 4–9 percent of the annual SSL was produced in any 3-day period during water years 2001–04. The monitoring station is located 3.5 miles downstream of Detroit Dam and 0.5 miles downstream of Big Cliff Dam. The two reservoirs function as traps for sediment transported in the upper basin, limiting sediment output downstream. Blowout Creek is the closest of the upper basin tributaries to Detroit Dam and therefore the most likely to transport sediment through the dam and downstream. In several instances, this effect could be noted when the highest SSL at Niagara closely followed the highest SSL events at Blowout.

Of the 64 suspended-sediment samples collected and used for the regression analysis at Niagara, 50 had instream turbidity less than 10 FNU. Any slight variability of SSC at low turbidity values can affect the regression model statistics. For Niagara, the R² value of the regression model was 0.76, the lowest of all monitoring stations.

Little North Santiam River near Mehama

The Little North Santiam River produced its highest SSL during the January 2004 storm. The 11,400 tons of sediment transported in 3 days represented 64 percent of the annual total SSL. Both the streamflow and turbidity at this station were greater than any of the other stations during this storm. The annual SSY values at Little North were the highest for all stations (126 and 99 tons per square mile, respectively) for water years 2003–04.

North Santiam River at Mehama

The turbidity and SSL at Mehama are heavily influenced by the Little North Santiam River. Mehama is located about 0.5 mile downstream of the confluence of the Little North Santiam River with the mainstem North Santiam River. As expected, the highest 3-day SSL at Mehama also was during the January 2004 storm. The 11,400 tons of sediment transported during this storm comprised only 42 percent of the annual total SSL. However, annual total SSL at Mehama was highest in water year 2002. The SSL of 35,200 tons was the second highest SSL for the period of study at any of the monitoring stations.

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