THREE CONCERNS ABOUT OAK CREEK BED-LOAD STUDY

Bed-material load in Oak Creek in the Oregon Coast Range near Corvalis, Oregon was sampled in 1970 and 1971. Many people have used the Oak Creek bed load transport data. This poster presents information on three concerns about the Oak Creek data that are mostly ignored by users of the data. The first concern is about the median size of the bed surface material (armour). One sample of the armour was reported at the time of the study as having a median size of 66 mm. This median size is based on area-by-weight sampling and analysis. The same data transformed to a equivalent of the Wolman sampling procedure would yield a median size of 44 mm. A value of the dimensionless shear stress of 0.021 has been used in flushing flow studies and was calculated using area-by-weight analysis of the armour. If the results of the Oak Creek study are used along with data from other studies where the median bed material size was obtained using the Wolman procedure the dimensionless shear stress should be 0.030. This is a significant difference. The second concern is the change in size of the armour during the sampling period. The most useful bed-load samples were obtained in the winter of 1971. At the beginning of the sampling period the median size of the armour was 52 mm, at the end 63 mm 9are by weight). This indicates the dimensionless shear stress was reduced by 82% during the sampling period. The third concern is how the streamflows in 1971 compare to a longer period. Long term data for Oak Creek does not exist. Analysis of data for the Luckiamute River suggest the period prior to 1971 may have been a period of relatively high flows. The ability of the Luckiamute River to transport sediment in 1971 is exceeded by 35% of the years in the period of record for the river.