01P080 – Tenmile Cr. abv Barrett Lake Technical Notes: 2006 Water Year Chuck Springer The telemetered stream gaging station on Bertrand Creek near the mouth operated throughout water year 2006 without interruption. During this time, eight discharge measurements were made and 30 discrete manual stage readings were taken at this station. Rating Curve This station began water year 2006 on Rating Table 5, which covers a range of discharge from 4.2 cubic feet per second (cfs) to 804 cfs. Three of the nine discharge measurements used to develop this rating were taken during water year 2006. The measured flows for this rating, ranging from 8.3 to 402 cfs, cover less than 50% of the rating curve, however; flows exceeded the measured range of flows only 4% of the time during water year 2006. Four percent of flows were less than the lowest measured flow, and no flows were greater than the highest measured flow. Flows less than 8.3 cfs or greater than 402 cfs were extrapolated using Johnson’s method to temporarily straighten the rating curve using a log offset (e = -0.2) calculated from the stage- discharge relationship. Tenmile Creek is dominated by reed-canary grass that proliferates in the riparian zone throughout the drainage. In terms of discharge, this means that the stage-discharge relationship shifts every summer as the grass on the banks grows to the point of impeding flow, and again every winter as the grass dies back. During late-fall 2005 and early-winter 2006, the grass die-off resulted in low flows that occurred at lower stages compared to those encountered during the summer. This shift is reflected in Rating Table 6, the transition to which is pro-rated from November 15, 2005 to January 1, 2006, assuming that the bulk of the die-off occurred during this period. Table 6 covers a range of discharge from 8.2 to 804 cfs. Three of the nine discharge measurements used to develop this rating were taken during water year 2006. The measured flows for this rating, ranging from 16.4 to 402 cfs, cover less than 50% of the rating curve. Flows exceeded the measured range of flows 25% of the time while this rating was in effect. Twenty-five percent of flows were less than the lowest measured flow, and no flows were greater than the highest measured flow. Flows less than 16.4 cfs or greater than 402 cfs were extrapolated using Johnson’s method to temporarily straighten the rating curve using a log offset (e = -19.9) calculated from the stage-discharge relationship. During late-spring and early-summer 2006, the grass grew on the banks, once again impeding flows and resulting in low flows that occurred at not only higher stages than during the winter, but at higher stage heights than the previous summer. This shift is reflected in Rating Table 7, the transition to which is prorated such that discharge predicted by the rating curve on August 8, 2006 matches the discharge measured on that date. Table 7 covers a range of discharge from 2.2 to 804 cfs. Three of the ten discharge measurements used to develop this rating were taken during water year 2006. The measured flows for this rating, ranging from 4.4 to 402 cfs, cover less than 50% of the rating curve. Flows exceeded the lowest measured flow 30% of the time during water year 2006. Flows less than 4.4 cfs or greater than 402 cfs were extrapolated using Johnson’s method to temporarily straighten the rating curve using a log offset (e = 1.7) calculated from the stage-discharge relationship. The station finished the water year in transition to rating table 8, which reflects a further “filling” of the rating curve, likely caused by beaver dam-building activity in the late-summer and early- fall. Rating Table 8 will be discussed in the technical notes for water year 2007. As part of the rating curve review process for water year 2006 for this station, historic rating curves for water years 2003-2005 were recalculated using current methods, resulting in changes to the previously reported discharge record for those years. The calculated potential error for flows derived from rating curve used during water year 2006 for this station is ±11% for Rating Tables 5 and 6, and ±14% for Rating Table 7. Stage Record The station logged continuously throughout water year 2006 without interruption. The staff gage at this site is generally readable to within 0.01 ft throughout the range of flows. Conditions surrounding the terminal end of the bubbler orifice are very similar to those around the staff gage. The stage height readings during the high flow season typically differed from manual staff gage readings by 0.01-0.03 ft, and were as high as 0.06 ft. During the summer of 2006, the differences were only as high as 0.02 ft. Time-weighted corrective adjustments were made to the continuous stage record whenever there was a difference between logged and measured stage values. All adjustments are documented in the Hydstra Data Workbench. The calculated potential error of the continuous stage data for this station is ±2%. Future Efforts Ongoing frequent discharge measurements are necessary at this site in order to adequately capture the seasonal changes in the stage-discharge relationship.