01Q070 - Dakota Creek @ Giles Rd. Technical Notes: 2007 Water Year Chuck Springer The instantaneous stream gaging station on Dakota Creek at Giles Road operated throughout water year 2007. During the water year, nine discharge measurements were made and 30 discrete manual stage readings were taken at this station. Rating Curve This station began water year 2007 in transition from Rating Table 3 to Rating Table 4 due to the construction of a beaver dam downstream of the gage. Table 4 covers a range of discharge from 1.2 cubic feet per second (cfs) to 974 cfs. Three of the 15 discharge measurements used to develop this rating were taken during water year 2007. The measured flows for this rating, ranging from 2.4 to 487 cfs, cover only 50% of the rating curve. However, only one of the discrete observations exceeded the lowest measured flow for Rating Table 4. The rating curve was interpolated between discharge measurements and extrapolated to half the lowest measured flow and twice the highest measured flow using Johnson’s method to temporarily straighten the rating curve using a log offset (e=3.4) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±15%. Sometime during winter 2007, the beaver dam was broken apart by high flows, shifting the stage-discharge relationship. This shift is represented by Rating Table 5. Not knowing which of the many winter storm events caused the shift, Table 5 is “phased in” from January to April. Table 5 covers a range of discharge from 0 to 974 cfs. Six of the 15 discharge measurements used to develop this rating were taken during water year 2007. The measured flows for this rating, ranging from point of zero flow (PZF) to 487 cfs, cover only 50% of the rating curve. However, none of the discrete observations exceeded the measured range of flows while Table 5 was in effect. The rating curve was interpolated between discharge measurements and extrapolated to an estimated PZF using Johnson’s method to temporarily straighten the rating curve using a log offset (e=2.5=PZF) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±21%. During summer 2007, a gradual “filling” occurred due to beaver activity downstream of the gage. This shift is represented by Rating Table 6, which is “phased in” between early July and late August 2007. Table 6 covers a range of discharge from 1.4 to 974 cfs. Two of the eight discharge measurements used to develop this rating were taken during water year 2007. The measured flows for this rating, ranging from 2.8 to 487 cfs, cover only 50% of the rating curve. However, only two of the discrete observations exceeded the lowest measured flows while Table 6 was in effect during water year 2007. The rating curve was interpolated between discharge measurements and extrapolated to half the lowest measured flow using Johnson’s method to temporarily straighten the rating curve using a log offset (e=2.5) calculated from the stage- discharge relationship. The potential error for flows derived from this rating curve is ±13%. Stage Record The stage record for this station is comprised of discrete (instantaneous) staff gage readings taken during weekly visits. A linear regression of staff gage readings versus depth-to-water surface, or “tape down” readings taken from a reference point on the Giles Road Bridge yielded one outlier during water year 2007. It was determined that the staff reading was most likely erroneous; however, because this could not be determined conclusively, the staff gage reading on the date in question was flagged as an estimate. In addition, three of the discrete staff gage observations taken during water year 2007 were qualified as estimates due to the staff gage being dirty and difficult to read. Future Efforts This station tends to see beaver activity every summer, followed by washouts of those dams during winter storm events. Ongoing frequent discharge measurements and, when possible, point of zero flow measurements, will be necessary to monitor these shifts.