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Discharge.-- PLATTE RIVER BASIN 06695000 SOUTH PLATTE RIVER ABOVE ELEVENMILE RESERVOIR Location.-- Drainage and Period of Record.-- Lat. 38°58'03",Long. 105°34'51", in NE¼ sec. 32, T.12 S., R.73 W., Park County, Hydrologic Unit 10190001, on left bank 200 ft downstream from highway bridge, 2.5 mi upstream from water line of Elevenmile Canyon Reservoir, at elevation 8,561 ft. and 13 mi southeast of Hartsel. 880 mi²; 1933 to present. Stevens A-71 graphic water-stage recorder and Sutron SatLink 2 satellite Data Collection Platform (DCP) with a Sutron 56- 0540 shaft encoder (SE) at a 25-foot concrete Parshall flume. The equipment is in a wooden shelter over a concrete well with inside electric tape gage and a staff gage on the right side at the Ha location. Facilities are owned and maintained by the Denver Water Board. Satellite instrumentation is owned and maintained by the State Engineers Office. Equipment.-- Hydrographic Conditions.-- Gage-Height Record.-- Datum Corrections.-- Rating.-- Discharge.-- Special Computations.-- Remarks.-- Recommendations.-- The gage is approximately two miles below Spinney Mountain Reservoir, and flow is controlled by reservoir releases. A small drainage empties in above the gage that is not controlled by the reservoir. This drainage can contribute significant flow after severe local rain events. The record is flat with step changes. The primary record is hourly averages of 15 minute satellite data with chart back up. The primary record agrees with the chart to within 0.02 ft. Generally releases above about 80 cfs from Spinney Mountain Reservoir keep this gage open year round. Lower flows can see ice affect at the flume in 2 ways: Ice jams downstream can cause backwater into the flume which can result in ice forming on the crest and walls of the flume. Also, ice jams upstream can cause a drop in flow followed by a surge. This can result in an accumulation of ice above the normal water line for that release period, as each brief surge hits the frozen flume walls. In this situation, the baseline GH’s will be lower than the ice layers and be good record, but the higher surge GH’s will be ice-affected. In either case, the GH record will not be the flat release expected and the computed flows will be higher than either Spinney release data or computed good record for that Spinney release period. Ice affects seen on days when a change is made in Spinney release are more complicated since there is a delay between gate changes at Spinney Reservoir and the gage, as return flows are seen following gate change reductions. The record is complete and reliable except for the following periods when the stage discharge relationship was affected by ice: November 15, 24-26, 29, 2008. December 1-6, 9-12, 15-21, 24, 27, 28, 2008, January 5-7, 13, 27-29, 31, 2009. Moss was cleaned from the flume on two occasions causing a drop in gage height following cleaning. Measurements were taken following the moss removal and datum corrections were utilized to account for the moss growth and applied back to the previous measurement on the following dates: November 3, 2008 (-0.01 ft) July 24, 2009 (-0.03 ft) Levels were run on August 27, 2008. No instrument corrections were needed. The control is a 25 ft. concrete Parshall flume. A standard rating has been used since the flume was installed in 1940. Rating No. 15, dated Oct. 1, 1970 is an expansion of the standard equation. Any flow by-passing the flume would be accounted for separately and not computed as part of this rating. Shifts are caused by moss growth at low stages and by approach conditions at higher stages. Eighteen measurements (Nos. 840 - 856) were made this year, ranging in discharge from 38.5 to 458 cfs. Peak flow of 475 cfs was recorded at 1730 June 3, 2009 at a gage height of 2.70 ft. with a shift a 0.04 ft. It exceeded measurement number 840 made that same day by 0.13 ft. in stage. Using the USBR "Water Measurement Manual" Third Edition, a 25' Parshall Flume has an accurate measuring range from 15 cfs to 1200 cfs. Anything above or below this range is outside the +/- 5% accuracy. All flows during the water year were within this range. Shifting control method was used all year. Measurements show shifts ranging between -0.10 and 0.04 ft. Shifts were applied as follows: October 1, 2008 to June 3, 2009: Stage-shift table PLAHARCOVST1 was used. Measurements 840- 849 made during the period were all used, No. 842 being adjusted 1% to fit the table. June 3 to June 24, 2009: Shifts were run by time with consideration to stage, holding the +0.04 shift from June 3 to June 10. June 24 to July 24: Shifts were run by time to account for moss growth. Flume was cleaned on July 24, and gravel traveling through the flume was observed to be reducing moss effects. July 24 to September 30, 2009: Stage shift table PLAHARCOVST1 was again used. Measurements 853-856 made during the period fit into the table when Nos. 853 and 855 were adjusted 2% and 4%. Due to the filling of the weir pool following peak flows for the year, velocities have increased and become inconsistent in the measurement section of the flume. Measurements 855 was adjusted less than five percent to match the release from Spinney Mountain Reservoir. The discharge at the time was known to come from the new 42” mag meter which has proven accuracy. Extremely high flows can bypass the gage through a culvert to the south. This did not occur this year. Shifting by time with consideration to stage between measurements 849 (June 3) and 850 (June 24) was actually accomplished by extending the shift table used prior to June 3 through to June 10. It was assumed that the moss effects increased as the stage decreased below the peak levels measured on June 3. Many ice days were estimated without loss accuracy, since these ice days fell in periods when the release by Spinney Reservoir remained constant. The remaining ice days were estimated from adjacent reliable record. Record is rated good, except for periods of ice, which are considered fair: November 30, December 7, 14, 23, 31, 2008, January 4, 10, 15, 16, 18, 21, 2009. Station maintained and record developed by Mike Wild. Weir pool cleaning has been scheduled for early 2010. 2009Water Year