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Discharge.-- PLATTE RIVER BASIN 06714000 SOUTH PLATTE RIVER AT DENVER Location.-- Drainage and Period of Record.-- Lat. 39°45'35",Long. 105°00'10", in NW¼SE¼ sec. 28, T.3 S., R.68 W., Denver County, Hydrologic Unit 10190003, on right bank 90 ft Upstream from Nineteenth Street Bridge in Denver and 0.4 mi downstream from Cherry Creek. 3,861 mi². May 1889 to Oct. 1890 sporadic record. July 1895 to current year continuous. Monthly data only for some periods. Graphic water stage recorder, shaft encoder and Sutron 8210 high date rate DCP in a 72 inch by 72 inch precast concrete structure with a 48 inch corrugated steel well. Primary reference gaeg is an electric drop tape, with a supplemental outside wire weight gage. A city water line is plumbed to the gage for flushing the inlets. USACE is a cooperator on the gage, and UDFCD has Alert instrumentation installed. Additionally, the UDFCD contracts with the USGS to operate a water quality sampler there. Equipment.-- Hydrologic Conditions.-- Gage-Height Record.-- Datum Corrections.-- Rating.-- Discharge.-- Special Computations.-- Remarks.-- Recommendations.-- The primary record is hourly averages of fifteen minute data taken from satellite monitoring with chart backup. Daily maximum and minimum stages for the satellite record checked to within 0.02 ft with the chart. The record is compete and reliable, except for the following periods: December 9-10, 2009, when the record appeared to be affected by ice; and, May 17-18, 2010, when the DCP failed and chart back up data were not available. Levels were last run on September 24, 2009, but were inconclusive due to conflicting readings from RM 1 and RM 2. No correction was made at that time. The control is a rock gabion dam approximately 50 feet below the gage. Rating No. 34 was begun in use on October 1, 2008. It is defined by measurements from 39.4 to 5340 cfs. Rating No. 34 was extended to 12,600 cfs using a peak flow on July 25, 1998 that was indirectly calculated using records from downstream gages. Eighteen measurements (Nos. 985- 1002) ranging in discharge from 103 to 1120 cfs were made this year. A measurement of 1450 cfs was made during the high flow period but was not used. The peak flow of 6170 cfs occurred 1315 on April 23, 2010 at a gage height of 8.57 ft (gage height correction of -0.01 ft applied) with a shift of 0.00 ft. It exceeded Measurement 996, made on June 17, by 3.22 ft in stage. Peak flow at this gage usually defies measurement — often occurring as a sharp, transitory rise in the evening. Shifting control method was used all year. The channel has a sand bottom that is continually scouring and filling in the gage pool formed by the control, causing both positive and negative shifts. Measurements show unadjusted shifts varying between -0.14 to +0.04 feet. All measurements were given full weight, except No. 986, 989-991, 993, 994, 996, 1000 and 1001 which were adjusted up to 3% to smooth shift distribution. The highest measurement (No. 996) was adjusted 2%, and was considered poor. A large gage height change occurred during the measurement due to an interruption for a rescue operation. Shifts were prorated by time, considering movement of sand to be the primary shift mechanism. Unadjusted high flow measurements indicate possible negative shifting at higher stages, but reliability of the high measurements was not considered good enough to justify using stage-shift tables. Also, the rating is new, and time shifting puts the peak closer to the rating. Stage tables may be used in future years if other measurements support the trend hinted at in WY2010. In general, consideration to stage is given when time shifting at this gage, in order to give weight to effects of storm peaks. While no special shifting was done this year, each storm event was analyzed to verify that the peak flow had a shift that was consistent with any possible stage distribution. The “spill”, as added to some of the measurements in the past, is the regulating discharge from the Farmers and Gardeners Ditch. The spill is just upstream and across the channel from the gage. The ditch and associated Parshall flume are covered and buried, so the spill emerges from a culvert. Normally there is no place below the spill where a good measurement can be made, so upstream measurements are made with this “Spill” added to the total. There wasn’t any spill during measurements this year, so none was added. It is impossible to actually measure the spill since it shoots out from under a gate and sluices down to the river. Velocities are supercritical and the flow has air in it. Often 10 cfs is estimated, based on a ditch rider statement that the head-gate will take a maximum of 35 cfs from the river and the ditch is decreed at 24 cfs. At low flow, this estimate can have a significant effect on measurement accuracy. Extreme low flow measurements have been made downstream of the Farmers and Gardeners Ditch spill and just upstream of the control pool. Normally this section is too deep to wade. Discharge for December 9-10 was estimated from adjacent non-ice affected record. Discharge for May 17-18 was estimated from good record trends before and after the period of missing data. The record is good, except for periods of ice effect and missing record, which are estimated and poor. Station maintained by Jana Ash and Tony Arnett and record developed by Tony Arnett. Run levels in the spring of 2011 and summarize past observations in a levels summary spreadsheet. 2010Water Year