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Discharge.-- <br />PLATTE RIVER BASIN <br />SOUTH PLATTE RIVER BELOW CHATFIELD RESERVOIR <br />Location.-- <br />Drainage and Period of Record.-- <br />Lat. 39°33'45",Long. 105°03'35", SE¼, sec. 1, T.6 S., R.69 W., Jefferson County, Hydrologic Unit 10190002. <br />3,018 mi². 1985 to present. <br />Graphic water stage recorder and satellite monitoring DCP and shaft encoder in a formed concrete well and shelter. The <br />DCP is owned and maintained by the DWR. An electric drop tape is used for referencing. There is no outside reference. <br />A cableway for high flow measurement is located just upstream of the gage. <br />Equipment.-- <br />Hydrographic Conditions.-- <br />Gage-Height Record.-- <br />Datum Corrections.-- <br />Rating.-- <br />Discharge.-- <br />Special Computations.-- <br />Remarks.-- <br />Recommendations.-- <br />Native flow is regulated by Chatfield Reservoir, as well as by many reservoirs further upstream. Releases from Chatfield <br />during flood events are regulated to limit the total flow at the Henderson gage (downstream from Denver) to 5000 cfs. <br />There are no minimum streamflow requirements for Chatfield releases. On many days the reservoir outlets are completely <br />closed and flow at the gage consists of tiny amounts of seepage or gate leakage. <br />The primary record is hourly averages of 15 minute satellite data with chart back up. The record is complete and reliable. <br />Daily max and min values agreed with the corrected chart values to within +-0.02 foot. One (+.01 ft) correction to the <br />encoder data was applied. <br />Levels were run on Sept. 23, 2009 using RM No. 1, and showed the new elevation of the RP to be 0.12 ft higher, at 20.09 <br />ft, which was also the case for levels run July 26, 2006. No corrections were made, as shifts are close to zero. <br />Investigation needs to be done to determine if the 20.09 ft reference elevation needs to be updated. <br />The control is a large sloped concrete dam approximately 50 feet below the gage, with a stilling basin that is about 1000 ft. <br />long back to the outlet pipe. Aside from some possible negative shifts due to anchor-type moss at low flows, very little <br />should affect the rating for this structure. Unfortunately, measurement conditions near the gage are not good. The channel <br />is very rocky and flow is deep and often extremely slow. The initial and subsequent ratings have incorporated a lot of <br />scatter in the measurements. Variations in shifts seen since 1987 are probably more due to measurement error than to <br />conditions affecting the control. In November of 2007, the gage pool was drained and rocks under the cableway <br />measurement section were removed for a more consistent measurement section. At this time a pzf was observed to be <br />0.27 ft. A small degree of systematic rating error due to stage-shifts is also likely to still exist in the current Rating No. 3. If <br />enough measurements are made with the highest possible precision, it should be possible to develop a table that does not <br />require shifts. Once such a table is established, it would be better not to measure the gage at all unless the measurements <br />are done with roughly twice as many sections as normal and with the best possible meters. Rating No. 3 was continued in <br />use this year and is well defined to 2500 cfs. Thirteen measurements (No. 390-402) were made this year, ranging in <br />discharge from 25.0 to 1760 cfs. These measurements contain the range in flow with exception of low flow days on Oct. 1, <br />2008-Jan. 8, 2009; Jan. 21-Feb. 23; March 3-10, 14-26, 31; April 1-3, 9-16; May 26-27, Aug. 25-26; Sept. 2-13, 27-30 and <br />high flow days of June 4-5. The peak flow of 2060 cfs occurred at 1300 June 4, 2009 at a gage height of 5.13 ft.with a <br />shift of 0.00 ft. It exceeded the stage of measurement no. 395 made June 4, 2009 by 0.27 ft. <br />Shifting control method was used all year. However, the control is a fairly massive structure and if kept clean of moss <br />would have no source for shifting. The main source of shifts at this gage is very likely to be measurement error. This year’s <br />measurements show shifts ranging from -0.01 ft to 0.00 ft. Shifts were distributed by stage using one stage shift table, <br />PLACHACOSC1, for the entire 2009 water year The depths for measurement #395 were adjusted because it was <br />discovered the meter was in the wrong hole when making the cable measurement. This measurement was downgraded to <br />fair. All measurements were given full weight with exception of measurements 390, 396, 401 and 402 which were adjusted <br />slightly to match surrounding shifts. <br />None. <br />The record is good. Station maintained and record developed by Jana Ash. <br />Work should continue on getting a flushing system working at the gage, now that flush water is available from the last <br />chance ditch pipeline. Pro-active measures should be taken with regard to shift variations at high flow: A submergence <br />staff should be installed below the control to check that high flow measurements are not encountering backwater. <br /> Cableway markings should be verified using a tagline at water level and a horizontal tape strung between the A- <br />Frames. Additionally, levels should be run to confirm elevations of the RM’s and PZF, and reconcile any tape length <br />problem. <br />2009Water Year