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Discharge.-- PLATTE RIVER BASIN SOUTH PLATTE RIVER BELOW CHATFIELD RESERVOIR Location.-- Drainage and Period of Record.-- Lat. 39°33'45",Long. 105°03'35", SE¼, sec. 1, T.6 S., R.69 W., Jefferson County, Hydrologic Unit 10190002. 3,018 mi². 1985 to present. Graphic water stage recorder and satellite monitoring DCP with shaft encoder installed in a formed concrete well and shelter. An electric drop tape is used for referencing. There is no outside reference. A cableway is located at the gage. Satellite equipment is owned and maintained by CO DWR. Equipment.-- Hydrologic Conditions.-- Gage-Height Record.-- Datum Corrections.-- Rating.-- Discharge.-- Special Computations.-- Remarks.-- Recommendations.-- Native flow is regulated by Chatfield Reservoir, as well as by many reservoirs further upstream. Releases from Chatfield during flood events are regulated to limit the total flow at the Henderson gage (downstream from Denver) to 5000 cfs. There are no minimum streamflow requirements for Chatfield releases. On many days the reservoir outlets are completely closed and flow at the gage consists of small amounts of seepage or gate leakage. The primary record is hourly averages of 15 minute satellite data with chart back up. Daily max and min values agreed with the corrected chart values to within +-0.02 foot. No encoder data corrections were applied. Record for March 23 and 24 were taken from the chart. The record is complete and reliable. 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 ft, which was also the case for levels run July 26, 2006. No corrections were made, as shifts are correlating close to zero. Further investigation needs to be done to determine if the 20.09 ft reference elevation needs to be updated. The control is a massive sloped concrete dam approximately 50 feet below the gage, with a deep stilling basin that is about 1000 ft. long back to the reservoir outlet pipe. This is a critical head measurement structure with a perfectly stilled pool at low flows. Anchor moss may grow at low flows, but this is easily removed. Measurement conditions near the gage are not good. The channel is very rocky and flow is deep and often extremely slow. The initial and subsequent ratings have incorporated a lot of scatter in the measurements. Variations in shifts at lower (wade-able) flows are probably more due to measurement error than to conditions affecting the control. If enough measurements are made with the highest possible precision, it should be possible to develop a table that does not require shifts, or measurements at lower flows. In fact, shift variations at low flows have at times made administration of the release problematic. Shift variations have been seen for high flow, cable measurements. Possible sources for this could be partial submergence of the control and growth of pond weed in the stilling basin. The extreme low end of the rating has not been verified in a long time. The PZF was observed to be 0.27 ft when the gage pool was drained and cleaned in November, 2007. Seepage flows (usually less than 0.5 cfs) are not measureable, and the near zero portions of the rating have been extensions to PZF from measurements in the 5 cfs range. Flows below 0.5 cfs should be considered estimates. Rating No. 3 was continued in use this year and is well defined to 2500 cfs. Eighteen measurements (No. 403-420) were made this year, ranging in discharge from 27.6 to 1570 cfs. These measurements contain the range in flow with exception of low flow days on Oct. 1-5, 8-12, 29-31, 2009; Nov. 1- 30; Dec. 1, 2, 2009; Jan.1-8; Feb 3-8; Mar 1-4, July 14-18; Sep. 2, 3, 5-9, 13-30, 2010. Measurement 413 captured the water year high flow day of April 27. The peak flow of 1580 cfs occurred at 1115 on April 27, 2010 at a gage height of 4.65 ft. with a shift of 0.03 ft. It exceeded measurement no. 413 (made the same day) by 0.01 ft in stage. Shifting control method was used all year. However, the control is a fairly massive structure and if kept clean of moss should have no source for shifting. The main source of shifts at this gage is very likely to be some degree of measurement error, combined with some degree of stage shifting. Big shifts have been seen at higher flows and are currently under investigation. This year’s measurements show unadjusted shifts ranging from -0.07 ft to 0.03 ft. Shifts were distributed by time with consideration of stage for the periods: October 1, 2009 through April 21, 2010, using Msmts 403-412, which were given full weight, except pairs 408-409 and 411-412, which were adjusted up to 3%. The pairs were training measurements made at the same time and were adjusted so that a single shift was used for each pair. And, June 1- September 30, using Msmts 414-421, with Nos. 419 and 420 being adjusted 2% to smooth distribution. Shifts were distributed by stage using two variable stage shift relationships: PLACHACO10-1 based on Msmts 412-413, both given full weight, and applied April 21-27, 2010. This table defined the year’s peak flow. And, PLACHACO10-2, based on Msmts 413-414, with No. 414 adjusted 2%, and applied April 27-June 1, 2010. The record is good, except for seepage flows below 0.5 cfs which are estimated and poor. Station maintained by Jana Ash and Tony Arnett and record developed by Tony Arnett. Pro-active measures should be taken with regard to shift variations at high flow. A staff should be installed below the control to check that high flow measurements are not encountering backwater. Cableway markings should be verified using a tagline at water level and a horizontal tape strung between the A-Frames. Vegetative growth in the stilling pool should be observed. Levels should be run to confirm elevations of the RM’s and PZF, and reconcile any tape length problems. Finally, the flushing system should be operated at least once, to check the flush water supply plumbed in from the Last Chance Ditch pipeline. 2010Water Year