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Discharge.-- PLATTE RIVER BASIN 06724000 SAINT VRAIN CREEK AT LYONS, CO Location.-- Drainage and Period of Record.-- Lat. 40°13'05",Long. 105°15'34", in NW¼NW¼ sec. 20, T.3 N,, R.70 W., Boulder County, Hydrologic Unit 10190005, on left bank 75 ft southwest of U.S. Highway 36 (State Highways 7 and 66) at southeast edge of Lyons, 400 ft upstream from St. Vrain Supply Canal, and 0.4 mi downstream from confluence of North and South St. Vrain Creeks. 212 mi². Aug. 1887 to Sep. 1891, June 1895 to current year. Monthly only data for some periods. Water quality data available from Oct. 1977 to Feb. 1981. On March 23, 2003, the gage was moved approximately 0.2 mi upstream. In the new location, the gage is above the Supply Ditch diversion, whereas the old location was below this diversion. Graphical water stage recorder and an incremental shaft encoder connected to a high data rate Data Collection Platform (DCP) in a 6-foot by 6-foot wooden shelter* with 42-inch precast concrete stilling well upstream of a low head concrete diversion dam. An Electric Tape Gage (ETG) located on the instrument shelf is the primary reference with a supplementary cantilever chain gage located 10 feet downstream of the shelter. The stilling well is connected to the channel via four 2-inch inlets, three of which are equipped with valves and flushing equipment. A secondary shaft encoder is installed on the instrument shelf of the shelter. This shaft encoder is used for the Highland Ditch Company’s Supervisory Control and Data Acquisition (SCADA) system. This instrument is maintained by the Colorado Division of Water Resources (DWR) and operated such that the instruments stage reading is set to the base gage stage plus or minus last measured shift. *On May 27, 2009 the timber shelter was replaced with a 6-foot by 6-foot exposed aggregate precast concrete building . Equipment.-- Hydrographic Conditions.-- Gage-Height Record.-- Datum Corrections.-- Rating.-- Discharge.-- 216 square miles of drainage from USGS topographic maps; mainly comprised of forested and grassy areas with varying topography. Gage is located below the confluence of the south and north forks of the Saint Vrain and below most of Lyons Colorado. Beaver Creek and Button Rock Reservoirs are upstream of this gage as well as numerous other diversions of varying magnitude. This station is susceptible to rapid increases in stage due to storm runoff events from hardened surfaces within the Town of Lyons, CO. The primary record is hourly averages of telemetered 15-minute data with chart record as backup. Frequent visits show good agreement between primary and backup records, and the ETG. No flush corrections or inlet plugging events occurred this year. However, debris accumulation on the diversion dam was a problem. Accumulated debris was removed from the control on April 6, 2009 resulting in a -0.02 foot change to the gage-height. Otherwise, the gage-height record is complete and reliable except: December 4,5 and 10-13, 2008, February 28, 2009, March 1, March 11-14 and 26-28, 2009, when ice affected the stage-discharge relationship; May 27, 2009, when the gage was offline for 14 hours for shelter replacement and no gage-height information available; and June 3 to 23, 2009, when there was debris accumulation on control, unspecified date debris was accumulated and shed. The gage was closed for the winter from December 14, 2008 to February 25, 2009. Prior to the shelter replacement, levels were last run on September 22, 2006 and found to be within allowable tolerances. Levels were run on May 28 on the new shelter. At that time the gage datum was misinterpreted and the gage was set to read 1.08 ft higher than previously. Levels were run again on September 22, and the gage was found to be reading 1.09 ft high. A correction of -1.09 ft. correction was applied to the record and measurement GH’s from the June 8 measurement to the time of the correction on September 22. A correction of -1.08 ft. was applied to the May 28 record and measurement GH. Between May 28 and June 8, the correction was prorated from -1.08 to -1.09 ft. The 0.01 ft difference was presumed to be due to settling of the shelter on the sealer used between the shelter and the floor. The gage datum was restored to 5301.080 feet following the September 3, 2009 levels circuit. The control for low to mid level stages is a low-head concrete diversion dam for the Supply Ditch approximately 570 feet below the gage. At higher stages the gage reverts to channel control; which, has not been fully defined since the gage was relocated in 2003. The diversion dam and ditch check structure approximately 1000 feet below the gage can gather debris and cause back water conditions at the gage under certain operational circumstances. Fill and scour conditions as well as debris accumulation on the low water control contributed to shifting away from the rating. Rating No. 25 in use since October 1, 2007 is defined by measurements from 11.8 to 976 cubic feet per second (cfs). Twenty-five measurements (Nos. 549-573) were performed this water year ranging in discharge from 21.1 to 758 cfs. Six measurements (Nos. 561- 564, 566, 567) were performed with a Teledyne RD Instruments StreamPro Acoustic Doppler Current Profiler (ADCP). Measurements made this water year cover the range in stage experienced this year well. The peak discharge of 890 cfs occurred at 0515 June 27, 2009 at a gage height of 3.45 feet (datum correction of -1.09 feet applied) with a shift of -0.05 ft. It exceed the stage of Measurement No. 566 by 0.41 ft. Shifting control method was used this year. Unadjusted shifts ranged from 0.06 to -0.09 feet. Shifts were applied as follows: October 1 through December 14, 2008: time dependent shifting as defined by Msmt. Nos. 548-551; February 25 through April 21, 2009: time dependent shifting as defined by Msmt. Nos. 555-557; April 21 through June 3, 2009: stage dependent shifting utilizing variable shift table SVCLYOCOVS09-01, which is based of nine measurements (Nos. 557-560, 564-567). Nos. 557-560 were performed during the period of use and Nos. 564-567 were performed following the period of use but were evaluated to better define the upper regions of the table during the period June 3 through June 23, 2009. Time dependent shifting as defined by Msmt. Nos. 560-564. Nos. 561-563 were performed on June 8, 2009 using the ADCP instrument. Nos. 561 and 562 were adjusted 1% and -2% to a measurement series mean shift of +0.05 feet. The measurements verified a relatively large positive shift change from the preceding and following measurements and associated stage-shift tables. This shift was conjectured to be due to siphonic action at the control, possibly involving light debris accumulation between the May 28 and June 23 measurements. June 3 was taken to be the end point for the previous table and the beginning of time shifting to the June 8 measurement shifts, since June 3 saw a significant stage rise. June 23 through September 18, 2009; stage dependent shifting utilizing variable shift table SVCLYOCOVS09-02, which is based on ten measurements (Nos. 564-573) made during the period of use. Nos. 564, 565, 567, 568, 571 were adjusted 15, -5%, 4%, -1% and -2% respectively to better fit the shift distribution September 18 through the end of the water year: time dependent shifting as defined by Msmt. Nos. 573-574 . 2009Water Year