DWR_2669034

• Title: STREAM FLOW DATA FOR COLORADO
• Year: 2010
• Document Type - Publications and Reports: Streamflow Report

Discharge.--
PLATTE RIVER BASIN
06707501 SOUTH PLATTE RIVER BELOW STRONTIA SPRINGS
Location.--
Drainage and Period of Record.--
Lat. 39°26'00",Long. 105°07'30", SW¼SW¼ sec. 16, T.7 S., R.69 W., Douglas County, on right bank 1/4 mi downstream from
Strontia Springs Dam.
2596 sq mi. 1983 to present.
Graphic water stage recorder, satellite monitoring data collection platform with digital shaft encoder installed in a formed
concrete shelter. An adjustable reference point with a graduated tape on the float drive for the recorder is used for
referencing. An adjustable reference for a drop tape is located below the floor hatch to the well. An outside staff gage is
used as an additional reference. The gage and well are one-piece cast concrete, set on bedrock.
Equipment.--
Hydrologic Conditions.--
Gage-Height Record.--
Datum Corrections.--
Rating.--
Discharge.--
Special Computations.--
Remarks.--
Recommendations.--
The flow is controlled by reservoir releases most of the time, including flows released from further upstream at Cheeseman
Reservoir and Robert’s Tunnel. The flows will reflect extreme basin conditions when the reservoirs are very low or
completely full. Strontia Dam is approximately 1500 feet upstream. The record usually runs in steps from the releases. A
period of free river occurred in June this year during which Strontia gates were closed and the reservoir spilled all inflow.
Primary record is hourly averages of 15-minute satellite data with chart data used for backup. Record is complete and
reliable, except for the following days when the inlets were partially plugged: April 2, 3; May 12; June 6-8. The encoder
maintained calibration so that no corrections were necessary to the record.
In the past, it was believed that since the gage is cast on bedrock, no RP to BM checks were needed because the most
stable BM sites would be all attached to the gage. Levels were first run on September 23, 2009. Three reference marks
were established. No corrections were made.
The control is a boulder and cobble riffle channel. The grade drops approximately 170 feet below the gage (just below the
cableway). The riffle below the cableway is considered to be the major control for flows under approximately 800 cfs. At
flows above approximately 800 cfs the entire channel becomes the control. Rating No. 4, dated March 19, 2008 was
continued for 2010. It is defined by measurements to 1670 cfs. Frequent measurements at high flows are needed since the
channel does change, but additional measurements are particularly desirable around 1000 cfs, as computed flows in this
range sometimes do not balance well with downstream gages. Twenty measurements (457-476) were made ranging in flow
from 32.7 to 946 cfs. They cover the range in discharge except for lower daily flows on January 29 and February 5; and
higher daily flows on June 13-15, 2010. The peak flow of 1010 cfs occurred at 1745 on June 13, 2010 at a gage height of
5.29 ft. with a shift of -.03 ft. It exceeded the stage of high measurement No.468 made June 13, 2010 by 0.08 ft.
Shifting control method was used all year. Shifts at low and medium flows are caused by scour and fill through the section
control below the gage. High flow shifts are influenced by downstream channel gradients and impedance factors.
Measurements showed shifts ranging from -0.04 to +0.05 feet. Measurements 462, 467, 468, 470, and 475 were
discounted 7%, 2%, -1%, 2% and -4% respectively to smooth shift distribution. Measurement 464 was discounted due to a
follow-up measurement. Shifts were distributed by time with consideration of stage for the periods: October 1-March 31,
and Sept 16-30. Shifts were distributed by stage using three variable stage shift relationships: March 31-April 30 (variable
shift table 1 based on Msmts. 462-465, and high flow Msmts. 468, 469 and 472). April 30 was an event peak used for table
transition. April 30-August 9 (variable shift table 2 based on Msmts 466-472, with Nos. 467, 470 and 472 being adjusted up
to 2% to fit the table). August 9- September 16 (variable shift table 3, based on Msmts: 472-475, with No. 475 being
adjusted 4% to fit the table).
Estimates for the days of partially plugged inlets were computed using known gate change times and the step-change
pattern for releases. Strontia Springs Reservoir typically releases constant amounts for long periods of time and this helped
to confirm the record. Also, the Strontia reports show max/min temperatures just below their Strontia Outflow numbers and
this spares the need to examine the chart closely. The Caretakers at Srontia Springs Dam rely heavily on the correlation of
electrical output of their generators to cfs values downstream.
The record is considered good, exception for estimated days of April 2, 3, May 12, and June 8, which are considered fair;
and June 6 and 7 which are poor. Station maintained by Jana Ash and Tony Arnett and record developed by Tony Arnett.
The Strontia—Chatfield gages need to be measured with the highest possible accuracy. Otherwise the shifts can cause
bad water balances and lots of complaints. These gages need to be operated by very experienced personnel who are
familiar with stage-shift relationships and the diversion flows that are balanced by the gage figures. The stilling well of this
gage needs to be inspected for excessive sediment accumulation as there seems to be a sluggish reaction to gate changes
of the Strontia Springs Dam. More frequent intake flushes also may be required to address the stilling-well response to
changes in water levels. The description needs a photo showing the float tape and indicator as the primary reference.
2010Water Year