Extended record — reconstructed, not measured

This gage's own record starts in 1957. A retired gage nearby, TENMILE CREEK AT DILLON, CO., ran earlier and overlapped it, so its measurements can be translated onto this gage's scale to extend the picture back roughly 37.0 years. The result before 1957 is a reconstruction. It is not what this gage measured, because this gage did not exist.

Donor gage	TENMILE CREEK AT DILLON, CO., 5.9 km away
Overlap used to fit	1,308 days, 1957-10-01 to 1961-04-30
Agreement (held out)	r² 0.937
Typical error (held out)	11.6%, bias 7.6%
Grade	fair

How this was built, and what it cannot tell you

• The two gages had to overlap. Without shared days there is nothing to measure the relationship against, and no extension is offered at all — that rule rejected the overwhelming majority of candidate pairs.
• The relationship is fitted in log space, because the ratio between two gages changes with flow. On Clear Creek the downstream gage reads 95% of the upstream one at low flow but 88% at high flow, because ditches in between take more when there is more to take. A single scale factor would get both ends wrong.
• It is scored out of sample: fitted on half the overlap, tested on the other half. The numbers above are from the half it never saw.
• Reconstruction never replaces a real observation. Every day in the series is labelled measured or reconstructed, and the overlap itself stays measured.
• The trends published on this site still use the measured record only. This extension is offered for reading the longer shape, not as an input to the statistics.
• DWR's published record is the authority. A reconstruction is an estimate of what this location would have read, not a measurement of it.

What the longer record says

Record used	Years	Trend	p	Change / decade
Measured only (what the site publishes)	69 1958–2026	no trend	0.97106	0.0 cfs
TENDILCO alone (measured, no reconstruction)	40 1911–1960	no trend	0.12124	-5.594 cfs
Extended (with reconstruction)	106 1911–2026	no trend	0.0814	-0.082 cfs

Both records reach the same conclusion, over 37 more years.

Extending changes the conclusion for roughly half the gages that have a reconstruction — which is why both rows are here and neither replaces the other. Every trend published elsewhere on this site uses the measured record only.

The record

Readings come from the weekly-refreshed cache, never fetched live — the date beside the value says exactly how fresh it is. The live gage is one click up.

Provenance

DWR station abbreviation KEYGUDCO; recorded station name “KEYSTONE GULCH NEAR DILLON, CO.”; operated by USGS. Daily flows from the DWR HydroBase snapshot plus CDSS REST updates, refreshed on the weekly cycle. Trend statistics are Mann-Kendall (Hamed-Rao corrected) with Sen's slope in native cfs, the same numbers as the basin Trends tab.