Extended record — reconstructed, not measured

This gage's own record starts in 1962. A retired gage nearby, PIEDRA RIVER NEAR PIEDRA, CO., ran earlier and overlapped it, so its measurements can be translated onto this gage's scale to extend the picture back roughly 22.9 years. The result before 1962 is a reconstruction. It is not what this gage measured, because this gage did not exist.

Donor gage	PIEDRA RIVER NEAR PIEDRA, CO., 15.3 km away
Overlap used to fit	3,956 days, 1962-09-01 to 1973-06-30
Agreement (held out)	r² 0.955
Typical error (held out)	8.1%, bias -0.3%
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)	60 1963–2026	no trend	0.31054	-14.568 cfs
PIEPIECO alone (measured, no reconstruction)	33 1940–1972	no trend	0.29921	-24.459 cfs
Extended (with reconstruction)	83 1940–2026	no trend	0.6596	-3.882 cfs

Both records reach the same conclusion, over 23 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.

Seasonal runoff outlook

NRCS calls it even odds that this gage carries at least 14 thousand acre-feetA volume: one acre covered a foot deep, about 326,000 gallons -- roughly a suburban household's outdoor use for a year. over 06-01 to 07-31 — 24% of a normal season; almost certainly at least 6, probably not more than 40. Issued 2026-06-01 — the season’s final; new forecasts begin in January.

90% chance of at least	70% chance of at least	50% chance of at least	30% chance of at least	10% chance of at least	Normal
6 kAF	10 kAF	14 kAF	25 kAF	40 kAF	58 kAF

The row is one season as a range of odds (exceedanceA forecast stated as odds of AT LEAST this much: the 90% exceedance value is a near-floor the river beats nine years in ten, the 10% value a near-ceiling it reaches one year in ten, and 50% is even odds -- as likely to come in above as below. values): the left end is a near-floor the river beats nine years in ten, the right a near-ceiling. How to read these numbers → Forecast by the NRCS National Water and Climate Center; shown beside the gage’s own record, which is what it is verified against on the report card.

Provenance

DWR station abbreviation PIEARBCO; recorded station name “PIEDRA RIVER NEAR ARBOLES, 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.