Extended record — reconstructed, not measured

This gage's own record starts in 1940. A retired gage nearby, COLORADO RIVER AT GLENWOOD SPRINGS, CO., ran earlier and overlapped it, so its measurements can be translated onto this gage's scale to extend the picture back roughly 41.2 years. The result before 1940 is a reconstruction. It is not what this gage measured, because this gage did not exist.

Donor gage	COLORADO RIVER AT GLENWOOD SPRINGS, CO., 19.4 km away
Overlap used to fit	9,435 days, 1940-12-01 to 1966-09-30
Agreement (held out)	r² 0.99
Typical error (held out)	4.6%, bias -3.7%
Grade	strong

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)	83 1942–2026	no trend	0.09779	-48.542 cfs
COLAGSCO alone (measured, no reconstruction)	67 1900–1966	decreasing	1e-05	-211.696 cfs
Extended (with reconstruction)	125 1900–2026	decreasing	0.0	-94.409 cfs

The strongest evidence needs no reconstruction at all. COLAGSCO shows decreasing across its own 67 measured years (1900–1966, p = 1e-05) — that is a measurement of this reach, not an estimate. The extended record mostly inherits it, which is why the long view differs from the 83-year modern one.

The longer record reaches a different conclusion, and the reconstruction's bias does not explain it: a -3.7% bias would tend to manufacture a increasing trend, which is not what appears. The likeliest reading is statistical power — 83 years cannot detect what 125 can.

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.

The water itself

USGS keeps a sensor here. Latest readings, with the last 30 days of daily means beside each. Provisional: not yet reviewed by USGS, and revised when it is.

Fetched 2026-10-19 09:20 UTC from USGS NWIS (site 09070500). The USGS page has the full sensor record; every Colorado sensor site is listed on one page.

Seasonal runoff outlook

NRCS calls it even odds that this gage carries at least 165 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 — 21% of a normal season; almost certainly at least 90, probably not more than 295. 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
90 kAF	125 kAF	165 kAF	225 kAF	295 kAF	800 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.

Record-adjusted April benchmark: re-anchoring NRCS’s April number (490 kAF) on this river’s own history moves it to 265 kAF; the river delivered 273. Computed by this site, shown only where it beats NRCS in replay (45% → 24% typical miss over 61 seasons). Method →

Provenance

DWR station abbreviation COLDOTCO; recorded station name “COLORADO RIVER NEAR DOTSERO, 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.