Extended record — reconstructed, not measured

This gage's own record starts in 1996. A retired gage nearby, ALVA B. ADAMS TUNNEL AT EAST PORTAL NEAR ESTES PARK, ran earlier and overlapped it, so its measurements can be translated onto this gage's scale to extend the picture back roughly 49.2 years. The result before 1996 is a reconstruction. It is not what this gage measured, because this gage did not exist.

Donor gage	ALVA B. ADAMS TUNNEL AT EAST PORTAL NEAR ESTES PARK, 0.0 km away
Overlap used to fit	10,025 days, 1996-10-01 to 2025-09-30
Agreement (held out)	r² 0.996
Typical error (held out)	0.6%, bias -0.2%
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)	28 1997–2025	no trend	0.67822	5.075 cfs
ADATUNCO alone (measured, no reconstruction)	78 1948–2026	increasing	0.02356	9.279 cfs
Extended (with reconstruction)	71 1953–2025	no trend	0.35364	-2.604 cfs

Both records reach the same conclusion, over 43 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.

What this gage meters

This is not a natural stream record. The Alva B. Adams Tunnel moves water from Colorado River headwaters at Grand Lake (Div 5) into Big Thompson River at Estes Park (Div 1), operated by Northern Water / USBR (Colorado-Big Thompson) — so this record is a ledger of those deliveries. Trends here reflect operational and demand decisions, not the basin's hydrology, and the basin trends page marks it accordingly (⇆). All of Colorado's curated transbasin diversions are collected at /transbasin.

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 ADANETCO; recorded station name “ADAMS TUNNEL AT EAST PORTAL-COMPUTED FLOW”; operated by DWR. 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.