Extended record — reconstructed, not measured

This gage's own record starts in 1985. A retired gage nearby, NF SOUTH PLATTE R BELOW GENEVA C, AT GRANT, CO., ran earlier and overlapped it, so its measurements can be translated onto this gage's scale to extend the picture back roughly 47.7 years. The result before 1985 is a reconstruction. It is not what this gage measured, because this gage did not exist.

Donor gage	NF SOUTH PLATTE R BELOW GENEVA C, AT GRANT, CO., 0.1 km away
Overlap used to fit	4,643 days, 1985-04-29 to 1998-09-30
Agreement (held out)	r² 0.993
Typical error (held out)	4.9%, bias 3.4%
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)	40 1987–2026	no trend	0.95355	0.915 cfs
NORGENCO alone (measured, no reconstruction)	61 1909–1998	increasing	0.00011	11.372 cfs
Extended (with reconstruction)	87 1910–2026	increasing	4e-05	11.658 cfs

The strongest evidence needs no reconstruction at all. NORGENCO shows increasing across its own 61 measured years (1909–1998, p = 0.00011) — 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 40-year modern one.

The longer record reaches a different conclusion, and the reconstruction's bias does not explain it: a 3.4% bias would tend to manufacture a decreasing trend, which is not what appears. The likeliest reading is statistical power — 40 years cannot detect what 87 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.

Provenance

DWR station abbreviation PLAGRACO; recorded station name “NORTH FORK SOUTH PLATTE RIVER AT GRANT”; 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.