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Big Thompson River

Division 1 · Water District 4 · As of 2026-09-26 05:20 MDT · 2 weeks ago
Live · machine-built

Big Thompson River on the ground — the ditches, wells, reservoirs and gages of Water District 4 · clicktap anything for its record

Browse this district’s ditches, gages, reservoirs, wells and springs as a list.

24% of normal flow, week of 2026-09-21
1 call the record places on it, 6 unsettled

The Big Thompson River is critically low for late September -- 24% of normal flow. 1 call stands against it right now; the most senior right calling dates to 1873. DWR's statements do not settle whether 6 more calls reach it. This year's runoff outlook, issued 2026-06-01, called for 61% of normal.

Snow and runoff

Source Independent agency measurement · fetched 2026-10-12 · verify at NRCS report generator ↗ Source Our computation, scored against a reference · NRCS forecast, graded here against the gage record

Snow-water equivalent, WY2027

A trace of snow at 2 of 2 sites (0.1 in of water at most); the median for the date is zero, so there is no percent to show yet. The snow year begins in October. The new water year began October 1; the chart fills in a day at a time as NRCS publishes each reading.

SNOTEL siteElev (ft)SWE (in)Median (in)% of medianTypical peak
Bear Lake9490 0.10.0 — April 27 (19.8 in)
Willow Park10710 0.10.0 — April 29 (21.0 in)

NRCS SNOTEL, daily. Sites are assigned to the district whose boundary contains them — the snow that feeds this district can also sit in a neighboring headwater district.

Two ways a snow year goes wrong (1981–2026)

A bad snow year here comes in two flavors (both called snow droughtA winter that shorts the snowpack. It happens two ways: a dry one, where the snow never comes, and a warm one, where near-normal snow falls but melts off weeks early -- and the river feels them differently.), and the river feels them differently. In 6 of the last 46 winters the snow never came — the pack peaked below 80% of its median. In 1 more, the snow came but left early — a near-normal peak melted out two weeks or more ahead of its usual date, sending the water down before the season needed it.

The snow never came: 1987, 1991, 1992, 2002, 2012, 2026. The snow left early: 1981.

Computed by this site from 2 SNOTEL sites’ full daily records (NRCS AWDB, back to 1981). Each year is measured against each site’s own multi-decade median — not the 1991–2020 normal the chart above uses — so the 1980s grade against a fair baseline. Peak and melt-out need 85% of the season’s days on the record; thin years are dropped, not guessed. Derived from snow measurements alone — it cannot tell dry-from-warm causes apart, only what the snowpack did.

Runoff outlook — Big Thompson R at Canyon Mouth

NRCS's final issue (2026-06-01) calls it even odds that at least 48 kAF arrives over 06-01 to 09-30 — 61% of a normal season; almost certainly at least 38, probably not more than 62. New forecasts begin in January. How to read these numbers →

90% chance of at least70% chance of at least50% chance of at least30% chance of at least10% chance of at leastNormal
3843485362 79

How good are these forecasts here? (1951–2012)

Grading every April issue against what the river then delivered, measured at this gage’s own record — 61 seasons: the median miss of the 50% forecast is 78%, with a high bias of 78%, and the observed volume landed inside the stated 10–90% band 16% of the time (a calibrated forecast would say 80%).

Computed by this site: NRCS's archived April issues (their period as issued each year) against the observed volume in DWR’s published record; years with under 90% daily coverage are dropped, not summed. A grade of the forecast, never of the forecasters — these are hard rivers. How this point ranks among every graded Colorado forecast point →

What April snow has meant for this river (1981–2013)

Across 33 measured years, the season’s water has loosely followed the April snowpack: a year opening April with 80% of median snow has typically gone on to deliver about 44% of a normal season (a snow-to-runoff elasticityHow strongly one number moves with another. A snow-to-runoff elasticity of 1.5 means a 10% shortfall in April snowpack has typically become a 15% shortfall in the season's water. of 1.2). In warm, early-melt years the river delivered a median of 41% of a normal season, against 70% in typical years — the timing of the melt matters, not just its size.

Computed by this site: the district’s April 1 SWE (% of each SNOTEL site’s own-record median) against the observed April–July volume at this gage (% of its median season) — a fixed window every year, unlike the forecast grades above, which use each issue’s own period. The slope is fit so single wild years can’t steer it (Theil–Sen, log space); years missing 10% of their days are dropped.

Drought

Source Our computation, no independent reference · computed here from the screened DWR record Source Independent agency measurement · fetched 2026-10-06 · verify at U.S. Drought Monitor ↗

Right now

This is the 24th-driest week 39 in 101 years of record at BIG THOMPSON RIVER AT MOUTH NEAR LA SALLE (as of 2026-09-21; this week’s flow sits at the 24th percentileRank against the record: a river at its 5th percentile is lower than it was in 95 of the past 100 years on this same date. for the date). On this week’s US Drought Monitor, Larimer County is 26% in D4 exceptional drought (2026-10-06). The law agrees: the most senior active call leaves 100% of this district’s decreed flow rights junior to the calling right this week.

Every year since 1914, week by week

Each thin row is a year at BIG THOMPSON RIVER AT MOUTH NEAR LA SALLE; each cell is one week’s flow ranked against the same calendar week of every other year — warm means dry for the date, blue wet, so seasonality cancels and droughts appear as warm bands. Gaps are missing record, never a dry river. Click a year to reconstruct it on the Historical tab.

This year against its analogs

Over the 39 weeks recorded so far, 2026’s weekly trace most resembles 2002 (mean gap 12.9 percentile points; next closest 2008 (14.8), 2006 (15.7)). The analog’s remainder is drawn as context — its ending is what happened to it, not a forecast for this year.

The Drought Monitor’s view since 2000

Mean share of area in each class across Boulder, Larimer, Weld counties — the counties this district’s gages sit in, which overlap the district imperfectly. The Drought Monitor weighs soils, vegetation and local reports; it is CONTEXT beside the river’s own record above, not the water-rights picture.

The law’s own drought gauge

When water runs short, administration answers: calls reach further back in time, and more decreed rights go junior to the calling right. This is that response, weekly since 2004 — the share of this district’s decreed flow rights junior to the most senior active call. Watch it spike in the named droughts below, and linger after the river recovers.

Computed from DWR’s call record (2300 calls touching this district) against the district’s rights index. Scoping is deliberately coarse — a call’s own district, or districts named in its comments — and this is administration’s RESPONSE, not a per-structure verdict (each dossier’s curtailment card does that, with stream miles). A deepening call can also reflect growing demand, not only shrinking supply.

1569–1999: this river before anyone measured it

Tree rings record wet and dry years centuries before the first gage. This is the published reconstruction of annual flow at Big Thompson River at Canyon Mouth — 431 years, with its uncertainty band drawn, not implied — and the green line is the measured record at the same gage, water-year totals from DWR’s daily data, carrying the story through 2013. A reconstruction is an estimate, not a measurement: it answers “is this decade unusual in four centuries?”, and nothing in this site’s statistics uses it.

Why the two lines disagree on purpose: the trees were calibrated against naturalized flow — upstream diversions and storage added back — while the gage measures what the ditches left in the river. So the measured line running below the reconstruction is not an error; the gap is, roughly, human use. Over the 66 years both series cover, the river carried a median of 80147% of its estimated natural flow past this gage. And because the reconstruction was fit to gage-era data, the overlap shows how well trees track the river (the authors report the models explain 63–76% of gage variance) — it is a calibration, not an independent test.

Woodhouse, C.A. and J.J. Lukas (2006), Multi-century tree-ring reconstructions of Colorado streamflow for water resource planning, Climatic Change 78; data via NOAA NCEI World Data Service for Paleoclimatology / TreeFlow. Source data ↗

This basin’s droughts, by its own record

DroughtSpanWeeks Trough percentileFlows recovered Water table back
the 2020-22 drought 2020-12-21 → 2021-05-24 22 25th (2021-03-01) 5 weeks —
the 2012-13 drought 2013-01-14 → 2013-07-29 28 20th (2013-03-18) 10 weeks —
unnamed dry spell 2007-09-17 → 2009-08-17 100 21st (2009-03-23) 25 weeks —
unnamed dry spell 2006-07-10 → 2007-04-16 40 19th (2006-09-04) 147 weeks —
the 2002 drought 2004-03-01 → 2004-09-13 28 21st (2004-05-03) 1 week —
the 2002 drought 2002-04-15 → 2003-07-07 64 11th (2002-12-23) 6 weeks —
the 1950s drought 1956-01-23 → 1957-06-10 72 15th (1956-04-02) 2 weeks —
the 1950s drought 1955-01-03 → 1955-09-26 38 14th (1955-02-14) 91 weeks —
the 1950s drought 1954-03-22 → 1954-09-13 25 22nd (1954-06-07) 145 weeks —
the 1950s drought 1950-01-02 → 1951-10-01 91 16th (1951-05-07) 9 weeks —
unnamed dry spell 1948-10-11 → 1949-08-15 44 15th (1949-02-14) 120 weeks —
unnamed dry spell 1944-10-30 → 1947-02-10 119 14th (1946-06-24) 23 weeks —
unnamed dry spell 1943-11-29 → 1944-06-26 30 15th (1944-03-13) 160 weeks —
the 1930s Dust Bowl drought 1939-08-07 → 1942-06-22 150 3rd (1940-01-08) 3 weeks —

Detected mechanically: the 26-week smoothed percentile below the 30th for at least 20 weeks; recovered on regaining the 50th. The water-table clock compares each monitoring well against its own pre-drought median (the 3 years before onset) and dates the year the median well got back — the aquifer remembers longest, and “still below” is a finding, not a gap. The knobs are stated because the episode list depends on them — a different reasonable choice merges or splits episodes at the edges. Names are attached where an episode overlaps a drought Colorado already talks about.

What drought has cost here

Since 1991, federal crop insurance has paid $38,748,835 in drought-caused claims across Boulder, Larimer, Weld counties — 15.2% of all insured crop losses there. The worst year was 2022 ($8,829,713).

USDA crop-insurance indemnities where the recorded cause of loss is drought, summed over the counties this district's gages sit in. Counties overlap water districts imperfectly, and insured-crop claims are a floor on drought damage, not a total. Source: USDA RMA cause-of-loss files.

When this basin claimed its water

Every bar is a year’s new appropriations (1,087 rights across 102 years of shared record), colored by that year’s wetness from the barcode above. Here the correlation is +0.17 — essentially no wet-dry pattern; here, law and settlement set the pace, not the weather. Across the state both behaviors exist: ephemeral southwest basins file after wet demonstrations, transbasin-source basins under drought — and the biggest statewide filing waves sat in the 1950s drought and after 2002, not in the wet 1980s–90s.

Appropriation dates are the CLAIMED initiation of a right, not the courthouse filing date; the record is survivor-biased (abandoned claims are absent, so failed dry-era scrambles are undercounted); and the big 1960s and 2000s spikes sit on legal events — the pre-1969-Act adjudication era and 2000s legislation — not on weather. A correlation here is a description, never a cause.

New to how a dry winter becomes a call on the river? Anatomy of a drought walks the chain in four minutes — snow to rivers to reservoirs to calls to the water table.

Snowpack — no snow on the ground at 2 SNOTEL sites — normal for the date; the snow year begins in October and typically peaks around April 27. Runoff outlook — Big Thompson R at Canyon Mouth: even odds of at least 48 kAF over 06-01 to 09-30 — 61% of a normal season, per NRCS, issued 2026-06-01 — the season’s final; new forecasts begin in January. Full snow & runoff picture — charts, forecast history and how the forecasts have verified →

Sites, exceedance bands and sources
SNOTEL siteElev (ft)SWE (in)Median (in)% of medianTypical peak
Bear Lake9490 0.10.0 — April 27 (19.8 in)
Willow Park10710 0.10.0 — April 29 (21.0 in)

Big Thompson R at Canyon Mouth — 06-01 to 09-30 volume, thousand acre-feet (chance the volume is AT LEAST this):

90%70%50%30%10%Normal
3843485362 79

Snow-water equivalent and medians (1991–2020) from NRCS SNOTEL; seasonal volume forecasts from the NRCS National Water and Climate Center. Sites and forecast points are assigned to the district whose boundary contains them — snowpack that feeds this district can also sit in a neighboring headwater district. Toggle the Snow chip on the map above to see the sites.

Topology cross-check: 9 parent disagreements, 9 sizable streams not on this network (vs DWR's route framework — under review)

This flow network was built mechanically; DWR's Source Water Route Framework disagrees with it in the places below. Each row is a question a human reviewer has not yet answered — not an error verdict. Checked 23 streams, 14 agree. Know this water? Tell us →

StreamThis site routes it toDWR routes it to
Twin Sisters Creek the mainstem West Fork Little Thompson River
North Fork Fish Creek the mainstem Fish Creek
Fish Creek the mainstem Buckhorn Creek
Stove Prairie Creek the mainstem Buckhorn Creek
Dry Creek the mainstem Cedar Creek
Wind River the mainstem Aspen Brook
Dry Creek the mainstem Little Thompson River
Dry Creek the mainstem Little Thompson River

Sizable streams DWR maps here that this network does not carry:

Cedar Creek (11 mi) · Glacier Creek (10 mi) · Ryan Gulch (6 mi) · Dry Gulch (5 mi) · Thornton Gulch (5 mi) · Hayden Creek (4 mi) · Aspen Brook (4 mi) · Spruce Creek (4 mi)

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