Big Thompson River

Source Our computation, no independent reference · computed here from the screened DWR record Source Our computation, no independent reference · no independent reference; see /methods

This year, in context

In the weeks around 2026-09-27 (the newest reading on file), flows at the 46 ranked gages are running at a median 70% of their own long-term norm for this window — 32 below a typical year, 14 at or above. Driest relative to its record: BLOWER DITCH, ranking 1st lowest of 16 years.

Gage flow trend & changepoints

Of the 72 gages with enough record, 10 are trending down and 5 up, and 11 carry a detected shift. The clearest is Saint Vrain Supply Canal T.O. to L. Thompson River, its annual mean flow changing -1.86 cfs per decade (-40.1% of its median annual flow) over 43 years (p<0.0001). The other 56 show no clear signal either way.

Drought history, cost, and the four-century context moved to the Drought tab.

⚠️ 1 gage flagged: their records may have quietly shifted against their neighbours, so treat their trends with caution.

56 other gage(s) show no significant trend or changepoint (not shown).

How the measurement network itself changed

This basin was not always measured the way it is today — the API-era record is dense and recent, the early record sparse. 51 gage(s) and 123 structure(s) report in 2026, but structure reporting only reached half its modern level around ~1994. Every historical claim below is checked against this: long-term shifts that co-move with the growth of the network carry an explicit measurement-evolution caution, and abandonment screens whose idleness begins before the modern reporting era are flagged †.

Mass-balance (network closure) trend

Structure divrec-history backfill coverage as of the last check: 5% of 1753 schematic-relevant structures. 0 year(s) clear the 30% coverage gate so far -- the trend activates automatically once qualifying years span a full decade. No action needed; the nightly backfill keeps extending the record.

Reach gain/loss shifts (1950–2026)

Mass-balance kinks computed for every summer month back to 1950 (5701 reach-snapshots) — which reaches gain or lose water, and how that has shifted across decades. Each shift is cross-checked against the observation record above: as more diversions became metered, "unaccounted" water shrinks mechanically, which can masquerade as a hydrologic shift.

• HANSEN FEEDER CANAL LOVELAND TURNOUT: reach shifted from -172 cfs to -21 cfs summer mean (more gaining, p=0.000, 26 yr) [alluvial valley: real alluvial aquifer -- a decades-long shift toward losing is the classic signature of alluvial pumping and urbanization]
• USBR POWER PLANT AT BIG THOMPSON CANYON MOUTH: reach shifted from +221 cfs to +117 cfs summer mean (more losing, p=0.006, 30 yr) [sedimentary plains/foothills: moderate groundwater connectivity -- gain/loss shifts can be real aquifer response; corroborate with nearby well records]
• MINER LONGAN DITCH: reach shifted from -143 cfs to -135 cfs summer mean (more gaining, p=0.043, 11 yr) [alluvial valley: real alluvial aquifer -- a decades-long shift toward losing is the classic signature of alluvial pumping and urbanization] [measurement-evolution caution: this shift co-moves with the growth of the gage/metering network (rho=0.82) -- part of it may be unaccounted water becoming accounted, not hydrologic change]
• RYANS GULCH EXCHANGE DITCH: reach shifted from -60 cfs to -17 cfs summer mean (more gaining, p=0.001, 17 yr) [sedimentary plains/foothills: moderate groundwater connectivity -- gain/loss shifts can be real aquifer response; corroborate with nearby well records]
• WIND RIVER NEAR ESTES PARK: reach shifted from +185 cfs to -41 cfs summer mean (more losing, p=0.009, 65 yr) [mountain crystalline canyon: minimal groundwater storage -- large or shifting unaccounted losses here are more consistent with data/measurement issues or unmodeled plumbing than with aquifer exchange] [measurement-evolution caution: this shift co-moves with the growth of the gage/metering network (rho=-0.54) -- part of it may be unaccounted water becoming accounted, not hydrologic change]
• BIG THOMPSON RIVER AT MOUTH OF CANYON NEAR DRAKE: reach shifted from +222 cfs to -61 cfs summer mean (more losing, p=0.000, 63 yr) [mountain crystalline canyon: minimal groundwater storage -- large or shifting unaccounted losses here are more consistent with data/measurement issues or unmodeled plumbing than with aquifer exchange] [measurement-evolution caution: this shift co-moves with the growth of the gage/metering network (rho=-0.52) -- part of it may be unaccounted water becoming accounted, not hydrologic change]

Groundwater now has its own tab: 91 monitored well(s), water-table trends, stream corridors, and the advisory depletion screen.

Reservoirs now has its own tab: 38 reservoir(s), end-of-year storage back to 1950, click-to-chart histories.

Cross-gage pattern fingerprint

Over the 80-year window with full coverage (1947–2026), the dominant shared pattern across 3 gage(s) explains 45% of year-to-year variance (almost certainly a basin-wide climate/snowpack driver).

BTBLESCO doesn't move with that shared pattern -- a real, localized signal (a new diversion, land-use change, or a gage-specific data problem) worth a closer look, not necessarily noise.

Structures most implicated by reconciliation (as of 2026-09-27)

Which structures' data absorbed the most correction when the current mass-balance snapshot was reconciled -- see the Historical tab for the full picture on any date.

• FT MORGAN OUTING CLUB PL (diversion): +7.4 cfs (20% of that reach's correction)
• ESTES PARK DITCH 1 (diversion): +7.4 cfs (20% of that reach's correction)
• LAKE ESTES (diversion): +7.4 cfs (20% of that reach's correction)
• ESTES PARK DITCH 4 (diversion): +7.4 cfs (20% of that reach's correction)
• OLYMPUS TUNNEL (diversion): +7.4 cfs (20% of that reach's correction)
• LARIMER CO SPRING E (return): -7.4 cfs (17% of that reach's correction)
• LARIMER CO SPRING F (return): -7.4 cfs (17% of that reach's correction)
• DILLE TUNNEL (diversion): -7.4 cfs (17% of that reach's correction)
• ARKINS SUPPLY CNL (diversion): -7.4 cfs (17% of that reach's correction)
• HANSEN FEEDER CNL PUMP (diversion): -7.4 cfs (17% of that reach's correction)

Source DWR administrative record · verify at DWR groundwater wells ↗ Source DWR administrative record · verify at DWR well permits ↗

6,963 water-level measurements across 91 monitored well(s) — 60 alluvial, 5 bedrock — plus 16 administered well(s) reporting metered pumping.

Well map

3D aquifer x-ray

Every aquifer unit as its own head surface, animated through time.

Water-table trends

Of 4 well(s) with enough record for a trend test: 4 declining, 0 rising.

Stream corridors — the surface connection

Where a reach's gain or loss has shifted, the wells within two miles of its anchoring gage and what their water table did over the same era.

MINER LONGAN DITCH: 1 alluvial well(s) nearby. Glover screen: 0.0 cfs of nearby metered alluvial pumping → est. lagged depletion 0.0–0.0 cfs (uncalibrated T/S ranges; flagged for SPDSS calibration, AWAS cross-check, and expert review — DWR's AWAS is the authority).

RYANS GULCH EXCHANGE DITCH: 6 alluvial well(s) nearby. Glover screen: 0.2 cfs of nearby metered alluvial pumping → est. lagged depletion 0.1–0.2 cfs (uncalibrated T/S ranges; flagged for SPDSS calibration, AWAS cross-check, and expert review — DWR's AWAS is the authority).

WIND RIVER NEAR ESTES PARK: 0 alluvial well(s) nearby. Glover screen: 0.0 cfs of nearby metered alluvial pumping → est. lagged depletion 0.0–0.0 cfs (uncalibrated T/S ranges; flagged for SPDSS calibration, AWAS cross-check, and expert review — DWR's AWAS is the authority).

BIG THOMPSON RIVER BELOW HILLSBOROUGH DIVERSION: 6 alluvial well(s) nearby; pooled water table -2.2 ft (1941–1986, decreasing, p=0.0017)

BIG THOMPSON RIVER ABOVE LAKE ESTES: 0 alluvial well(s) nearby

BIG THOMPSON RIVER AT LOVELAND, CO.: 5 alluvial well(s) nearby. Glover screen: 0.0 cfs of nearby metered alluvial pumping → est. lagged depletion 0.0–0.0 cfs (uncalibrated T/S ranges; flagged for SPDSS calibration, AWAS cross-check, and expert review — DWR's AWAS is the authority).

Wells labeled by administrative world before anything else is said about them: tributary alluvium participates in the priority system; designated-basin and Denver Basin bedrock aquifers are governed differently and are excluded from stream-connection reasoning. Read bedrock declines differently too: deep bedrock aquifers recharge negligibly on human timescales, so a falling bedrock level is storage mining — effectively permanent — not a drought response that a wet year undoes. All screens advisory.

Source DWR administrative record · verify at DWR structure records ↗

38 reservoir(s) with 17,304 storage measurement(s), 1950–2026. Basin storage as of 2025-10-31 (the record's own end — reservoir records publish ~annually): 145,945 AF across 27 reservoir(s).

Reservoir map — how full is each vessel?

Basin storage, end of each water year

End-of-water-year totals; per-year reservoir counts shown because reporting coverage varies -- a total is only comparable to years with similar counts.

Largest reservoirs on record

Click a row to chart its full storage history, or open its dossier for rights, documents and dam-safety facts.

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.

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 →

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.

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, Boulder County is 24% 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

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.

The water itself, in this district

Four registers, each on its own schedule: what USGS sensors read right now, where samples have been taken, which stretches the state lists as impaired, and how the tap water of the systems here stands with EPA. None is a DWR record; each links its own page.

Right now

Source Provisional telemetry · fetched 2026-10-13 · verify at USGS current conditions ↗

1 USGS sensor reports temperature, conductance, dissolved oxygen or turbidity from inside this district; provisional readings, never merged into the verified record.

Where water has been sampled

Source Register of another agency · fetched 2026-10-08 · verify at Water Quality Portal ↗

The portal lists 973 sampling stations in this district's watershed (Big Thompson 10190006), of which 85 reported a sample in the last 5 years on the state and tribal side (USGS activity is in the station count, not in that figure). Most active: Colorado Dept. of Public Health & Environment-WQCD (35), USGS Colorado Water Science Center (33), Colorado State University Agricultural Water Quality Program (Volunteer) (12). Most-sampled groups: Physical, Nutrient, Inorganics, Major, Metals, Inorganics, Minor, Metals. Every station as a file ↗

Results themselves are not fetched here; USGS samples added after March 2024 may not be in the portal's legacy profiles yet.

Drinking water

Source Register of another agency · fetched 2026-10-07 · verify at EPA SDWIS ↗

21 community systems touch this district. 1 carries an open health-based violation on EPA's record. EPA sampled 11 of them for PFAS in 2023–25; 1 had PFOA or PFOS at or above the 2024 limit.

and 11 more; the full directory filters by district.

EPA's record carries violations and lead/copper percentiles; nitrate and disinfection by-product results are not in it. UCMR 5 samples are monitoring, not compliance, and the 2024 PFOA/PFOS limits equal its reporting limit, so any detection of those two is at or above them.

Impaired waters

Source Register of another agency · verify at EPA ATTAINS ↗

The state's 303(d) impaired-waters list has not been joined to this basin's own watershed boundary yet, so nothing is shown rather than a guess; the basins with a join carry the full list here. Why the join is geometry, not a name match.