The September 2013 Boulder Flood: What Happened and Why

calendar May 25, 2026 · 3 min read · boulder flood 2013 boulder creek flood history natural disaster boulder county lyons colorado longmont colorado jamestown colorado  ·
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The Storm — September 9–15, 2013

A stalled upper-level low pressure system parked over the Great Basin tapped tropical moisture pulled north from a decaying Pacific tropical system, feeding eight straight days of orographic rainfall against the Front Range foothills. The NWS Denver/Boulder forecast office recorded 9.08 inches at the Boulder station on September 12 alone — more than the city's typical September total in a single 24-hour window. Storm-total rainfall reached 17–20 inches in the foothills above Jamestown and Pinewood Springs, the highest five-day totals on record for the central Front Range.

Peak Stream Flows

Boulder Creek crested at the USGS gauge in Boulder at roughly 5,000 cubic feet per second on September 12, more than five times the typical September flow. The St. Vrain Creek through Lyons hit 8,000–10,000 cfs at peak — a flow with a recurrence interval estimated above the 500-year mark for that drainage. Left Hand Creek, James Creek, and Fourmile Creek all overtopped historical records for their respective gauges. Lower Boulder Creek through Longmont peaked at flows that pushed water out of the channelized urban corridor and into adjacent neighborhoods, while Hygiene saw road inundation on St. Vrain Road for several days.

Why the Storm Caused This Much Damage So Quickly

Three factors compounded:

  1. Pre-saturated soils. August 2013 had been wetter than average across the foothills; by September 9 the soil column couldn't absorb additional rainfall. Runoff began within hours of the first heavy bands.
  2. Stationary orographic forcing. The atmospheric setup held in place for 96+ hours. Most Front Range flood events end when the upper-level pattern shifts in 24–48 hours; this one didn't.
  3. Steep, narrow drainages. Boulder, Fourmile, and James Creeks all run through narrow canyons before reaching the plains. Water accumulating in those drainages moved fast and hit communities at the canyon mouths — Jamestown, Lyons, and the western edge of Boulder — with very little warning.

The Affected Communities

Jamestown was effectively cut off when both road accesses washed out; nearly half the town's structures sustained major damage. Lyons lost its water and sewer systems for weeks and saw the entire downtown business district flood. Longmont evacuated 18,000 residents from the South Pratt Parkway / Roger's Grove corridor. Hygiene saw isolated road and pasture flooding. In Boulder itself, flooding hit hardest along the South Boulder Creek and Bear Creek corridors, with significant damage in Frasier Meadows and along the U.S. 36 alignment.

Across the eight-day event, ten lives were lost and FEMA disaster declarations covered 14 Colorado counties. Recovery costs exceeded $4 billion.

The Post-Event Record

The 2013 flood reshaped FEMA flood-zone mapping across Boulder County. The Boulder County Office of Emergency Management led a multi-year remapping effort with FEMA that expanded the AE zone along South Boulder Creek, James Creek, and the St. Vrain corridor through Lyons. Several thousand parcels that had been mapped X (minimal flood risk) before September 2013 are now mapped AE or A — a permanent change to insurance requirements and building-code constraints.

Stream-gauge records from the event are archived at waterdata.usgs.gov and inform every flood-risk model for the central Front Range produced since. The 2013 event is now the calibration anchor for what a 500-year flood actually looks like in this drainage basin.

References

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