Starting on September 11, 2013, a slow-moving cold front stalled over Colorado, clashing with warm humid monsoonal air from the south.

The National Weather Service's Hydrometeorological Design Studies Center stated in a document that the annual exceedance probability (AEP) for the entire rainfall event was as low as 1/1000 (0.1%) in places.

[10] Governor John Hickenlooper declared a disaster emergency on September 12, 2013, in 14 counties: Adams, Arapahoe, Broomfield, Boulder, Denver, El Paso, Fremont, Jefferson, Larimer, Logan, Morgan, Pueblo, Washington and Weld.

"[12] This resulted in rainfall totals exceeding 20 inches in parts of Boulder County, along with numerous flash floods, property destruction and loss of life.

In 2013, The Big Thompson River experienced peak flow rates near Loveland, CO of 4,500 CFS (127.43 cubic meters per second) before the measuring gauge was destroyed by floodwaters.

[citation needed] At least eight deaths were reported by the Colorado Office of Emergency Management, with two more missing and presumed dead and hundreds remaining unaccounted for.

[10] The town of Lyons in Boulder County was isolated by the flooding of St. Vrain Creek,[19] and several earth dams along the Front Range burst or were over-topped.

[22][23][24] Rescue efforts were hampered by continuing rain and a low cloud ceiling, which grounded National Guard helicopters September 15.

[27] Miles of freight and passenger rail lines were washed out or submerged, including a section servicing Amtrak's iconic California Zephyr.

[29] Lower-lying agricultural land in northeast Colorado was affected as flood waters surged down rivers and creeks, inundating fields and pastures.

[51][52] A spill from flood-damaged storage tanks in Milliken was reported September 18, which released 5,250 US gallons (19,900 L; 4,370 imp gal) of crude oil into the South Platte River.

Contamination from sewage, oil, and waste water containing toxic substances can delay natural succession processes if not alter them entirely.

For example, waste water from flooded fracking wells could introduce levels of lead into a freshwater system, keeping a particular strain of algae from developing in usual successional form.

This authorized federal search and rescue teams, as well as supplies such as food, water, cots, generators, and emergency flood control measures.

[60] Obama also declared a major disaster specifically for Boulder County, which provides for federal recovery assistance such as temporary housing, home repairs, and low-cost loans.

[61] On September 25, 2013, Rep. Cory Gardner (R-CO) introduced the bill To authorize the Secretary of Transportation to obligate funds for emergency relief projects arising from damage caused by severe weather events in 2013 (H.R.

[65] The shutdown compromise signed on October 17, 2013 includes funding for Colorado relief efforts, specifically referencing Rep. Gardner's bill H.R.