On September 9, 2013, rain began to fall in eastern Colorado as a large low-pressure system pulled plumes of tropical moisture northward from the Pacific Ocean and the Gulf of Mexico. By September 16, 2013, as much as 12 to 20 inches of rain had fallen in the foothills of the Front Range of the Southern Rocky Mountains and adjacent plains near Colorado Springs, Colorado, north to the Colorado-Wyoming border. The rain caused major flooding during September 9–18, 2013, in a large part of the South Platte River Basin and in the Fountain Creek Basin. The floods resulted in several fatalities, more than 31,000 damaged or destroyed structures, and an estimated 3 billion dollars in damages. The U.S. Geological Survey (USGS) documented peak stage, streamflow, or both from the flood event for 80 sites located on selected rivers and streams in the South Platte River and Fountain Creek Basins and on the Platte River in Nebraska. The majority of flood-peak streamflows occurred on September 12 or 13, 2013, coinciding with the period of maximum rainfall. The flood resulted in new record peak streamflows at 17 streamgages having at least 10 years of record; 13 in the South Platte River Basin and 4 in the Fountain Creek Basin.

Flooding in the South Platte River Basin was primarily contained to select streams in Aurora and the Denver metropolitan area, most of the mountain tributaries joining the main stem South Platte River from Denver to Greeley, and in the main stem South Platte River from Denver to the Colorado-Nebraska State line. In Aurora, where about 15 inches of rain fell, streamflow peaked at 5,470 cubic feet per second (ft³/s) in Toll Gate Creek, a tributary to Sand Creek. Downstream from Aurora near the confluence with the South Platte River, Sand Creek peaked at 14,900 ft³/s, which was the highest streamflow since at least 1992, but less than the peak of 25,500 ft³/s in 1957 that occurred 4 miles upstream from the mouth. Flood-peak streamflows in the Denver metropolitan area were generally below historic records. The peak of 3,930 ft³/s on September 12 at the State of Colorado streamgage South Platte River at Denver ranked 59 out of 116 peaks and was less than the 1965 peak of 40,300 ft³/s. Ten of the 13 streamgages in the South Platte River Basin with new record peak streamflows were located on the mountain tributaries; Bear Creek, Fourmile Creek, Boulder Creek, St. Vrain Creek, the Big Thompson River, and the Cache la Poudre River. A daily average streamflow of 8,910 ft³/s on September 13 in Boulder Creek at the confluence with St. Vrain Creek was more than twice the previous instantaneous peak of 4,410 ft³/s from 1938. The USGS calculated a peak streamflow of 23,800 ft³/s for the St. Vrain Creek at Lyons; the highest streamflow on record at this State of Colorado streamgage (122 years of record) is 10,500 ft³/s from 1941. A peak streamflow of 16,200 ft³/s was calculated for the Big Thompson River at mouth of canyon near Drake streamgage, which is the second highest peak in 90 years of record and about one-half the magnitude of the peak of 31,200 ft³/s from July 31, 1976. A streamflow of 60,000 ft³/s in the South Platte River at Fort Morgan (September 15, 2013) suggests that a new record streamflow occurred in the main stem in the Greeley area, about 45 miles upstream from Fort Morgan. The current peak of record at a State of Colorado streamgage at Kersey, about 6.5 miles downstream from Greeley, is 31,500 ft³/s from 1973. Given that there was minimal inflow between Kersey and Fort Morgan, the USGS estimates there was probably at least 60,000 ft³/s at Kersey, which would be almost double the peak streamflow of record from 1973.

Flooding in the Fountain Creek Basin was primarily contained to Fountain Creek from southern Colorado Springs to its confluence with the Arkansas River in Pueblo, in lower Monument Creek, and in several mountain tributaries. New record peak streamflows occurred at four mountain tributary streamgages having at least 10 years of record; Bear Creek, Cheyenne Creek, Rock Creek, and Little Fountain Creek. Five streamgages with at least 10 years of record in a 32-mile reach of Fountain Creek extending from Colorado Springs to Piñon had peak streamflows in the top five for the period of record. A peak of 15,300 ft³/s at Fountain Creek near Fountain was the highest streamflow recorded in the Fountain Creek Basin during the September 2013 event and ranks the third highest peak in 46 years. Near the mouth of the basin, a peak of 11,800 ft³/s in Pueblo was only the thirteenth highest annual peak in 74 years. A new Colorado record for daily rainfall of 11.85 inches was recorded at a USGS rain gage in the Little Fountain Creek Basin on September 12, 2013.