Colorado Flood History

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On 31 July 1999 thunderstorms with heavy rain caused widespread damage to the small county of Ouray, Colorado's tenth least-populated county. Rainfall exceeding the 100-year event criterion contributed to the damage of nearly two miles of County Highway 24, and Colorado State Highway 62 was undercut in three places by floodwater. Dallas Creek, a tributary of the Uncompahgre River, swelled to 51 times its normal volume, resulting in the destruction of the waterway's only river gaging station. Four public bridges were damaged, and a fifth was rendered impassable, isolating several families on Log Hill Mesa and hampering access of emergency vehicles. Total public damage from these storms totaled over $750,000, half of the county's highway budget and nearly a quarter of the entire county annual budget. Private property damages included $100,000 to bridges and structures, and $20,000 to livestock and hay. No fatalities were associated with this event, but seven people had to be evacuated from an inundated barn by a local mountain rescue team. This study identifies the unique meteorological, hydrological and geographical factors that worked in tandem to create a devastating and financially-crippling flood in this sparsely populated location.

The 1999 Summer Monsoon season began in western Colorado during the second week of July. In the San Juan mountains, thunderstorm activity increased dramatically afterwards. Due to an increased flow of moist air from the eastern Pacific Ocean, precipitable water values were above normal for much of the month. Cooperative observing stations around the Dallas Creek area reported measurable precipitation nearly every day during the last half of the month. Total July rainfall was in excess of 200% above normal at several sites in southwest Colorado; Telluride, just south of the Dallas Creek basin, measured a monthly total of 159.8 mm (250% of normal), setting a July record. The period of heaviest July rainfall in the Dallas Creek basin occurred during the last week of the month. Thunderstorm activity with heavy rain was observed in or near the basin each day and numerous mudslides were reported in the adjacent San Miguel River basin to the southwest.

The heaviest rain in this area fell on 30-31 July, a period with precipitable water values in excess of 130% of normal. Placerville, located a few miles to the southwest of the Dallas Creek headwaters, measured 79.5 mm on 30 July alone. Two of the ten USBR DCP meteorological station rain gages in the Dallas Creek drainage also reported heavy rain on 30 July, with even heavier rain the following day. Pleasant Valley Meteorological Station, at an elevation of 2296 m in western Ouray County, is located in the drainage of Pleasant Valley Creek. The West Fork Dallas Creek Meteorological Station, located in southwest Ouray County at an elevation of 2823 m, is representative of rainfall that falls into both Cottonwood and West Fork Dallas Creeks.

The Pleasant Valley Meteorological Station received 95.8 mm of rain from thunderstorms on 30-31 July. Rainfall during this 48 hour period accounted for 72% of the July total. This site was located closest to the axis of heavy precipitation on 31 July, based on radar images and local reports. With nearby soils at or near saturation, over 25 mm of rain fell between 2000 and 2100, followed by an additional 25 mm the following hour. By 2200, rainfall rates had fallen to around 12.5 mm per hour, then ended shortly before 0000. Between 2000 and 2300, a total of 69.9 mm was recorded, an amount in excess of a 100 year rainfall for that location. The diminishing rainfall rates at Pleasant Valley was a result of a slight southern progression of the heavy rain band. The West Fork Dallas Creek Meteorological Station, approximately 11 km south of Pleasant Valley, recorded only light amounts of rain through 2200. Heavier rain totaling 33.3 mm had fallen at this gage on 30 July, and surrounding soils were also at or near saturation. After 31 July 2200 heavy rain again began to fall, with 30.7 mm recorded at the site by 2300. Lesser amounts of less than 6.5 mm fell between 2300 and 0000. The meteorological stations located near the easternmost tributaries, Beaver and East Fork Dallas Creeks, reported less than 6.5 mm during the afternoon of 31 July.

With rainfall rates of over 25 mm an hour for two hours in Pleasant Valley, combined with already saturated soils, Pleasant Valley Creek came out of its banks. Local reports and damage patterns to surrounding hayfields indicate that a large volume of overland flow swept across the valley and inundated the normally tiny stream. The post-flood debris field near the Route 24D bridge indicated that the creek, normally only a meter across, swelled to over 30 m wide. The bridge, about 3.7 m above the stream bed, sustained moderate damage when, according to eyewitnesses, it was overtopped by water levels that extended to nearly a meter above the bridge surface.

As the heavy rain shifted south, Cottonwood and West Fork Dallas Creeks also became inundated and came out of their banks. Their combined water was forced into the Dallas Creek main channel, which merges with Pleasant Valley Creek about 3 km downstream. Downstream from this confluence, the main stem Dallas Creek, normally 3 meters wide, expanded to 60 to 90 meters across. A concrete bridge on route 24A that traverses the creek about one mile downstream from the Pleasant Valley Creek confluence was undercut and damaged.

At the onset of this flood event, flows on Dallas Creek were averaging between 130 and 140 cfs, nearly double the monthly normal due to recent thunderstorm activity. By 2200 small rises on began to be recorded at the USGS gage. Telemetered data indicated that the flow had risen to around 250 cfs. One hour later, water volumes doubled to over 500 cfs. By 1 Aug 0000, Dallas Creek had increased to nearly 1900 cfs, exceeding the old 1923 record of 1120 cfs. Water levels continued to rapidly increase until 0045 when all data from the gage ceased. Estimates by the USGS through floodmarks indicated that Dallas Creek peaked around 3960 cfs, at a stage of 2.3 m (7.58 feet). This volume of water was 354% greater than the previous 1923 flood of record, and over 5100% of the July average flow

Previous flooding along South Boulder Creek occurred in 1894, 1938 and 1969.

During the flood of May 29-June 2, 1894, the South Boulder Creek basin received up to 6 inches of precipitation, washing out bridges, buildings, roads and railroads. This event occurred at the same time as the largest recorded flood on nearby Boulder Creek.

From Aug. 31 - Sep. 4, 1938, more than 6 inches of rainfall was recorded west of Eldorado Springs causing the largest recorded South Boulder Creek flood. Floodwaters of 7,390 cubic feet per second passed through Eldorado Springs causing heavy damage and destroying numerous buildings including the Eldorado Springs Resort and leaving a path of destruction in the valley downstream.

Flooding from May 4-8, 1969 was caused by a combination of rainfall and snowmelt. Precipitation of 8.11 inches at Eldorado Springs and 10.05 inches at Gross Reservoir caused a peak discharge of 1690 cubic feet per second at Eldorado Springs and inundated many areas downstream including US 36 at South Boulder Road. Of note during this event was the fact that Gross Reservoir captured the entire runoff generated above the dam, averting what might have been a much greater flood in the South Boulder Creek valley.