Upper South Platte Watershed Drought and Flood Data

Upper South Platte Watershed Drought 

The Upper South Platte Watershed has a continental-type climate modified by topography, in which there are large temperature ranges and irregular seasonal and annual precipitation. Areas along the Continental Divide average 30" or more of precipitation annually, which includes snowfall in excess of 25'. In contrast, the annual precipitation on the plains east of Denver, Colorado, and in the South Park area in the southwest part of the basin, ranges from 7" to 15" (USGS, 1995).

Most of the precipitation on the plains occurs as rain, which typically falls between April and September, whereas most of the precipitation in the mountains occurs as snow, which typically falls between October and March.

With its beautiful lush green vegetation and mild weather, most assume that Colorado is a well balanced state. The truth is that Colorado is a dry state. 

Droughts regularly occur in the Upper South Platte Watershed as with the rest of Colorado. Statewide, in the 1900's alone, there were four prolonged dry spells that occurred. One of them caused the infamous "Black Sunday" - the worst storm of the Dust Bowl. 

In this century, the most severe Colorado drought since 1723 occurred in 2002. It all started in the fall of 2001, instead of cooling down, summer temperatures continued through the winter months. By the early months of 2002, the state had not received nearly as much snow as it normally would. By late spring/early summer, the state encountered some heavy rains. However, the ground was so hard, the vegetation on grazed lands was so short and the rains fell for such a short period of time, that little of the moisture soaked in and vegetation remained parched. 

The effect of the drought was severe wildfires, extremely low streamflows, rapid depletion of reservoirs, severe agricultural damages, urban water restrictions, and intense heat which caused serious health issues. By September of 2002, the weather began to turn around and Colorado finally experienced a cooling period. 

Colorado Drought Data

The data above demonstrates drought severity in Colorado from 2000 to 2020. 

• D0 - Abnormally Dry
• D1 - Moderate Drought
• D2 - Severe Drought
• D3 - Extreme Drought
• D4 - Exceptional Drought

The data below demonstrates drought severity for the Upper South Platte Watershed

According to the United States Drought Monitor, the Upper South Platte Watershed can be classified anywhere between abnormally dry (D0) to extreme drought (D3)

Upper South Platte Watershed Flood

Due to the fact that Colorado is an arid state and prone to wildfires, it is also subject to flooding. In 1996, the Buffalo Creek Fire burned approximately 12,000 acres within the Upper South Platte Watershed, resulting in the loss of several homes and essential forest cover on highly erodible soils. Following the fire, heavy rainfall led to floods, which resulted in substantial erosion and sedimentation that caused severe damage to downstream reservoirs used to supply drinking water for the Denver Metro Area.

During the first week of September 2013, Colorado was exceptionally warm and dry. Two weeks later, everything had changed. Flood conditions stretched about 150 miles, from Colorado Springs to Ft. Collins. Soils were completely saturated with water, ponds, lakes, rivers, and creeks began to overflow, roads were washed out, and homes were flooded. By September 16, authorities had confirmed six deaths, and more than 1,000 people remained missing (Denver Post, n.d.).

1965 South Platte Flood

On the evening of June 16, 1965, 14 inches of rain near Larkspur, CO, sent a violent wall of water, 20 feet high in some places, down Plum Creek and into the South Platte River, wiping out bridges, roads and homes as it tore through Denver. The South Platte, itself swollen by rain, crested at 25 feet above normal.

A floating logjam of debris, including lumber, trees, old cars and refrigerators, became a battering ram as it moved downstream, destroying roads and 26 bridges in its path. A massive rescue operation by owners, trainers and jockeys saved about 140 horses at the Centennial Racetrack. Power plants along the river were shut down, and emergency circuits became waterlogged and shorted out. As the flood continued north, other tributaries added their water: Sand Creek, Clear Creek, the Bijou, Little Beaver and the Cache La Poudre River. The course of the South Platte River to the Colorado-Nebraska border was a mud-encased, wreckage-strewn landscape of desolation. The communities of Sterling, Fort Morgan and Brush became isolated as the waters spread out over a quarter-million acres of farmland. Denver's water supply, which consisted mainly of a series of wells along the river, was nearly destroyed.

The flood crest did not pass the most-downstream gaging station on the South Platte River in Colorado until June 20, 1965. Damage estimates came to about $540 million. The storm claimed 28 lives. Peak discharges at several gauging stations had recurrence intervals that exceeded 100 years. The peak discharge on the South Platte River at Denver was 40,300 feet cfs. In the wake of the flood, plans were quickly finalized and construction began on the Chatfield Dam, completed in 1972 (9News, 2013).

1986 Big Thompson Canyon Flood

Saturday, July 31, Colorado was celebrating 100 years of statehood and, by late afternoon, an estimated 2,500 to 3,500 people were driving into or already staying in one of Colorado's most scenic and natural river valleys, the Big Thompson Canyon. A thunderstorm lifted along the Front Range of the Rockies and began to dump heavy rain on the region. Light winds in the upper atmosphere allowed the storm to remain virtually stationary over the Big Thompson Valley. Heavy rain fell over a 70-square-mile area in the central portion of the Big Thompson watershed from 6:30-11:00 pm. The most intense rainfall, between 12 and 14 inches, fell on slopes in the western end of the canyon. Water in the river quickly rose over banks and raged through the canyon corridors.

The flood washed out all stream and rain gages so accurate measurements were not possible. The flood crest on the Big Thompson River moved through the 7.6 miles between Drake and the canyon mouth in about 30 minutes, at an average speed of about 15 miles per hour. At times the torrent of water was 19 feet high. Flash flooding destroyed recreation and commercial buildings built along the rivers banks. U.S. Highway 34, the only road that exits the canyon, was washed out. The floodwater was so powerful that 10-foot-wide boulders were lifted and carried down-river. Cars were easily swept away. Many of the lives lost were campers who had set their camps near the river. The flood of July 31-August 1, 1976, on the Big Thompson and Cache la Poudre Rivers, resulted in 143 deaths, 150 injuries, and total damage of about $39 million (Big Thompson Coalition, 2018).

2013 Colorado Flood

Through the first week of September 2013, Colorado was exceptionally warm and dry. By September 12, everything had changed. Flood conditions stretched about 150 miles, from Colorado Springs north to Ft. Collins. Saturated soils left water with no place to go, and puddles turned to ponds throughout the densely populated Colorado Front Range. Rainwater swelled rivers and creeks, overtopped dams, flooded basements, and washed out roads. By September 16, authorities had confirmed six deaths, and more than 1,000 people remained missing.

Among the hardest-hit communities was Boulder, located on the northwestern end of the Denver metropolitan area. Boulder's Daily Camera reported that heavy rains started on the evening of September 11 and continued through the following morning. The National Weather Service recorded rainfall amounts exceeding 8 inches in Boulder on September 12, and amounts exceeding 4 inches the next day. Meteorologist Jeff Masters noted on his blog that the three-day rainfall recorded by the evening of September 12 exceeded the monthly total for any month since rainfall records began in 1897. Similarly high rainfall totals occurred in other spots along the Front Range. Masters exclaimed, "These are the sort of rains one expects on the coast in a tropical storm, not in the interior of North America!"

Russ Schumacher of Colorado State University concluded that the precipitation in Boulder County and other parts of the state qualified as a 1,000-year event, meaning that any one year has just a 1-in-1,000 chance of experiencing such heavy precipitation. Ranking the actual flooding is more challenging than quantifying the precipitation because over time, people use the land differently. Bob Henson of the University Corporation for Atmospheric Research remarked in AtmosNews, "An identical weather event a century ago might produce a much different flood than the same event today."

On September 12, the Boulder Creek, which flows roughly eastward through town, crested in downtown Boulder at 7.78 feet—the highest water level observed at that location since 1894. The main highway running through Boulder was partially closed southeast of town, and partially destroyed northwest of town, isolating the nearby mountain community of Lyons. Thousands of residents faced power outages and evacuation orders in the Denver-Boulder area as officials called in the National Guard to assist rescue efforts. Schools, businesses, and government offices closed. Many roads remained closed and impassable, so multiple mountain communities remained isolated. And the rain kept falling.

As the rain continued, heavy humidity hung in the air. Precipitable water—the height of liquid water that would result if all the water vapor in the atmospheric column were condensed—showed record-high measurements for this time of year, as recorded by balloon soundings from the National Weather Service Storm Prediction Center. Masters noted that the region's highest September precipitable water measurements, dating back to 1948, occurred on September 12-13, 2013 (COState, 2015).