Colorado history

Wednesday, September 30, 2026

A president blesses a dam on the river that named the state, Denver's missing Interstate gets a federal stamp of no, and September files one last 25-degree morning next to a 91-degree afternoon.

Roosevelt stands on the wall and tells the Colorado River it is employed now

1935 · Black Canyon of the Colorado River, Arizona–Nevada line

On Sept. 30, 1935, Franklin Roosevelt dedicated Boulder Dam — later Hoover Dam — on the Colorado River. He looked 726 feet down a concrete arch and told a radio nation that the river which had been running unused to the sea was now a national possession.

The river keeps the state's name even when it is 700 miles from Denver. By the time it reaches Black Canyon it has already left Grand Lake, Glenwood, the Utah line, and the Grand Canyon. What it had not done, for most of human time, was stay put. Flood, drought, flood. The Colorado River Compact of 1922 tried to divide the water among seven states. The dam tried to make the division real.

Roosevelt arrived by train after stops in Nebraska, Wyoming, and Utah. About 10,000 people stood on the Nevada rim. Interior Secretary Harold Ickes ran the program. The president, who had never seen the hole before that morning, borrowed Caesar and then improved him: "I came, I saw and I was conquered." He pressed a button tied to the first generator. Twelve jets of water answered. A White House operator wired the dam to ask if the signal had landed. The dam wired back: "Yes. There's water all over the place."

The wall was the tallest dam on earth that year. Lake Mead, still filling, would become the largest reservoir in the country. The Bureau of Reclamation billed the dam and powerhouses at $108 million, to be paid back from power sales. Construction from 1931 to 1936 killed nearly a hundred men, a number the official ledger still argues with. Congress had authorized the project under Coolidge. Hoover's name was on the original paperwork. Roosevelt's people called it Boulder Dam anyway. In 1947 Congress put Hoover back on the map.

Colorado does not own the dam. It owns the headwaters argument. Every acre-foot that leaves the Front Range and the Western Slope is still traveling toward that wall. The dedication was a New Deal sermon about putting idle water to work. The river has been on the clock ever since.

Sources: American Presidency Project — FDR dedication address · Bureau of Reclamation / Reclamation Era, Oct. 1935 · White House Historical Association — 1935 western tour · Associated Press via Tucson.com — 1935 dedication

Washington takes I-470 off the Interstate map and Denver invents the 470s instead

1977 · Southwest metro Denver

On Sept. 30, 1977, the Federal Highway Administration approved Colorado's request to pull proposed Interstate 470 — a 26.3-mile southwest beltway — out of the Interstate system. Governor Dick Lamm had promised to drive a silver stake through it. The stake held. The beltway did not disappear. It changed letters.

The 1968 highway act had put I-470 on the federal books as a 90-percent-paid loop along the hogback from Golden toward I-25. Environmental statements followed. The Colorado Department of Health said the road would fight the Clean Air Act. Lamm won the governorship in 1974 on a platform that included killing the project. In May 1975 he ordered the planning to stop. An ad-hoc commission spent 1976 taking testimony and, that December, voted unanimously to withdraw the route and spend the money on a parkway plus other transportation.

Lamm sent the formal request on July 28, 1977. On this date Federal Highway Administrator William Cox and Urban Mass Transportation Administrator Richard Page signed the withdrawal. Colorado's Interstate construction account was trimmed by about $4 million on paper. The real pot — later tallied near $264 million in withdrawal value — went to substitute work: the southwest "Centennial Parkway," a wider Santa Fe Drive, and transit that included downtown's 16th Street Mall.

The parkway did not stay a parkway. Suburbs kept arriving. Designers put in interchanges. Construction started in the early 1980s. The last piece of what everyone now calls C-470 opened in October 1990. East of I-25 the missing arc became E-470, a toll authority road that reached I-25 north in 2003 and learned to bill your plate. Northwest Parkway closed another gap. The northwest quadrant is still the argument that will not sit down.

Lamm was right that a six-lane Interstate would rearrange the land. He was also watching the land rearrange itself. If you drive the hogback at rush hour you are on the ghost of I-470, paying state and toll money for a federal idea that this date officially declined.

Sources: FHWA — Interstate withdrawal-substitution, I-470 Denver · Matthew Salek — Denver's 470 saga · Interstate Guide — former I-470 Colorado · AARoads Wiki — Colorado State Highway 470

September's last morning can still be 25 degrees, and the afternoon can still be 91

1985 · Denver

On the morning of Sept. 30, 1985, Denver bottomed out at 25 degrees, a date record and the last shiver of the coldest September air mass the official books admit. The same square on the calendar once refused to climb above 35 and, in 2023, reached 91.

The 17-degree reading on the 29th is still the September floor. This date kept the sequel: 25 at dawn, a high of only 40, and a city that had already shoveled 8 to 12 inches of powder off trees that still had leaves. Record cold hung around through Oct. 1. If you lived here that week you remember the sound of limbs before you remember the number.

Twenty-six years earlier the date was the back half of another September snow. Rain on the 28th of 1959 turned wet and heavy. Stapleton measured 5.9 inches from the 30th into Oct. 1, on top of the 8.2 already in the book for the 29th. The official high on this date in 1959 was 35, still the lowest maximum the square has allowed. Date-record snowfall is a modest 1.5 inches. The storm around it was not modest. Colorado Springs took 27 to 34. Trees in leaf snapped from the Springs to Denver.

The date also practices summer. In 2023 the official high hit 91, the warmest this calendar box has been. In 1980 it was 90. In 1940 a late thunderstorm killed a man by lightning inside the city. In 1898 south winds held 41 and gusted to 54. Boulder clocked 60 in 1981. South Aurora took inch hail in 2004. Average high is still in the low 70s, which is the reading almost nobody writes down.

Close the month however you like. This square has already been a freeze warning, a snow day, a wind event, and a short-sleeve record. October can take notes.

Sources: Extreme Weather Watch — Denver Sept. 30 · ThorntonWeather — Sept. 27 to Oct. 3 · NWS Boulder public information statement · WeatherBug — 1959 Front Range snow