Donner Summit

Interstate 80 crosses the Sierra Nevada at Donner Summit, about 7,239 feet up on the westbound side and a little lower eastbound. The highway tops out at Euer Saddle, roughly two miles north of the historic Donner Pass (7,056 ft) that gave the place its name. This is the main all-weather truck route across the northern Sierra. If it shuts down, the next interstate-grade crossing for a loaded rig is about 400 miles south near Tehachapi, so a Donner closure backs freight up across two states.

The numbers say why it matters. Caltrans counts about 30,000 cars and 6,200 commercial vehicles over the summit on an average day, and rates I-80 here a primary freight route carrying more than $4.7 million in goods per hour. The grade is not the steepest in the West, but it is long: a sustained 3 to 6 percent for roughly 30 miles of climb and descent. Westbound, dropping toward Sacramento, is the side that kills brakes. There is a brake-check area between Nyack and Blue Canyon and runaway-truck ramps below it. Pick a low gear before you start down and let the engine hold you back.

Snow is the other half of the story. The Central Sierra Snow Lab at Soda Springs, right by the summit, averages around 360 inches a season, and the wider Donner Pass area runs higher. A single storm can drop several feet and bury the lanes faster than the plows clear them. Caltrans posts chain controls from about November through April, and rigs over 6,500 pounds have to carry chains in the Sierra all winter whether or not controls are up.

• Summit about 7,239 ft on I-80, at Euer Saddle just north of the original 7,056 ft Donner Pass
• About 30,000 cars and 6,200 trucks cross on an average day; a primary freight route (Caltrans District 3)
• Sustained 3 to 6 percent grade for roughly 30 miles; brake-check between Nyack and Blue Canyon, runaway ramps below
• Around 360 inches of snow in an average season at the Central Sierra Snow Lab (UC Berkeley)
• Chain controls roughly November to April; rigs over 6,500 lb must carry chains in the Sierra all winter (Caltrans)

Caltrans runs three chain levels on I-80. R-1 requires chains on everything except light vehicles under 6,000 pounds with snow tires. R-2 is the one that catches trucks: chains on all commercial vehicles, with an exception only for 4WD or AWD under 6,500 pounds on four snow tires. R-3 means chains on every vehicle, no exceptions. Trucks over 6,500 pounds chain up under R-1 and R-2 regardless of tires, and a rig pulling a trailer needs chains on at least one drive axle. When controls go up, the chain-up points are usually eastbound from Kingvale to the Donner Lake interchange at Truckee, and westbound from Truckee down to Rainbow. The pass itself closes several times in a typical winter, and Caltrans notes a road will often be shut before conditions reach R-3. Closures run from a few hours to more than a day in a long storm. Check Caltrans QuickMap or call the Caltrans Highway Information line at 1-800-427-7623 before you commit to the climb.

The pass carries the name of the Donner Party, a wagon train of emigrants trapped here by early heavy snow in the winter of 1846 to 1847, near what is now Donner Lake. Accounts differ on the exact count, but roughly half of the group of more than 80 died, and the survivors resorted to cannibalism. The Central Pacific Railroad punched through the crest in 1868, boring the 1,659-foot Tunnel 6 through granite with crews made up largely of Chinese laborers, then armoring the line with miles of timber snowsheds to keep it open through Sierra winters.

The graceful Rainbow Bridge on the old US-40 alignment opened in 1926 and still stands above Donner Lake. The winter of 1951 to 1952 is still the benchmark: a January storm dropped 154 inches at the summit with 80 mph winds, US-40 stayed blocked for 28 straight days, and the streamliner City of San Francisco sat snowbound near the summit with about 226 people aboard.