Authorities charged the stepgrandmother of a 1-year-old Idaho boy for his death due to being locked in a hot car for five hours on Wednesday evening.

His dead body was discovered by passers-by on a day when temperatures approached 100 degrees in Orofino, Idaho.

The heat wave continued moving across the West on Thursday as residents headed to rivers and lakes seeking relief from triple-digit temperatures expected to set records through at least Friday.

Some office workers were given the option to float on innertubes down the Boise River instead of sitting at stuffy desks, with temps in Boise expected to reach 105 degrees. Forecasters predicted a high of 107 on Friday — six degrees higher than the 101 record for that date set in 1985.

"Once it gets that high — 105, 107, 109 — it just feels hot," said Rick Overton, a copywriter who arranged the float trip for the digital marketing firm Wirestone. "I'm going to keep a tube under my desk for the whole summer and whenever it gets this hot I'm going to escape."

But temperatures in part of the West were climbing so high that authorities warned residents of southern Nevada, southeastern California and northwestern Arizona that outdoor activities could be dangerous except during the cooler early morning hours.

St. George, Utah, hit 111 by 1:30 p.m., a day after a nearby weather sensor recorded an unofficial reading of 118, which would top the the state's all-time record of 117 set in St. George in 1985. Summer temperatures across Utah are running 10 to 15 degrees above normal, meteorologist Brandon Smith said.

"To be honest, as far as temperatures, for as far out as we can see there's no relief," he said.

Around Las Vegas — where temperatures reached 109 degrees before 1 p.m. Thursday — transformers were overheating and causing electrical pole fires because of all the people switching on their air conditioners, said Scott Allison with the Clark County Fire Department.

In Montana, farmers anxiously watched their crops and thermometers. High temperatures for a handful of days can harm crop yield.

"Prolonged heat is devastating. Four or five days of it is going to be hard," said wheat farmer Lynn Nordwick near Poplar, Mont.

Even Stanley, Idaho, which at more than 6,000 feet elevation is routinely the coldest place in the lower 48 states, was seeing record highs, the National Weather Service said. The remote town in the Sawtooth Mountains was expected to reach 93 degrees Thursday, and 92 degrees Friday.

Hardly anyone in the tiny town has air conditioning, said Nancy Anderson, Stanley deputy city clerk. The City Hall offices are also without that amenity.

"They're all going to the lakes and the rivers and trying to find the shade," Anderson said.

At least 150,000 people were expected to flock to the Lake Mead National Recreation Area in Nevada and Arizona in hopes of cooling off in the water this weekend, said Roxanne Dey, recreation area spokeswoman.

"For some people, we're the only affordable alternative for a place to cool off," Dey said.

In Phoenix, which hit 115 Thursday, 42-year-old laborer Russ Waldrip wiped sweat from his face as he unloaded large windows from the back of a truck.

"When it gets this hot I pour water over my head all day," Waldrip said. "Sometimes I can't wait to jump in the pool, but I don't even have the energy to do that."

Arizona emergency rooms don't see many patients when it's this hot, said Dr. Ann-Michelle Ruha at Banner Thunderbird Medical Center in Glendale. "People in Arizona seem aware of the problems that come with the heat," Ruha said.

In Spokane, Wash., the temperature was expected to soar past 100, breaking a record for July 5 set in 1975. In the northern Idaho lake city of Sandpoint, a forecast temperature of 103 would break a record for the date set in 1926, the National Weather Service said.

Northeastern Oregon residents were experiencing what was expected to be the hottest day of the year on Thursday, with temperatures hitting 107 in Hermiston and Pendleton.

The heat and a dry spring raised concern among firefighters.

"We're really primed to burn right now," said Dennis Winkler, an assistant fire management officer for the Wallowa-Whitman National Forest. "We're well above average in terms of fire danger for this time of year."

An index that helps fire officials estimate how fast flames would spread was at its highest point for early July in the past decade, said Dave Quinn, who manages an interagency dispatch center at the La Grande Airport in Oregon.

The heat wave began last week after a large high pressure center developed over Arizona, said National Weather Service forecaster Paul Flatt in Boise. A weather pattern was pushing that high-pressure center north into Canada, Flatt said, but most of the West is expected to experience high temperatures into next week.