The state’s department of forestry and fire protection, Cal Fire, says more than two million acres have burned, more than the size of Delaware.
One fire, El Dorado, which has spread over 7,000 acres, was started by a gender reveal party, officials say.
California is currently experiencing a record heatwave.
Los Angeles County reported its highest ever temperature of 49.4C (121F) on Sunday. Although temperatures are expected to drop from Tuesday onwards it may bring strong winds which could fan the flames, the National Weather Service warns.
The largest blaze, known as the Creek Fire, has burned more than 78,000 acres since it broke out in the Sierra Mountains on Friday, and the authorities said none of it had been contained.
The fire has burned at least two dozen dwellings in the town of Big Creek, the Los Angeles Times reports. More than 200 hikers had to be airlifted out of the popular Mammoth Pool Reservoir after becoming trapped by flames on Saturday.
Valley Fire in San Diego County has burned through more than 10,000 acres, and prompted the evacuation of the remote town of Alpine; while Bobcat fire in Angeles National Forest has destroyed nearly 5,000 acres and saw the evacuation of the Mount Wilson Observatory.
Cal Fire blamed a “smoke-generating pyrotechnic device, used during a gender reveal party” for starting the The El Dorado fire in San Bernadino County. Gender reveal parties are celebrations announcing whether expecting parents are going to have a girl or a boy. In recent years, several large-scale parties have gone wrong, even resulting in the death of a woman in 2019.
“Cal Fire reminds the public that with the dry conditions and critical fire weather, it doesn’t take much to start a wildfire”, the tweet read.
People who cause fires “can be held financially and criminally responsible”, it added.
California has seen nearly 1,000 wildfires since 15 August, often started by lightning strikes.
Cal Fire said that, not only had two million acres already burned this year, but there had been eight fatalities and more than 3,300 structures destroyed. In 2018, 1.96 million acres were burned – the highest since Cal Fire began tracking numbers in 1987 – the Associated Press reports.
Cal Fire spokesman Lynne Tolmachoff told AP that it was alarming how early the fires had begun this year. “It’s a little unnerving because September and October are historically our worst months for fires. It’s usually hot, and the fuels really dry out. And we see more wind events”.
States of emergency have been issued in five Californian counties – Fresno, Madera, Mariposa, San Bernardino and San Diego – and people have been urged not to hike, after one hiker died.